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资源说明：Random erlang stuff
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Running erlang programs
=======================
For simple programs, use escript
Here is a simple hello world in escript::
#!/usr/bin/env escript
-export([main/1]).
main([]) -> io:format("Hello, World!~n").
for complex programs however, compile the standard way
source code ::
-module(hello).
-export([start/0]).
start() ->
io:format("Hello, World!").
from the shell comple & run like so ::
$erl
>c(hello).
>hello:start().
to compile from a Emakefile do ::
$ cd /path/to/emakefile
$ erl -make
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Common syntax mistakes
======================
* Not using comma in case of multi line statement::
X = 32 <-- missing comma
Y = 38
compiles to give the followoing error::
lists_bit_syntax.erl:7: syntax error before: Y
* Forgetting that io:format takes a list to substitute values.::
io:format("Value of X and Y are ~p , ~p" , X,Y) % Wrong, X,Y should be in a list
io:format("Value of X and Y are ~p , ~p" , [X,Y]) % Correct
* Not closing function clauses with semi-colon ( last one will close with a ".")::
valid_time({Y,M,D),{H,Min,S})->
io_format("Good") . <-- There should be a semi-colon here instead of dot as it is part of clause
valid_time(_) ->
io:format("Invalid input"). <- last function in the clause ends with dot
* look out for less than equals and greater than equals::
<= wrong =< right
>= right => wrong
* forgetting semi-colon in both `if` and `case ... of` clauses and adding semi-clon in the last clause::
if X == vala ->
Res = call_some_function(X),
Res; <--- this semi-colon should be there for all-but-last if clause
if X == valb ->
Res = call_some_other_function(X),
Res <--- last clause should not have anything. It is the end that completes the if block
end,
% case can let us do pattern matching
case X of ->
vala ->
Res = call_some_function(X),
Res;
valb ->
Res = call_some_other_function(X),
Res
end.
* ending the last statement of an anonymous funtion with a period. In a normal fuction last statement is followed by a period::
% Normal function
do_something(X) ->
io:format("Printing value of X ~p" ,[ X ] ),
io:format("Doing somethiing~n"). % <-------- notice the period here
DoSomething = fun(A) ->
io:format("Printing value of X ~p" ,[ X ] ),
io:format("Doing somethiing~n") %<-------- no period is required
end % <------ , or . depending on where it is
* putting -> after `receive` or leaving -> after `after` ::
% this is a bit weird but have to live with the syntax
% there should be no '->' after the receive and there should be
% a '->' after the 'after'
receive
{ From, Msg } -> do_something(Msg)
after 2000 ->
timeout
end,
* be very careful with records ::
% observe how records are used in the function head and the return
% see the '=' sign
-record( state, { name="testing",
value=0,
flags }.
increase_val( S=#state{ value=V, name=Name}, IncVal) -> % notice the '=' here
%increases the value of state by IncVal amount
io:format("increasing value of ~p by ~p~n",[Name,IncVal]),
S#state{ value=V+Inc}. % no '=' in the return value
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Useful libraries and man pages
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* `io:format `_ man page
* `Coding standards `_ for erlang
* `list of functions/bifs `_ allowed in guards
* Erlang `design principles http://www.erlang.org/doc/design_principles/applications.html#id71171>`_
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Resources and other stuff
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* books
- `learn you some erlang `_ book
- `Erlang programming `_ on Wikibooks
- `Erlang programming book
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