This is a "kick start" tutorial to get you started with Erlang. Everything here is true, but only part of the truth. For example, I'll only tell you the simplest form of the syntax, not all esoteric forms. Where I've greatly oversimplified things I'll write *manual* which means there is lots more information to be found in the Erlang book or in the manual. I also assume that this isn't the first time you have touched a computer and you have a basic idea about how they are programmed. Don't worry, I won't assume you're a wizard programmer. The Erlang Shell ---------------- Most operating systems have a command interpreter or shell, Unix and Linux have many, Windows has the Command Prompt. Erlang has it's own shell where you can directly write bits of Erlang code and evaluate (run) them to see what happens (*manual*). Start the Erlang shell (in Linux or UNIX) by starting a shell or command interpreter in your operating system and typing "erl", you will see something like this. $ erl Erlang (BEAM) emulator version 5.2 [source] [hipe] Eshell V5.2 (abort with ^G) 1> Now Type in 1 + 1. as shown below. 1> 1 + 1. 2 2> In Windows, the shell is started by double-clicking on the Erlang shell icon. You'll notice that the Erlang shell has numbered the lines that can be entered, (as 1> 2>) and that it has correctly told you that 1 + 1 is 2! Also notice that you have to tell it you are done entering code by finishing with a full stop "." and a carriage return. If you make mistakes writing things in the shell, you can delete things by using the backspace key as in most shells. There are many more editing commands in the shell (*manual*). (note: you will find a lot of line numbers given by the shell out of sequence in this tutorial as it was written and the code tested in several sessions). Now let's try a more complex calculation. 2> (42 + 77) * 66 / 3. 2618.00 3> Here you can see the use of brackets and the multiplication operator "*" and division operator "/", just as in normal arithmetic *manual*. To shutdown the Erlang system and the Erlang shell type Control-C. You will see the following output: BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded (v)ersion (k)ill (D)b-tables (d)istribution a Type "a" to leave the Erlang system. A programming language isn't much use if you can just run code from the shell. So here is a small Erlang program. Enter it into a file called tut.erl (the file name "tut.erl" is important, also make sure that it is in the same directory as the one where you started "erl" (*manual*) using a suitable text editor. If you are lucky your editor will have an Erlang mode which will make it easier for you to enter and format your code nicely (*manual*), but you can manage perfectly well without. Here's the code to enter: -module(tut). -export([double/1]). double(X) -> 2 * X. It's not hard to guess that this "program" doubles the value of numbers. I'll get back to the first two lines later. Let's compile the program. This can be done in your Erlang shell as shown below: 3> c(tut). {ok,tut} 4> The {ok, tut} tells you that the compilation was OK. If it said "error" instead, you have made some mistake in the text you entered and there will also be error messages to give you some idea as to what has gone wrong so you can change what you have written and try again. Now lets run the program. 4> tut:double(10). 20 5> As expected double of 10 is 20. Now let's get back to the first two lines. Erlang programs are written in files. Each file contains what we call an Erlang "module". The first line of code in the module tells us the name of the module (*manual*). -module(tut). This tells us that the module is called "tut". Note the "." at the end of the line. The files which are used to store the module must have the same name as the module but with the extension ".erl". In our case the file name is "tut.erl". When we use a function in another module, we use the syntax, module_name:function_name(arguments). So 4> tut:double(10). means call function "double" in module "tut" with argument "10". The second line: -export([double/1]). says that the module tut contains a function called double which takes one argument ("X" in our example) and that this function can be called from outside the module tut. More about this later. Again note the "." at the end of the line. Now for a more complicated example, the factorial of a number (e.g. factorial of 4 is 4 * 3 * 2 * 1). Enter the following code in a file called tut1.erl. -module(tut1). -export([fac/1]). fac(1) -> 1; fac(N) -> N * fac(N - 1). Compile the file 5> c(tut1). {ok,tut1} 6> And now calculate the factorial of 4. 6> tut1:fac(4). 24 7> The first part: fac(1) -> 1; Says that the factorial of 1 is 1. Note that we end this part with a ";" which indicates that there is more of this function to come. The second part: fac(N) -> N * fac(N - 1). Says that the factorial of N is N multiplied by the factorial of N - 1. Note that this part ends with a "." saying that there are no more parts of this function. A function can have many arguments. Let's expand the module tut1 with the rather stupid function to multiply two numbers: -module(tut1). -export([fac/1, mult/2]). fac(1) -> 1; fac(N) -> N * fac(N - 1). mult(X, Y) -> X * Y. Note that we have also had to expand the "-export" line with the information that there is another function "mult" with two arguments. Compile: 7> c(tut1). {ok,tut1} 8> and try it out: 8> tut1:mult(3,4). 12 9> In the example above the numbers are integers and the arguments in the functions in the code, N, X, Y are called variables. Note that variables must start with a capital letter (*manual*). Examples of variable could be Number, ShoeSize, Age etc. Atoms are another data type in Erlang. Atoms start with a small letter (*manual*), for example: charles, centimeter, inch. Atoms are simply names, nothing else. They are not like variables which can have a value. Enter the next program (file: tut2.erl) which could be useful for converting from inches to centimeters and vice versa: -module(tut2). -export([convert/2]). convert(M, inch) -> M / 2.54; convert(N, centimeter) -> N * 2.54. Compile and test: 9> c(tut2). {ok,tut2} 10> tut2:convert(3, inch). 1.18110 11> tut2:convert(7, centimeter). 17.7800 12> Notice that I have introduced decimals (floating point numbers) without any explanation, but I guess you can cope with that. See what happens if I enter something other than centimeter or inch in the convert function: 13> tut2:convert(3, miles). =ERROR REPORT==== 28-May-2003::18:36:27 === Error in process <0.25.0> with exit value: {function_clause,[{tut2,convert,[3,miles]},{erl_eval,expr,3},{erl_eval,exprs,4},{shell,eval_loop,2}]} ** exited: {function_clause,[{tut2,convert,[3,miles]}, {erl_eval,expr,3}, {erl_eval,exprs,4}, {shell,eval_loop,2}]} ** 14> The two parts of the convert function are called its clauses. Here we see that "miles" is not part of either of the clauses. The Erlang system can't "match" either of the clauses so we get an error message "function_clause". The above output looks rather a mess, but with a little practice, you can see from it exactly where in the code the error occurred. Now the tut2 program is hardly good programming style. Consider: tut2:convert(3, inch). Does this mean that 3 is in inches? or that 3 is in centimeters and we want to convert it to inches? So Erlang has a way to group things together to make things more understandable. We call these "tuples". Tuples are surrounded by "{" and "}". So we can write {inch, 3} to denote 3 inches and {centimeter, 5} to denote 5 centimeters. Now let's write a new program which converts centimeters to inches and vice versa. (file tut3.erl). -module(tut3). -export([convert_length/1]). convert_length({centimeter, X}) -> {inch, X / 2.54}; convert_length({inch, Y}) -> {centimeter, Y * 2.54}. compile and test: 14> c(tut3). {ok,tut3} 15> tut3:convert_length({inch, 5}). {centimeter, 12.7000} 16> tut3:convert_length(tut3:convert_length({inch, 5})). {inch, 5.00000} 17> Note on line 16 we convert 5 inches to centimeters and back again and reassuringly get back to the original value. I.e the argument to a function can be the result of another function. Pause for a moment and consider how line 16 (above) works. The argument we have given the function {inch, 5} is first matched against the first head clause of "convert_length" i.e. convert_length({centimeter, X}) where is can be seen that {centimeter, X} does not match {inch, 5} (the head is the bit before the "->"). this having failed, we try the head of the next clause i.e. convert_length({inch, Y}), this matches and Y get the value 5. We have shown tuples with two parts above, but tuples can have as many parts as we want and contain any valid Erlang "term". For example, to represent the temperature of various cities of the world we could write {moscow, {c, -10}} {cape_town, {f, 70}} {paris, {f, 28}} Tuples have a fixed number of things in them. We call each thing in a tuple an element. So in the tuple {moscow, {c, -10}}, element 1 is moscow and element 2 is {c, -10}. I have chosen c meaning Centigrade (or Celcius) and f meaning Fahrenheit. Whereas tuples group things together, we also want to be able to represent lists of things. List in Erlang are surrounded by "[" and "]". For example a list of the temperatures of various cities in the world could be: [{moscow, {c, -10}}, {cape_town, {f, 70}}, {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}] Note that this list was so long that it didn't fit on one line. This doesn't matter, Erlang allows line breaks at all "sensible places" but not, for example, in the middle of atoms, integers etc. A very useful way of looking at parts of lists, is by using "|". This is best explained by an example using the shell. 18> [First |TheRest] = [1,2,3,4,5]. [1,2,3,4,5] 19> First. 1 20> TheRest. [2,3,4,5] 21> We use | to separate the first elements of the list from the rest of the list. (First has got value 1 and TheRest value [2,3,4,5] Another example: 21> [E1, E2 | R] = [1,2,3,4,5,6,7]. [1,2,3,4,5,6,7] 22> E1. 1 23> E2. 2 24> R. [3,4,5,6,7] 25> Here we see the use of | to get the first two elements from the list. Of course if we try to get more elements from the list than there are elements in the list we will get an error. Note also the special case of the list with no elements []. 25> [A, B | C] = [1, 2]. [1,2] 26> A. 1 27> B. 2 28> C. [] 29> In all the examples above, I have been using new variable names, not reusing the old ones: First, TheRest, E1, E2, R, A, B, C. The reason for this is that a variable can only be given a value once in its context (scope). I'll get back to this later, it isn't so peculiar as it sounds! The following example shows how we find the length of a list: -module(tut4). -export([list_length/1]). list_length([]) -> 0; list_length([First | Rest]) -> 1 + list_length(Rest). Compile (file tut4.erl) and test: 29> c(tut4). {ok,tut4} 30> tut4:list_length([1,2,3,4,5,6,7]). 7 31> Explanation: list_length([]) -> 0; The length of an empty list is obviously 0 list_length([First | Rest]) -> 1 + list_length(Rest). The length of a list with the first element First and the remaining elements Rest is 1 + the length of Rest. (Advanced readers only: This is not tail recursive, there is a better way to write this function). In general we can say we use tuples where we would use "records" or "structs" in other languages and we use lists when we want to represent things which have varying sizes, (i.e. where we would use linked lists in other languages). Erlang does not have a "string" date type , instead strings can be represented by lists of ASCII characters. So the list [97,98,99] is equivalent to "abc". The Erlang shell is "clever" and guesses the what sort of list we mean and outputs it in what it thinks is the most appropriate form, for example: 31> [97,98,99]. "abc" 32> Erlang has a lot of standard modules to help you do things. For example, the module io contains a lot of functions to help you do formatted input/output. To look up information about standard modules, the command "erl -man" can be used at the operating shell or command prompt (i.e. at the same place as that where you started "erl"). Try the operating system shell command: erl -man io It's nice to be able to do formatted output in these example, so the next example shows a simple way to use to use the io:format function. Of course, just like all other exported functions, you can test the io:format function in the shell: 32> io:format("hello world~n", []). hello world ok 33> io:format("this outputs one Erlang term: ~w~n", [hello]). this outputs one Erlang term: hello ok 34> io:format("this outputs two Erlang terms: ~w~w~n", [hello, world]). this outputs two Erlang terms: helloworld ok 35> io:format("this outputs two Erlang terms: ~w ~w~n", [hello, world]). this outputs two Erlang terms: hello world ok 36> The function format/2 (i.e. format with two arguments) takes two lists. The first one is nearly always a list written between " ". This list is printed out as it stands, except that each ~w is replaced by a term taken in order from the second list. Each ~n is replaced by a new line. The io:format/2 function itself returns the atom ok of everything goes as planned. Like other functions in Erlang, it crashes if an error occurs. This is not a fault in Erlang, it is a deliberate policy. Erlang has sophisticated mechanisms to handle errors which we will show later. As an exercise, try to make io:format crash, it shouldn't be difficult. But notice that although the io:format crashes, the Erlang shell itself does not crash. Now for a larger example to consolidate what we have learnt so far. Assume we have a list of temperature readings from a number of cities in the world. Some of them are in Celsius (Centigrade) and some in Fahrenheit (as in the previous list). First let's convert them all to Celsius, then let's print out the data neatly. %% This module is in file tut5.erl -module(tut5). -export([format_temps/1]). %% Only this function is exported format_temps([])-> % No output for an empty list ok; format_temps([City | Rest]) -> print_temp(convert_to_celcius(City)), format_temps(Rest). convert_to_celcius({Name, {c, Temp}}) -> % No conversion needed {Name, {c, Temp}}; convert_to_celcius({Name, {f, Temp}}) -> % Do the conversion {Name, {c, (Temp - 32) * 5 / 9}}. print_temp({Name, {c, Temp}}) -> io:format("~-15w ~w c~n", [Name, Temp]). 36> c(tut5). {ok,tut5} 37> tut5:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}}, 37> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). 38> moscow -10 c cape_town 21.1111 c stockholm -4 c paris -2.22222 c london 2.22222 c ok 39> Before we look at how this program works, notice that we have added a few comments to the code. A comment starts with a % character and goes on to the end of the line. Note as well that the -export([format_temps/1]). line only includes the function format_temps/1, the other functions are "local" functions, i.e. they are not visible from outside the module tut5. Note as well that when testing the program from the shell, I had to spread the input over two lines as the line was too long. When we call format_temps the first time, City gets the value {moscow,{c, -10}} and Rest is the rest of the list. So we call the function print_temp(convert_to_celcius({moscow, {c, -10}})). Here we see a function call as convert_to_celcius({moscow, {c, -10}}) as the argument to the function print_temp. When we "nest" function calls like this we execute (evaluate) them from the inside out. I.e. we first evaluate convert_to_celcius({moscow, {c, -10}}) which gives the value {moscow, {c, -10}} as the temperature is already in Celcius and then we evaluate print_temp({moscow, {c, -10}}). The function convert_to_celcius works in a similar way to the convert_length function in the previous example. print_temp simply calls io:format in a similar way to what has been described above. Note that ~-15w says to print the "term" with a field length (width) of 15 and left adjust it. (*manual*). Now we call format_temps(Rest) with the rest of the list as an argument. This way of doing things is similar to the loop constructs other languages. (Yes, this is recursion, but don't let that worry you). So the same format_temps function is called again, this time City gets the value {cape_town, {f, 70}} and we repeat the same procedure as before. We go on doing this until the list becomes empty, i.e. [], which causes the first clause format_temps([]) match. This simply returns (results in) the atom ok, so the program ends. It could be useful to find the maximum and minimum temperature in lists like this. Before extending the program to do this, let's look at functions for finding the maximum value of the elements in a list: -module(tut6). -export([list_max/1]). list_max([Head|Rest]) -> list_max(Rest, Head). list_max([], Res) -> Res; list_max([Head|Rest], Result_so_far) when Head > Result_so_far -> list_max(Rest, Head); list_max([Head|Rest], Result_so_far) -> list_max(Rest, Result_so_far). 39> c(tut6). {ok,tut6} 40> tut6:list_max([1,2,3,4,5,7,4,3,2,1]). 7 41> First note that we have two functions here with the same name list_max. However each of these takes a different number of arguments (parameters). In Erlang these are regarded as completely different functions. Where we need to distinguish between these functions we write name/arity, where name is the name of the function and arity is the number of arguments, in this case list_max/1 and list_max/2. This is an example where we walk through a list "carrying" a value with us, in this case Result_so_far. list_max/1 simply assumes that the max value of the list is the head of the list and calls list_max/2 with this the rest of the list and the value of the head of the list, in the above this would be list_max([2,3,4,5,7,4,3,2,1], 1). If we tried to use list_max/1 with an empty list or tried to use it with something which isn't a list at all, we would cause an error. Note that the Erlang, philosophy is not to handle errors of this type in the function they occur, but to do so elsewhere. More about this later. In list_max/2 we walk down the list and use Head instead of Result_so_far when Head > Result_so_far. "when" is a special word we use before the -> in the function to say that we should only use this part of the function if the test which follows is true. We call tests of this type a "guard". If the "guard" isn't true (we say the guard fails), we try the next part of the function. In this case if Head isn't greater than Result_so_far then it must be smaller or equal to is, so we don't need a guard on the next part of the function. Some useful operators in guards are, < less than, > greater than, == equal, >= greater or equal, <= less or equal, /= not equal. (*manual*). To change the above program to one which works out the minimum value of the element in a list, all we would need to do is to write < instead of >. (But it would be wise to change the name of the function to list_min :-). Remember that I mentioned earlier that a variable could only be given a value once in its scope? In the above we see, for example, that Result_so_far has been given several values. This is OK since every time we call list_max/2 we create a new scope and one can regard the Result_so_far as a completely different variable in each scope. Another way of creating and giving a variable a value is by using the match operator = . So if I write M = 5, a variable called M will be created and given the value 5. If, in the same scope I then write M = 6, I'll get an error. Try this out in the shell 41> M = 5. 5 42> M = 6. ** exited: {{badmatch,6},[{erl_eval,expr,3}]} ** 43> M = M + 1. ** exited: {{badmatch,6},[{erl_eval,expr,3}]} ** 44> N = M + 1. 6 45> The use of the match operator is particularly useful for pulling apart Erlang terms and creating new ones. 45> {X, Y} = {paris, {f, 28}}. {paris,{f,28}} 46> X. paris 47> Y. {f,28} 48> Here we see that X gets the value paris and Y {f, 28}. Of course if we try to do the same again with another city, we get an error: 49> {X, Y} = {london, {f, 36}}. ** exited: {{badmatch,{london,{f,36}}},[{erl_eval,expr,3}]} ** 50> Variables can also be used to improve the readability of programs, for example, in the list_max/2 function above, we could write: list_max([Head|Rest], Result_so_far) when Head > Result_so_far -> New_result_far = Head, list_max(Rest, New_result_far); which is possibly a little clearer. Remember that the | operator can be used to get the head of a list: 50> [M1|T1] = [paris, london, rome]. [paris,london,rome] 51> M1. paris 52> T1. [london,rome] The | operator can also be used to add a head to a list: 53> L1 = [madrid | T1]. [madrid,london,rome] 54> L1. [madrid,london,rome] 55> Now an example of this when working with lists - reversing the order of a list: -module(tut8). -export([reverse/1]). reverse(List) -> reverse(List, []). reverse([Head | Rest], Reversed_List) -> reverse(Rest, [Head | Reversed_List]); reverse([], Reversed_List) -> Reversed_List. 56> c(tut8). {ok,tut8} 57> tut8:reverse([1,2,3]). [3,2,1] 58> Consider how Reversed_List is built. It starts as [], we then successively take off the heads of the list to be reversed and add them to the the Reversed_List, as shown in the following: reverse([1|2,3], []) -> reverse([2,3], [1|[]]) reverse([2|3], [1]) -> reverse([3], [2|[1]) reverse([3|[]], [2,1]) -> reverse([], [3|[2,1]]) reverse([], [3,2,1]) -> [3,2,1] The module "lists" contains a lot of functions for manipulating lists, for example for reversing them, so before you write a list manipulating function it is a good idea to check that one isn't already written for you. (*manual*). Now lets get back to the cities and temperatures, but take a more structured approach this time. First let's convert the whole list to Celcius as follows and test the function: -module(tut7). -export([format_temps/1]). format_temps(List_of_cities) -> convert_list_to_c(List_of_cities). convert_list_to_c([{Name, {f, F}} | Rest]) -> Converted_City = {Name, {c, (F -32)* 5 / 9}}, [Converted_City | convert_list_to_c(Rest)]; convert_list_to_c([City | Rest]) -> [City | convert_list_to_c(Rest)]; convert_list_to_c([]) -> []. 58> c(tut7). {ok, tut7}. 59> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}}, 59> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). [{moscow,{c,-10}}, {cape_town,{c,21.1111}}, {stockholm,{c,-4}}, {paris,{c,-2.22222}}, {london,{c,2.22222}}] 60> Looking at this bit by bit: format_temps(List_of_cities) -> convert_list_to_c(List_of_cities, []). Here we see that format_temps/1 calls convert_list_to_c/1. convert_list_to_c/1 takes off the head of the List_of_cities, converts it to Celcius if needed. The | operator is used to add the (maybe) converted to the converted rest of the list: [Converted_City | convert_list_to_c(Rest)]; or [City | convert_list_to_c(Rest)]; We go on doing this until we get to the end of the list (i.e. the list is empty: convert_list_to_c([]) -> []. Now we have converted the list, we add a function to print it: -module(tut7). -export([format_temps/1]). format_temps(List_of_cities) -> Converted_List = convert_list_to_c(List_of_cities), print_temp(Converted_List). convert_list_to_c([{Name, {f, F}} | Rest]) -> Converted_City = {Name, {c, (F -32)* 5 / 9}}, [Converted_City | convert_list_to_c(Rest)]; convert_list_to_c([City | Rest]) -> [City | convert_list_to_c(Rest)]; convert_list_to_c([]) -> []. print_temp([{Name, {c, Temp}} | Rest]) -> io:format("~-15w ~w c~n", [Name, Temp]), print_temp(Rest); print_temp([]) -> ok. 60>c(tut7). {ok,tut7} 61> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}}, 61> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). moscow -10 c cape_town 21.1111 c stockholm -4 c paris -2.22222 c london 2.22222 c ok 62> We now have to add a function to find the cities with the maximum and minimum temperatures. The program below isn't the most efficient way of doing this as we walk through the list of cities four times. But it is better to first strive for clarity and correctness and to make programs efficient only if really needed. -module(tut7). -export([format_temps/1]). format_temps(List_of_cities) -> Converted_List = convert_list_to_c(List_of_cities), print_temp(Converted_List), {Max_city, Min_city} = find_max_and_min(Converted_List), print_max_and_min(Max_city, Min_city). convert_list_to_c([{Name, {f, Temp}} | Rest]) -> Converted_City = {Name, {c, (Temp -32)* 5 / 9}}, [Converted_City | convert_list_to_c(Rest)]; convert_list_to_c([City | Rest]) -> [City | convert_list_to_c(Rest)]; convert_list_to_c([]) -> []. print_temp([{Name, {c, Temp}} | Rest]) -> io:format("~-15w ~w c~n", [Name, Temp]), print_temp(Rest); print_temp([]) -> ok. find_max_and_min([City | Rest]) -> find_max_and_min(Rest, City, City). find_max_and_min([{Name, {c, Temp}} | Rest], {Max_Name, {c, Max_Temp}}, {Min_Name, {c, Min_Temp}}) -> if Temp > Max_Temp -> Max_City = {Name, {c, Temp}}; % Change true -> Max_City = {Max_Name, {c, Max_Temp}} % Unchanged end, if Temp < Min_Temp -> Min_City = {Name, {c, Temp}}; % Change true -> Min_City = {Min_Name, {c, Min_Temp}} % Unchanged end, find_max_and_min(Rest, Max_City, Min_City); find_max_and_min([], Max_City, Min_City) -> {Max_City, Min_City}. print_max_and_min({Max_name, {c, Max_temp}}, {Min_name, {c, Min_temp}}) -> io:format("Max temperature was ~w c in ~w~n", [Max_temp, Max_name]), io:format("Min temperature was ~w c in ~w~n", [Min_temp, Min_name]). 62> c(tut7). {ok, tut7} 63> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}}, 63> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). moscow -10 c cape_town 21.1111 c stockholm -4 c paris -2.22222 c london 2.22222 c Max temperature was 21.1111 c in cape_town Min temperature was -10 c in moscow ok 64> The function find_max_and_min works out the way maximum and minimum temperature. We have introduced a new construct here "if". If works as follows: if Condition 1 -> Action 1; Condition 2 -> Action 2; Condition 3 -> Action 3; Condition 4 -> Action 4 end Note there is no ";" before "end"! Conditions are the same as guards, tests which succeed or fail. Erlang starts at the top until it finds a condition which succeeds and then it evaluates (performs) the action following the condition and ignores all other conditions and action before the "end". If no condition matches, there will be a run-time failure. A condition which always is succeeds is the atom, "true" and this is often used last in an "if" meaning do the action following the "true" if all other conditions have failed. The following is a short program to show the workings of "if". -module(tut9). -export([test_if/2]). test_if(A, B) -> if A == 5 -> io:format("A = 5~n", []), a_equals_5; B == 6 -> io:format("B = 6~n", []), b_equals_6; A == 2, B == 3 -> %i.e. A equals 2 and B equals 3 io:format("A == 2, B == 3~n", []), a_equals_2_b_equals_3; A == 1 ; B == 7 -> %i.e. A equals 1 or B equals 7 io:format("A == 1 ; B == 7~n", []), a_equals_1_or_b_equals_7 end. Testing this program gives: 64> c(tut9). {ok,tut9} 65> tut9:test_if(5,33). A = 5 a_equals_5 66> tut9:test_if(33,6). B = 6 b_equals_6 67> tut9:test_if(2, 3). A == 2, B == 3 a_equals_2_b_equals_3 68> tut9:test_if(1, 33). A == 1 ; B == 7 a_equals_1_or_b_equals_7 69> tut9:test_if(33, 7). A == 1 ; B == 7 a_equals_1_or_b_equals_7 70> tut9:test_if(33, 33). =ERROR REPORT==== 11-Jun-2003::14:03:43 === Error in process <0.85.0> with exit value: {if_clause,[{tut9,test_if,2},{erl_eval,exprs,4},{shell,eval_loop,2}]} ** exited: {if_clause,[{tut9,test_if,2}, {erl_eval,exprs,4}, {shell,eval_loop,2}]} ** 71> Notice that tut9:test_if(33, 33) did not cause any condition to succeed so we got the rub time error "if_clause". See the (*manual*) for details of the many guard tests available. "case" is another construct in Erlang. Recall that we wrote the convert_length function as: convert_length({centimeter, X}) -> {inch, X / 2.54}; convert_length({inch, Y}) -> {centimeter, Y * 2.54}. we could also write the same program as: -module(tut10). -export([convert_length/1]). convert_length(Length) -> case Length of {centimeter, X} -> {inch, X / 2.54}; {inch, Y} -> {centimeter, Y * 2.54} end. 71> c(tut10). {ok,tut10} 72> tut10:convert_length({inch, 6}). {centimeter,15.2400} 73> tut10:convert_length({centimeter, 2.5}). {inch,0.98425} 74> Notice that both "case" and "if" have "return" values, i.e. in the above example "case" returned either {inch, X / 2.54} or {centimeter, Y * 2.54}. The behaviour of "case" can also be modified by using guards. An example should hopefully clarify this. The following example, tells us the length of a month, given the year. We need to know the year of course, since February has 29 days in a leap year. -module(tut11). -export([month_length/2]). month_length(Year, Month) -> %% All years divisible by 400 are leap %% Years divisible by 100 are not leap (except the 400 rule above) %% Years divisible by 4 are leap (except the 100 rule above) Leap = if trunc(Year / 400) * 400 == Year -> leap; trunc(Year / 100) * 100 == Year -> not_leap; trunc(Year / 4) * 4 == Year -> leap; true -> not_leap end, case Month of sep -> 30; apr -> 30; jun -> 30; nov -> 30; feb when Leap == leap -> 29; feb -> 28; jan -> 31; mar -> 31; may -> 31; jul -> 31; aug -> 31; oct -> 31; dec -> 31 end. 74> c(tut11). {ok,tut11} 75> tut11:month_length(2004, feb). 29 76> tut11:month_length(2003, feb). 28 77> tut11:month_length(1947, aug). 31 78> As you can see, we first find out if a year is leap or not. If a year is divisible by 400, it is a leap year. To find this out we fist divide the year by 400 and use the built in function "trunc" (more later) to cut off any decimals. We then multiply by 400 again and see if we get back the same value. For example, year 2004: 2004 / 400 = 5.01 trunc(5.01) = 5 5 * 400 = 2000 and we can see that we got back 2000 which is not the same as 2004, so 2004 isn't divisible by 400. Year 2000: 2000 / 400 = 5.0 trunc(5.0) = 5 5 * 400 = 2000 so we have a leap year. The next two tests if the year is divisible by 100 or 4 are done in the same way. The first "if" "returns" leap or not_leap which lands up in the variable Leap. We use this variable in the guard for "feb" in the following "case" which tells us how long the month is. This example showed the use of "trunc", an easier way would be to use the Erlang operator "rem" which gives the remainder after division. For example 2> 2004 rem 400. 4 so instead of writing trunc(Year / 400) * 400 == Year -> leap; we could write Year rem 400 == 0 -> leap; There are many other built in functions (BIF) such as "trunc". Only a few built in functions can be used in guards, and you cannot use functions you have defined yourself in guards. (*manual*) (aside for advanced readers: this is to ensure that guards don't have side effects). Let's play with a few of these functions in the shell. 78> trunc(5.6). 5 79> round(5.6). 6 80> length([a,b,c,d]). 4 81> float(5). 5.00000 82> is_atom(hello). true 83> is_atom("hello"). false 84> is_tuple({paris, {c, 30}}). true 85> is_tuple([paris, {c, 30}]). false 86> All the above can be used in guards. Now for some which can't be used in guards: 87> atom_to_list(hello). "hello" 88> list_to_atom("goodbye"). goodbye 89> integer_to_list(22). "22" 90> The BIFs above do conversions which would be difficult (or impossible) to do in Erlang. Erlang, like most modern functional programing languages, has higher order functions. We start with an example using the shell: 90> Xf = fun(X) -> X * 2 end. #Fun 91> Xf(5). 10 92> What we have done here is to define a function which doubles the value of number and assign this function to a variable. Thus Xf(5) returned the value 10. Two useful functions when working with lists are "foreach" and "map", which are defined as follows: foreach(Fun, [First|Rest]) -> Fun(First), foreach(Fun, Rest); foreach(Fun, []) -> ok. map(Fun, [First|Rest]) -> [Fun(First)|map(Fun,Rest)]; map(Fun, []) -> []. These two functions are provided in the standard module "lists". "foreach" takes a list and applies a fun to every element in the list, "map" creates a new list by applying a fun to every element in a list. Going back to the shell, we start by using "map" and a fun to add 3 to every element of a list. 92> Add_3 = fun(X) -> X + 3 end. #Fun 93> lists:map(Add_3, [1,2,3]). [4,5,6] 94> Now lets print out the temperatures in a list of cities (yet again). 94> Print_City = fun({City, {X, Temp}}) -> io:format("~-15w ~w ~w~n", 95> [City, X, Temp]) end. #Fun 96> lists:foreach(Print_City, [{moscow, {c, -10}}, {cape_town, {f, 70}}, 96> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). moscow c -10 cape_town f 70 stockholm c -4 paris f 28 london f 36 ok 97> We will now define a fun which can be used to go through a list of cities and temperatures and transform them all to Celcius. -module(tut13). -export([convert_list_to_c/1]). convert_to_c({Name, {f, Temp}}) -> {Name, {c, trunc((Temp - 32) * 5 / 9)}}; convert_to_c({Name, {c, Temp}}) -> {Name, {c, Temp}}. convert_list_to_c(List) -> lists:map(fun convert_to_c/1, List). 97> tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}}, 98> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). [{moscow,{c,-10}}, {cape_town,{c,21}}, {stockholm,{c,-4}}, {paris,{c,-2}}, {london,{c,2}}] 99> The convert_to_c function is the same as before, but we use it as a "fun". lists:map(fun convert_to_c/1, List) When we use a function defined elsewhere as a fun we can refer to it as Function/Arity (remember that Arity = number of arguments). So in the map call we write lists:map(fun convert_to_c/1, List). As you can see convert_list_to_c becomes much shorter and easier to understand. The standard module "lists" also contains a function sort(Fun, List) where Fun is a "fun" with two arguments. This "fun" should return true if the the first argument is less than the second argument, or else false. We add sorting to the convert_list_to_c: -module(tut13). -export([convert_list_to_c/1]). convert_to_c({Name, {f, Temp}}) -> {Name, {c, trunc((Temp - 32) * 5 / 9)}}; convert_to_c({Name, {c, Temp}}) -> {Name, {c, Temp}}. convert_list_to_c(List) -> New_list = lists:map(fun convert_to_c/1, List), lists:sort(fun({_, {c, Temp1}}, {_, {c, Temp2}}) -> Temp1 < Temp2 end, New_list). 99> c(tut13). {ok,tut13} 100> tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}}, 100> {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]). [{moscow,{c,-10}}, {stockholm,{c,-4}}, {paris,{c,-2}}, {london,{c,2}}, {cape_town,{c,21}}] 101> In sort we use the fun: fun({_, {c, Temp1}}, {_, {c, Temp2}}) -> Temp1 < Temp2 end, Here we introduce the concept of an anonymous variable "_", This is simply shorthand for a variable which is going to get a value, but we will ignore the value. This can be used anywhere suitable, not just in fun's. Temp1 < Temp2 returns true if Temp1 is less than Temp2. ---- One of the main reasons for using Erlang instead of other functional languages is Erlang's ability to handle concurrency and distributed programming. By concurrency we mean programs which can handle several threads of execution at the same time. For example, modern operating systems would allow you to use a word processor, a spreadsheet, a mail client and a print job all running at the same time. Of course each processor (CPU) in the system is probably only handling one thread (or job) at a time, but it swaps between the jobs a such a rate that it gives the illusion of running them all at the same time. It is easy to create parallel threads of execution in an Erlang program and it is easy to allow these threads to communicate with each other. In Erlang we call each thread of execution a "process". (Aside: the term "process" is usually used when the threads of execution share no data with each other and the term thread when they share data in some way. Threads of execution in Erlang share no data, that's why we call them processes). The Erlang BIF (built in function) "spawn" is used to create a new process: spawn(Module, Exported_Function, List of Argument). Consider the following module: -module(tut14). -export([start/1, say_something/2]). say_something(What, 0) -> done; say_something(What, Times) -> io:format("~p~n", [What]), say_something(What, Times - 1). start() -> spawn(tut14, say_something, [hello, 3]), spawn(tut14, say_something, [goodbye, 3]). 5> c(tut14). {ok,tut14} 6> tut14:say_something(hello, 3). hello hello hello done We can see that function say_something writes its first argument the number of times specified by second argument. Now look at the function start. It starts two Erlang processes, one which writes "hello" three times and one which writes "goodbye" three times. Both of these processes use the function "say_something". Note that a function used in this way by "spawn" to start a process must be exported from the module (i.e. in the -export at the start of the module). 9> tut14:start(). hello goodbye <0.63.0> hello goodbye hello goodbye Notice that it didn't write "hello" three times and then "goodbye" three times, but the first process wrote a "hello", the second a "goodbye", the first another "hello" and so forth. But where did the <0.63.0> come from? The return value of a function is of course the return value of the last "thing" in the function. The last thing in the function "start" is spawn(tut14, say_something, [goodbye, 3]). Spawn returns a "process identifier, or PID, which uniquely identifies the process. So <0.63.0> is the PID of the spawn function call above. We will see how to use PIDs in the next example. Note as well that we have used ~p instead of ~w in io:format To quote the manual: "~p Writes the data with standard syntax in the same way as ~w, but breaks terms whose printed representation is longer than one line into many lines and indents each line sensibly. It also tries to detect lists of printable characters and to output these as strings". In the following example we create two processes which send messages to each other a number of times. -module(tut15). -export([start/0, ping/2, pong/0]). ping(0, Pong_PID) -> Pong_PID ! finished, io:format("ping finished~n", []); ping(N, Pong_PID) -> Pong_PID ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_PID). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start() -> Pong_PID = spawn(tut15, pong, []), spawn(tut15, ping, [3, Pong_PID]). 1> c(tut15). {ok,tut15} 2> tut15: start(). <0.36.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong ping finished Pong finished The functions "start" first creates a process, let's call it "pong". Pong_PID = spawn(tut15, pong, []) this process executes tut15:pong(). Pong_PID is the process identity of the "pong" process. The function start now creates another process "ping" spawn(tut15, ping, [3, Pong_PID]), this process executes tut15:ping(3, Pong_PID) <0.36.0> (see above) is the return value from the "start" function. The process "pong" now does: receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. The receive construct is used to allow processes to wait for messages from other processes. It has the format receive pattern1 -> actions1; pattern2 -> actions2; .... patternN actionsN end. Note: no ";" before the "end". Messages between Erlang processes are simply Erlang "terms". I.e. they can be lists, tuples, integers, atoms, PID's etc and any combination thereof. Each process has its own input queue for messages it receives. New messages received are put at the end of the queue. When a process executes a "receive", the first message in the queue is matched against the first pattern in the "receive", if this matches, the message is removed from the queue and the actions corresponding to the the pattern are executed. However, if the first pattern does not match, the second pattern is tested, if this matches the message is removed from the queue and the actions corresponding to the second patter are executed. If the second pattern does not match the third is tried and so on until there are no more pattern to test. If there are no more patterns to test, the first message is kept in the queue and we try the second message instead. If this matches any pattern, the appropriate actions are executed and the second message is removed from the queue (keeping the first message and any other messages in the queue). If the second message does not match we try the third message and so on until we reach the end of the queue. If we reach the end of the queue, the process blocks (stops execution) and waits until a new message is received and this procedure is repeated. Of course the Erlang implementation is "clever" and minimizes the number of times each message is tested against the patterns in each "receive". Now back to the "ping-pong" example. Pong is waiting for messages. If the atom "finished" is received, pong writes "Pong finished" to the output and as it has nothing more to do, terminates. If it receives a message with the format {ping, Ping_PID} it write "Pong received ping" to the output and sends the atom "pong" to the process "ping", Ping_PID ! pong Note how the operator "!" is used to send messages. The syntax of "!" is PID ! Message I.e. Message (any Erlang term) is sent to the process with identity PID. After sending the message "pong", to the process "ping", pong calls itself pong() which causes it to get back to the "receive" again and wait for another message. Now let's look at the process "ping". Recall that it started by executing: tut15:ping(3, Pong_PID) Looking at the function ping/2 we see that the second "clause" of ping/2 is executed since the value of the first argument is 3 (not 0) (first clause head is ping(0, Pong_PID), second clause head is ping(N, Pong_PID), so N become 3). The second clause sends a message to "pong" Pong_PID ! {ping, self()}, self() returns the PID of the process which executes self(), in this case "ping". (recall the code for "pong", this will land up in the variable Ping_PID in the "receive" previously explained). Ping now waits for a reply from "pong" receive pong -> io:format("Ping received pong~n", []) end, and writes "Ping received pong" when this reply arrives, after which ping calls itself, ping(N - 1, Pong_PID) N - 1 causes the first argument to be decremented until it becomes "0". When this occurs, the first clause of ping/2 will be executed. ping(0, Pong_PID) -> Pong_PID ! finished, io:format("ping finished~n", []); The atom "finished" is sent to "pong" (causing it to terminate as described above) and "ping finished" is written to the output. Ping then itself terminates as it has nothing left to do. In the above example, we first created "pong" so as to be able to give the identity of ping when we started "ping". I.e. in some way "ping" must be able to know the identity of "pong" in order to be able to send a message to it. Sometimes processes which need to know each others identities are started completely independently of each other. Erlang thus provides a mechanism for processes to be given names so that these names can be used as identities instead of PIDs. This is done by using the "register" BIF. register(some_atom, PID) We will now re-write the ping pong example using this and giving the name "pong" to the pong process -module(tut16). -export([start/0, ping/1, pong/0]). ping(0) -> pong ! finished, io:format("ping finished~n", []); ping(N) -> pong ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start() -> register(pong, spawn(tut16, pong, [])), spawn(tut16, ping, [3]). 2> c(tut16). {ok, tut16} 3> tut16:start(). <0.38.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong ping finished Pong finished In the start/0 function, register(pong, spawn(tut16, pong, [])), both spawns the "pong" process and gives it the name pong. In the "ping" process we can now send messages to pong by: pong ! {ping, self()}, so that ping/2 now becomes ping/1 as we don't have to give the argument Pong_PID to ping. Now let's re-write the ping-pong program with "ping" and "pong" on different computers. Before we do this, there are a few things we need to set up to get this to work. The distributed Erlang implementation provides a basic security mechanism to prevent unauthorized access to an Erlang system on another computer (*manual*). Erlang systems which talk to each other must have the same "magic cookie". The easiest way to achieve this is by having a file called .erlang.cookie in your home directory on all machines which on which you are going to run erlang systems communicating with each other (on Windows systems the home directory is the directory where pointed to by the $HOME environment variable - you may need to set this. On Linux or Unix you can safely ignore this and simply create a file called .erlang.cookie in the directory you get to after executing the command "cd" without any argument). The each .erlang.cookie file should contain on line with the same atom. For example on Linux or Unix in the OS shell: [mike@bill mike]$ cd [mike@bill mike]$ cat > .erlang.cookie this_is_very_secret [mike@bill mike]$ chmod 400 .erlang.cookie The chmod above make the .erlang.cookie file accessible only by the owner of the file. This is a requirement. When you start an erlang system which is going to talk to other Erlang systems, you must give it a name, eg. erl -sname my_name We will see more details of this later (*manual*). If you want to experiment with distributed Erlang, but you only have one computer to work on, you can start two separate Erlang systems on the same computer but give them different names. Each Erlang system running on a computer is called an Erlang node. (note erl -sname assumes that all nodes are in the same IP domain and we can use only the first component of the IP address, if we want to node in different domains we use -name instead, but then all IP address must be given in full *manual*) Here is the ping pong example modified to run on two separate nodes: -module(tut17). -export([start_ping/1, start_pong/0, ping/2, pong/0]). ping(0, Pong_Node) -> {pong, Pong_Node} ! finished, io:format("ping finished~n", []); ping(N, Pong_Node) -> {pong, Pong_Node} ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_Node). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start_pong() -> register(pong, spawn(tut17, pong, [])). start_ping(Pong_Node) -> spawn(tut17, ping, [3, Pong_Node]). Let us assume we have two computers called gollum and kosken. We will start a node on kosken called pong and then a node on kosken called ping. On kosken (on a Linux/Unix system): kosken> erl -sname ping Erlang (BEAM) emulator version 5.2.3.7 [hipe] [threads:0] Eshell V5.2.3.7 (abort with ^G) (ping@kosken)1> On gollum: gollum> erl -sname pong Erlang (BEAM) emulator version 5.2.3.7 [hipe] [threads:0] Eshell V5.2.3.7 (abort with ^G) (pong@gollum)1> Now we start the pong process on gollum: (pong@gollum)1> tut17:start_pong(). true (pong@gollum)2> and start the ping process on kosken (from the code above you will see that a parameter of the start_ping function is the node name of the Erlang system where pong is running). (ping@kosken)1> tut17:start_ping(pong@gollum). <0.37.0> Ping received pong Ping received pong Ping received pong ping finished (ping@kosken)2> Here we see that the ping pong program has run, on the "pong" side we see: (pong@gollum)2> Pong received ping Pong received ping Pong received ping Pong finished (pong@gollum)2> Looking at the tut17 code we see that the pong function itself is unchanged, the lines: {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, work in the same way irrespective of on which node the ping process is executing. Thus Erlang PID's contain information about where the process executes so if you know the PID of a process, the "!" operator can be used to send it a message if the process is on the same node or on a different node. The other major difference is how we send messages to a registered process on another node: {pong, Pong_Node} ! {ping, self()}, we use a tuple {registered_name, node_name} instead of just the node_name. In the previous example, we started ping and pong from the shells of two separate Erlang nodes. "spawn" can also be used to start processes in other nodes. The next example is the ping pong program, yet again, but this time we will start "ping" in another node. -module(tut18). -export([start/1, ping/2, pong/0]). ping(0, Pong_Node) -> {pong, Pong_Node} ! finished, io:format("ping finished~n", []); ping(N, Pong_Node) -> {pong, Pong_Node} ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_Node). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start(Ping_Node) -> register(pong, spawn(tut17, pong, [])), spawn(Ping_Node, tut17, ping, [3, node()]). Assuming an Erlang system called ping (but not the ping process) has already been started on kosken, then on gollum we do: (pong@gollum)1> tut18:start(ping@kosken). <3934.39.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong Pong finished ping finished (pong@gollum)2> Notice we get all the output on gollum. This is because the io system finds out where the process are spawned from and sends all output there. Now for a larger example. We will make an extremely simple "messenger". The messenger is a program which allows users to log in on different nodes and send simple messages to each other. Before we start, let's note the following. * This example will just show the message passing logic no attempt at all has been made to provide a nice graphical user interface - this can of course also be done in Erlang - but that's another tutorial. * This sort of problem can be solved more easily if you use the facilities in OTP, which will also provide methods for updating code on the fly etc. But again, that's another tutorial * The first program we write will contain some inadequacies as regards handling of nodes which disappear, we will correct these in a later version of the program. We will set up the "Messenger" by allowing "clients" to connect to a central server and say who and where they are. I.e. a user won't need to know the name of the Erlang node to send a message. File messenger.erl %%% Message passing utility. %%% User interface: %%% logon(Name) %%% One user at a time can log in from each Erlang node in the %%% system messenger: and choose a suitable Name. If the Name %%% is already logged in at another node or if someone else is %%% already logged in at the same node, login will be rejected %%% with a suitable error message. %%% logoff() %%% Logs off anybody at at node %%% message(ToName, Message) %%% sends Message to ToName. Error messages if the user of this %%% function is not logged on or if ToName is not logged on at %%% any node. %%% %%% One node in the network of Erlang nodes runs a server which maintains %%% data about the logged on users. The server is registered as "messenger" %%% Each node where there is a user logged on runs a client process registered %%% as "mess_client" %%% %%% Protocol between the client processes and the server %%% ---------------------------------------------------- %%% %%% To server: {ClientPid, logon, UserName} %%% Reply {messenger, stop, user_exists_at_other_node} stops the client %%% Reply {messenger, logged_on} logon was successful %%% %%% To server: {ClientPid, logoff} %%% Reply: {messenger, logged_off} %%% %%% To server: {ClientPid, logoff} %%% Reply: no reply %%% %%% To server: {ClientPid, message_to, ToName, Message} send a message %%% Reply: {messenger, stop, you_are_not_logged_on} stops the client %%% Reply: {messenger, receiver_not_found} no user with this name logged on %%% Reply: {messenger, sent} Message has been sent (but no guarantee) %%% %%% To client: {message_from, Name, Message}, %%% %%% Protocol between the "commands" and the client %%% ---------------------------------------------- %%% %%% Started: messenger:client(Server_Node, Name) %%% To client: logoff %%% To client: {message_to, ToName, Message} %%% %%% Configuration: change the server_node() function to return the %%% name of the node where the messenger server runs -module(messenger). -export([start_server/0, server/1, logon/1, logoff/0, message/2, client/2]). %%% Change the function below to return the name of the node where the %%% messenger server runs server_node() -> messenger@bill. %%% This is the server process for the "messenger" %%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...] server(User_List) -> receive {From, logon, Name} -> New_User_List = server_logon(From, Name, User_List), server(New_User_List); {From, logoff} -> New_User_List = server_logoff(From, User_List), server(New_User_List); {From, message_to, To, Message} -> server_transfer(From, To, Message, User_List), io:format("list is now: ~p~n", [User_List]), server(User_List) end. %%% Start the server start_server() -> register(messenger, spawn(messenger, server, [[]])). %%% Server adds a new user to the user list server_logon(From, Name, User_List) -> %% check if logged on anywhere else case lists:keymember(Name, 2, User_List) of true -> From ! {messenger, stop, user_exists_at_other_node}, %reject logon User_List; false -> From ! {messenger, logged_on}, [{From, Name} | User_List] %add user to the list end. %%% Server deletes a user from the user list server_logoff(From, User_List) -> lists:keydelete(From, 1, User_List). %%% Server transfers a message between user server_transfer(From, To, Message, User_List) -> %% check that the user is logged on and who he is case lists:keysearch(From, 1, User_List) of false -> From ! {messenger, stop, you_are_not_logged_on}; {value, {From, Name}} -> server_transfer(From, Name, To, Message, User_List) end. %%% If the user exists, send the message server_transfer(From, Name, To, Message, User_List) -> %% Find the receiver and send the message case lists:keysearch(To, 2, User_List) of false -> From ! {messenger, receiver_not_found}; {value, {ToPid, To}} -> ToPid ! {message_from, Name, Message}, From ! {messenger, sent} end. %%% User Commands logon(Name) -> case whereis(mess_client) of undefined -> register(mess_client, spawn(messenger, client, [server_node(), Name])); _ -> already_logged_on end. logoff() -> mess_client ! logoff. message(ToName, Message) -> case whereis(mess_client) of % Test if the client is running undefined -> not_logged_on; _ -> mess_client ! {message_to, ToName, Message}, ok end. %%% The client process which runs on each server node client(Server_Node, Name) -> {messenger, Server_Node} ! {self(), logon, Name}, await_result(), client(Server_Node). client(Server_Node) -> receive logoff -> {messenger, Server_Node} ! {self(), logoff}, exit(normal); {message_to, ToName, Message} -> {messenger, Server_Node} ! {self(), message_to, ToName, Message}, await_result(); {message_from, FromName, Message} -> io:format("Message from ~p: ~p~n", [FromName, Message]) end, client(Server_Node). %%% wait for a response from the server await_result() -> receive {messenger, stop, Why} -> % Stop the client io:format("~p~n", [Why]), exit(normal); {messenger, What} -> % Normal response io:format("~p~n", [What]) end. ----- To use this program you need to: configure the server_node() function copy the compiled code (messenger.beam) to the directory on each computer where you start erlang. In the following example of use of this program, I have started nodes on four different computers, but if you don't have that many machines available on your network, you could start up several nodes on the same machine. We start up four Erlang nodes, messenger@super, c1@bilbo, c2@kosken, c3@gollum. First we start up a the sever at messenger@super (messenger@super)1> messenger:start_server(). true Now Peter logs on at c1@bilbo: (c1@bilbo)1> messenger:logon(peter). true logged_on James logs on at c2@kosken (c2@kosken)1> messenger:logon(james). true logged_on and Fred logs on at c3@gollum (c3@gollum)1> messenger:logon(fred). true logged_on Now Peter sends Fred a message: (c1@bilbo)2> messenger:message(fred, "hello"). ok sent And Fred receives the message and sends a message to Peter and logs off. Message from peter: "hello" (c3@gollum)2> messenger:message(peter, "go away, I'm busy"). ok sent (c3@gollum)3> messenger:logoff(). logoff James now tries to send a message to Fred (c2@kosken)2> messenger:message(fred, "peter doesn't like you"). ok receiver_not_found But this fails as Fred has already logged off. First let's look at some of the new concepts we have introduced. There are two versions of the server_transfer function, one with four arguments (server_transfer/4) and one with five (server_transfer/5). These are regarded by Erlang as two separate functions. Note how we write the server function so that it calls itself, server(User_List) and thus creates a loop. The Erlang compiler is "clever" and optimizes the code so that this really is a sort of loop and not a proper function call. But this only works if there is no code after the call, otherwise the compiler will expect the call to return and make a proper function call. This would result in the process getting bigger and bigger for every loop. We use functions in the lists module. This is a very useful module and a study the manual page is recommended (erl -man lists). lists:keymember(Key, Position, Lists) looks through a list of tuples and looks at each Position in the tuple to see if it is the same as Key. The first element is position 1. If it find a tuple where the element at position is the same, it returns true, otherwise false. 3> lists:keymember(a, 2, [{x,y,z}, {b,b,b}, {b, a, c}, {q, r, s}]). true ^ | here 4> lists:keymember(p, 2, [{x,y,z}, {b,b,b}, {b, a, c}, {q, r, s}]). false lists:keydelete works in the same way but deletes the first tuple found (if any) and returns the remaining list: 5> lists:keydelete(a, 2, [{x,y,z}, {b,b,b}, {b, a, c}, {q, r, s}]). [{x,y,z},{b,b,b},{q,r,s}] lists:keysearch is like lists_keymember, but it returns {value, Tuple_Found} or the atom "false". There are a lot more very useful functions in the lists module. An Erlang process will (conceptually) run until it does a "receive" and there is no message which it wants to receive in the message queue. I say "conceptually" because the Erlang system shares the CPU time between the active processes in the system. A process terminates when there is nothing more for it to do, i.e. the last function it calls simply returns and doesn't call another function. Another way for a process to terminate is for it to call exit/1. The argument to exit/1 has a special meaning which we will look at later. In this example we will do exit(normal) which has the same effect as a process running out of functions to call. The BIF whereis(RegisteredName) checks if a registered process of name RigisteredName exists and return the PID of the process if it does exist or the atom "undefined" if it does not. You should by now be able to understand most of the code above so I'll just go through one case: a message is sent from on user to another. The first user "sends" the message in the example above by: messenger:message(fred, "hello") After testing that the client process exists whereis(mess_client) and a message is sent to the mess_client mess_client ! {message_to, fred, "hello"} The client sends the message to the server by: {messenger, messenger@super} ! {self(), message_to, fred, "hello"}, and the client waits for a reply from the server. The server receives this message and calls: server_transfer(From, fred, "hello", User_List), which checks that the PID From is in the User_List: lists:keysearch(From, 1, User_List) If keysearch returns the atom "false", some sort of error has occurred and the server sends back the message: From ! {messenger, stop, you_are_not_logged_on} Which is received by the client which in turn does exit(normal) and terminates. If keysearch returns {value, From, Name} we know that the user is logged on and is his name (peter) is in variable Name. We now call: server_transfer(From, peter, fred, "hello", User_List) Note that as this is server_transfer/5 it is not the same as the previous function server_transfer/4. We do another keysearch on User_List to find the PID of the client corresponding to fred: lists:keysearch(fred, 2, User_List) This time we use argument 2 which is the second element in the tuple. If this returns the atom "false" we know that fred is not logged on and we send the message From ! {messenger, receiver_not_found}; which is received by the client, if keysearch returns {value, {ToPid, fred}} we send the message ToPid ! {message_from, peter, "hello"}, to fred's client and the message From ! {messenger, sent} to peter's client. Fred's client receives the message and prints it: {message_from, peter, "hello"} -> io:format("Message from ~p: ~p~n", [peter, "hello"]) and peter's client receives the message in the await_result function. There are several things which are wrong with the above program. For example if a node where a user is logged on goes down without doing a log off, the user will remain in the server's User_List but the client will disappear thus making it impossible for the user to log on again as the server thinks the user already logged on. Or what happens if the server goes down in the middle of sending a message leaving the sending client hanging for ever in the await_result function? Before improving the messenger program will will look into some general principles, using the ping pong program as an example. Recall that when ping finishes, it tells pong that it has done so by sending the atom "finished" as a message to pong so that pong could also finish. Another way to let pong finish, is to make pong exit if it does not receive a message from ping within a certain time, this can be done by adding a "timeout" to pong a shown in the following example: -module(tut19). -export([start_ping/1, start_pong/0, ping/2, pong/0]). ping(0, Pong_Node) -> io:format("ping finished~n", []); ping(N, Pong_Node) -> {pong, Pong_Node} ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_Node). pong() -> receive {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() after 5000 -> io:format("Pong timed out~n", []) end. start_pong() -> register(pong, spawn(tut19, pong, [])). start_ping(Pong_Node) -> spawn(tut19, ping, [3, Pong_Node]). After we have compiled this and copied the tut19.beam file to the necessary directories: On (pong@kosken): (pong@kosken)1> tut19:start_pong(). true Pong received ping Pong received ping Pong received ping Pong timed out (ping@gollum)1> tut19:start_ping(pong@kosken). <0.36.0> Ping received pong Ping received pong Ping received pong ping finished The timeout is set in: pong() -> receive {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() after 5000 -> io:format("Pong timed out~n", []) end. We start the timeout ("after 5000") when we enter "receive". The timeout is canceled if {ping, Ping_PID} is received. If {ping, Ping_PID} is not received, the actions following the timeout will be done after 5000 milliseconds. "after" must be last in the "receive", i.e. preceded by all other message reception specifications in the receive. Of course we could also call a function which returned an integer for the timeout. after pong_timeout() -> In general, there are better ways than using timeouts to supervise parts of a distributed Erlang system. Timeouts are usually appropriate to supervise external events, for example if you have expected a message from some external system within a specified time. For example, we could use a timeout to log a user out of the "messenger" system if they have not accessed it, for example, in ten minutes. Before we go into details of the supervision and error handling in an Erlang system, we need see how Erlang processes terminate, or in Erlang terminology, "exit". A process which executes exit(normal) or simply runs out of things to do has a "normal" exit. A process which encounters a runtime error (e.g. divide by zero, bad match, trying to call a function which doesn't exist etc) exits with an error, i.e. has an abnormal exit. A process which executes exit(Reason) (*manual*) where reason is any Erlang term except the atom "normal", also has an abnormal exit. An Erlang process can set up links to other Erlang processes. If a process calls link(Other_Pid) (*manual*) it sets up a bidirectional link between itself and the process called Other_Pid. When a process terminates its sends a special message called a "signal" to all the processes it has links to. The signal has the following format for normal exit {'EXIT', FromPID, normal} and for abnormal exit generated by exit(Reason) {'EXIT', FromPID, Reason} and for abnormal exit generated by SW error {'EXIT', FromPID, Error_Details} The default behaviour of a process which receives a normal EXIT is to ignore the signal. The default behaviour in the two other cases (i.e. abnormal exit) above is to bypass all other messages to the receiving process and to kill the it and to propagate the same error signal to the killed process' links. In this way you can connect all the processes in a transaction together using links and if one of the processes exits abnormally, all the processes in the transaction will be killed. As we often want to create a process and link to it at the same time, there is a special BIF, "spawn_link" which does the same as "spawn", but also creates a line to the spawned process. (*manual*). Now an example of the ping pong example using links to terminate pong: -module(tut20). -export([start/1, ping/2, pong/0]). ping(N, Pong_Pid) -> link(Pong_Pid), ping1(N, Pong_Pid). ping1(0, _) -> exit(ping); ping1(N, Pong_Pid) -> Pong_Pid ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping1(N - 1, Pong_Pid). pong() -> receive {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start(Ping_Node) -> PongPID = spawn(tut20, pong, []), spawn(Ping_Node, tut20, ping, [3, PongPID]). (s1@bill)3> tut20:start(s2@kosken). Pong received ping <3820.41.0> Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong This is slight modification of the ping pong program where both processes are spawned from the same start/1 function, where the ping process can be spawned on a separate node. Note the use of the spawn_link BIF. Ping calls exit(ping) when it finishes and this will cause an exit message to be sent to pong which will also terminate. It is possible to modify the default behaviour of a process so that it does not get killed when it receives abnormal exit signals, but all signals, including {'EXIT', FromPID, normal} will be turned into normal messages to the process and added to the end of the receiving processes mesage queue. This behaviour is set by: process_flag(trap_exit, true) There are several other process flags (*manual*). Changing the default behaviour of a process in this way is usually not done in standard user programs, but is left to the supervisory programs in OTP (but that's another tutorial). However we will modify the ping pong program to illustrate EXIT trapping, but be warned, this is just an illustration and not a good way to program in Erlang. -module(tut21). -export([start/1, ping/2, pong/0]). ping(N, Pong_Pid) -> link(Pong_Pid), ping1(N, Pong_Pid). ping1(0, _) -> exit(ping); ping1(N, Pong_Pid) -> Pong_Pid ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping1(N - 1, Pong_Pid). pong() -> process_flag(trap_exit, true), pong1(). pong1() -> receive {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong1(); {'EXIT', From, Reason} -> io:format("pong exiting, got ~p~n", [{'EXIT', From, Reason}]) end. start(Ping_Node) -> PongPID = spawn(tut21, pong, []), spawn(Ping_Node, tut21, ping, [3, PongPID]). (s1@bill)1> tut21:start(s2@gollum). <3820.39.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong pong exiting, got {'EXIT',<3820.39.0>,ping} Now we return to the messenger program and add changes which make it more robust. %%% Message passing utility. %%% User interface: %%% login(Name) %%% One user at a time can log in from each Erlang node in the %%% system messenger: and choose a suitable Name. If the Name %%% is already logged in at another node or if someone else is %%% already logged in at the same node, login will be rejected %%% with a suitable error message. %%% logoff() %%% Logs off anybody at at node %%% message(ToName, Message) %%% sends Message to ToName. Error messages if the user of this %%% function is not logged on or if ToName is not logged on at %%% any node. %%% %%% One node in the network of Erlang nodes runs a server which maintains %%% data about the logged on users. The server is registered as "messenger" %%% Each node where there is a user logged on runs a client process registered %%% as "mess_client" %%% %%% Protocol between the client processes and the server %%% ---------------------------------------------------- %%% %%% To server: {ClientPid, logon, UserName} %%% Reply {messenger, stop, user_exists_at_other_node} stops the client %%% Reply {messenger, logged_on} logon was successful %%% %%% When the client terminates for some reason %%% To server: {'EXIT', ClientPid, Reason} %%% %%% To server: {ClientPid, message_to, ToName, Message} send a message %%% Reply: {messenger, stop, you_are_not_logged_on} stops the client %%% Reply: {messenger, receiver_not_found} no user with this name logged on %%% Reply: {messenger, sent} Message has been sent (but no guarantee) %%% %%% To client: {message_from, Name, Message}, %%% %%% Protocol between the "commands" and the client %%% ---------------------------------------------- %%% %%% Started: messenger:client(Server_Node, Name) %%% To client: logoff %%% To client: {message_to, ToName, Message} %%% %%% Configuration: change the server_node() function to return the %%% name of the node where the messenger server runs -module(messenger). -export([start_server/0, server/0, logon/1, logoff/0, message/2, client/2]). %%% Change the function below to return the name of the node where the %%% messenger server runs server_node() -> messenger@super. %%% This is the server process for the "messenger" %%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...] server() -> process_flag(trap_exit, true), server([]). server(User_List) -> receive {From, logon, Name} -> New_User_List = server_logon(From, Name, User_List), server(New_User_List); {'EXIT', From, _} -> New_User_List = server_logoff(From, User_List), server(New_User_List); {From, message_to, To, Message} -> server_transfer(From, To, Message, User_List), io:format("list is now: ~p~n", [User_List]), server(User_List) end. %%% Start the server start_server() -> register(messenger, spawn(messenger, server, [])). %%% Server adds a new user to the user list server_logon(From, Name, User_List) -> %% check if logged on anywhere else case lists:keymember(Name, 2, User_List) of true -> From ! {messenger, stop, user_exists_at_other_node}, %reject logon User_List; false -> From ! {messenger, logged_on}, link(From), [{From, Name} | User_List] %add user to the list end. %%% Server deletes a user from the user list server_logoff(From, User_List) -> lists:keydelete(From, 1, User_List). %%% Server transfers a message between user server_transfer(From, To, Message, User_List) -> %% check that the user is logged on and who he is case lists:keysearch(From, 1, User_List) of false -> From ! {messenger, stop, you_are_not_logged_on}; {value, {_, Name}} -> server_transfer(From, Name, To, Message, User_List) end. %%% If the user exists, send the message server_transfer(From, Name, To, Message, User_List) -> %% Find the receiver and send the message case lists:keysearch(To, 2, User_List) of false -> From ! {messenger, receiver_not_found}; {value, {ToPid, To}} -> ToPid ! {message_from, Name, Message}, From ! {messenger, sent} end. %%% User Commands logon(Name) -> case whereis(mess_client) of undefined -> register(mess_client, spawn(messenger, client, [server_node(), Name])); _ -> already_logged_on end. logoff() -> mess_client ! logoff. message(ToName, Message) -> case whereis(mess_client) of % Test if the client is running undefined -> not_logged_on; _ -> mess_client ! {message_to, ToName, Message}, ok end. %%% The client process which runs on each user node client(Server_Node, Name) -> {messenger, Server_Node} ! {self(), logon, Name}, await_result(), client(Server_Node). client(Server_Node) -> receive logoff -> exit(normal); {message_to, ToName, Message} -> {messenger, Server_Node} ! {self(), message_to, ToName, Message}, await_result(); {message_from, FromName, Message} -> io:format("Message from ~p: ~p~n", [FromName, Message]) end, client(Server_Node). %%% wait for a response from the server await_result() -> receive {messenger, stop, Why} -> % Stop the client io:format("~p~n", [Why]), exit(normal); {messenger, What} -> % Normal response io:format("~p~n", [What]) after 5000 -> io:format("No response from server~n", []), exit(timeout) end. ---- We have added the following changes: The messenger server traps exits. If it receives an EXIT signal, {'EXIT', From, Reason} this means that a client process has terminated or is unreachable because: the user has logged off (we have removed the "logoff" message). the network connection to the client is broken the node on which the client process resides has gone down the client processes has done some illegal operation. If we receive an EXIT signal as above, we delete the tuple, {From, Name} from the servers User_List using the server_logoff function. If the node on which the server goes down, an exit signal (automatically generated by the system), will be sent to all of the client processes {'EXIT', MessengerPID, noconnection} causing all the client processes to terminate. We have also introduced a timeout of five seconds in the await_result function. I.e. if the server does not reply withing five seconds (5000 ms), the client terminates. This is really only needed in the logon sequence before the client and server are linked. An interesting case is if the client were to terminate before the server links to it. This is taken care of because linking to a non-existent process causes an exit signal, {'EXIT', From, noproc}, to be automatically generated as if the process terminated immediately after the link operation. ---- Larger programs are usually written as a collection of files with a well defined interface between the various parts. To illustrate this, we will divide the messenger example into four files. mess_config.hrl header file for configuration data mess_interface.hrl interface definitions between the client and the messenger user_interface.erl functions for the user interface mess_client.erl functions for the client side of the messenger mess_server.erl functions for the server side of the messenger While doing this we will also clean up the message passing interface between the shell, the client and the server and define it using "records", we will also introduce "macros". ----FILE mess_config.hrl---- %%% Configure the location of the server node, -define(server_node, messenger@super). ----END FILE---- ----FILE mess_interface.hrl---- %%% Message interface between client and server and client shell for %%% messenger program %%%Messages from Client to server received in server/1 function. -record(logon,{client_pid, username}). -record(message,{client_pid, to_name, message}). %%% {'EXIT', ClientPid, Reason} (client terminated or unreachable. %%% Messages from Server to Client, received in await_result/0 function -record(abort_client,{message}). %%% Messages are: user_exists_at_other_node, %%% you_are_not_logged_on -record(server_reply,{message}). %%% Messages are: logged_on %%% receiver_not_found %%% sent (Message has been sent (no guarantee) %%% Messages from Server to Client received in client/1 function -record(message_from,{from_name, message}). %%% Messages from shell to Client received in client/1 function %%% spawn(mess_client, client, [server_node(), Name]) -record(message_to,{to_name, message}). %%% logoff ----END FILE---- ----FILE user_interface.erl---- %%% User interface to the messenger program %%% login(Name) %%% One user at a time can log in from each Erlang node in the %%% system messenger: and choose a suitable Name. If the Name %%% is already logged in at another node or if someone else is %%% already logged in at the same node, login will be rejected %%% with a suitable error message. %%% logoff() %%% Logs off anybody at at node %%% message(ToName, Message) %%% sends Message to ToName. Error messages if the user of this %%% function is not logged on or if ToName is not logged on at %%% any node. -module(user_interface). -export([logon/1, logoff/0, message/2]). -include("mess_interface.hrl"). -include("mess_config.hrl"). logon(Name) -> case whereis(mess_client) of undefined -> register(mess_client, spawn(mess_client, client, [?server_node, Name])); _ -> already_logged_on end. logoff() -> mess_client ! logoff. message(ToName, Message) -> case whereis(mess_client) of % Test if the client is running undefined -> not_logged_on; _ -> mess_client ! #message_to{to_name=ToName, message=Message}, ok end. ----END FILE---- ----FILE mess_client.erl---- %%% The client process which runs on each user node -module(mess_client). -export([client/2]). -include("mess_interface.hrl"). client(Server_Node, Name) -> {messenger, Server_Node} ! #logon{client_pid=self(), username=Name}, await_result(), client(Server_Node). client(Server_Node) -> receive logoff -> exit(normal); #message_to{to_name=ToName, message=Message} -> {messenger, Server_Node} ! #message{client_pid=self(), to_name=ToName, message=Message}, await_result(); {message_from, FromName, Message} -> io:format("Message from ~p: ~p~n", [FromName, Message]) end, client(Server_Node). %%% wait for a response from the server await_result() -> receive #abort_client{message=Why} -> io:format("~p~n", [Why]), exit(normal); #server_reply{message=What} -> io:format("~p~n", [What]) after 5000 -> io:format("No response from server~n", []), exit(timeout) end. ----END FILE--- ----FILE mess_server.erl---- %%% This is the server process of the messneger service -module(mess_server). -export([start_server/0, server/0]). -include("mess_interface.hrl"). server() -> process_flag(trap_exit, true), server([]). %%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...] server(User_List) -> io:format("User list = ~p~n", [User_List]), receive #logon{client_pid=From, username=Name} -> New_User_List = server_logon(From, Name, User_List), server(New_User_List); {'EXIT', From, _} -> New_User_List = server_logoff(From, User_List), server(New_User_List); #message{client_pid=From, to_name=To, message=Message} -> server_transfer(From, To, Message, User_List), server(User_List) end. %%% Start the server start_server() -> register(messenger, spawn(?MODULE, server, [])). %%% Server adds a new user to the user list server_logon(From, Name, User_List) -> %% check if logged on anywhere else case lists:keymember(Name, 2, User_List) of true -> From ! #abort_client{message=user_exists_at_other_node}, User_List; false -> From ! #server_reply{message=logged_on}, link(From), [{From, Name} | User_List] %add user to the list end. %%% Server deletes a user from the user list server_logoff(From, User_List) -> lists:keydelete(From, 1, User_List). %%% Server transfers a message between user server_transfer(From, To, Message, User_List) -> %% check that the user is logged on and who he is case lists:keysearch(From, 1, User_List) of false -> From ! #abort_client{message=you_are_not_logged_on}; {value, {_, Name}} -> server_transfer(From, Name, To, Message, User_List) end. %%% If the user exists, send the message server_transfer(From, Name, To, Message, User_List) -> %% Find the receiver and send the message case lists:keysearch(To, 2, User_List) of false -> From ! #server_reply{message=receiver_not_found}; {value, {ToPid, To}} -> ToPid ! #message_from{from_name=Name, message=Message}, From ! #server_reply{message=sent} end. ----END FILE--- You will see some files above with extension ".hrl". These are header files which are included in the ".erl" files by -include("File_Name"). for example: -include("mess_interface.hrl"). In our case above the file is fetched from the same directory as all the other files in the messenger example. (*manual*). .hrl files can contain any valid Erlang code but are most often used for record and macro definitions. A record is defined as -record(name_of_record,{field_name1, field_name2, field_name3, ......}). for example: -record(message_to,{to_name, message}). This is exactly equivalent to: {message_to, To_Name, Message} Creating record, is best illustrated by an example: #message_to{message="hello", to_name=fred) this will create: {message_to, fred, "hello"} Note that you don't have to worry about the order you assign values to the various parts of the records when you create it. The advantage of using records is that by placing their definitions in header files you can conveniently define interfaces which are easy to change. For example, if you want to add a new field to the record, you will only have to change the code where the new field is used and not at every place the record is referred to. If you leave out a field when creating a record, it will get the value of the atom "undefined". (*manual*) Pattern matching with records is very similar to creating records. For example inside a "case" or "receive": #message_to{to_name=ToName, message=Message} -> is the same as: {message_to, ToName, Message} The other thing we have added to the messenger is macro. The file mess_config.hrl contains the definition: %%% Configure the location of the server node, -define(server_node, messenger@super). we include this file in mess_server.erl -include("mess_config.hrl"). Every occurrence of ?server_node will in mess_server.erl will now be replaced by messenger@super. The other place a macro is used is when we spawn the server process: spawn(?MODULE, server, []) This is a standard "macro" (i.e. defined by the system, not the user. ?MODULE is always replaced by the name of current module (i.e. the -module definition near the start of the file). There are more advanced ways of using macros with, for example parameters (*manual*). The three Erlang (.erl) files in the messenger example are individually compiled into object code file (.beam). The Erlang system loads and links these files into the system when they are referred to during execution of the code. In our case we simply have put them in the same directory which is our current working directory (i.e. the place we have done "cd" to). There are ways of putting the .beam files in other directories. In the messenger example, no assumptions have been made about what the message being sent is. It could be any valid Erlang term. ---- This is the end of this part of the tutorial, which has missed out some parts of basic Erlang. In particular the following has been omitted. References Local error handling (catch/throw) handling of binary data (binaries / bit syntax) list comprehension how to communicate with the outside world and/or software written in other languages (ports) Very few of the Erlang libraries have been touched on (for example file handling) OTP has been totally skipped and in consequence the Mnesia database has been skipped. Hash tables for Erlang terms (ETS) Changing code in running systems I'll try to rectify these omissions when I have the time / energy later on.