Erlang overview

Basic

Documentation: https://erlang.org/doc/search/

Functional language

Erlang is a functional language. Code need compile and running line by line.

Every line need finish by a .. like: A = 1..

Module

Every erlang file will consider as a module. You console will compile all module you want.

You have to add -module(<filename>). into first line. Module should be same with filename without suffix.

In erl console, run c(<filename>). to compile it. Run a function is like: <moduleName>: <functionName>(...<argument>)

Also you need export a list contained all function you expect to export. Like -export([append/2,distance/2, double/1, drop/2]). or -compile(export_all). to export all function.

append/2 means a function called append with 2 arguments.

Variable & item

Variable a word starting with upper case. like A or Result.

item is a word starting from lower case. like request or add.

Variable store the value.

item use for matching.

Bind

In erlang, No concept about assigning a value to a variable. It is actually bind a variable to a value. Once they are bound together you cannot to assign a new value to it.

A = 1. # right
B = 2. # right
A = B, # wrong. Because A is already bound.

Function

Sample function:

sample(A) -> A.

The last line of Function is the return value. Use ; to represent end point of a branch.

if
    (A == true) and (B == true) ->
        true;
    true ->
        false
end.

Multi-thread

Erlang use message passing module to deal with multi thread. Each thread have own mailbox, stack and heap.

thread id

Each thread have a id called Pid. You need use Pid as an address and sending the message through it.

receive and send message

echo() ->
    receive
        {echo, From, Ref, Msg} ->
    From ! {responds, self(), Ref, Msg}, echo();
        {stop} -> ok
end.

spawn

Reference: http://erlang.org/doc/man/erlang.html#spawn-1

We need spawn/1, spawn/2, spawn/3 to generate new thread, usually we use spawn/3. The basic idea is giving a function that running initially at new thread. This function can calling another function inside the module.

Pid = spawn(?MODULE, echo, []). This line means we generate a new thread with a function called echo in unknown module (usually self module). The argument list is empty and the thread id bind with Pid.

special

fold

This is a basic function which is same as reduce() in javascript.

In standard library, erlang have lists:foldl(F,A,[H|T]).

foldl is folding the list by a given function from left to right. foldr is from right to left.

foldl(_F, A, []) -> A;
foldl(F, A, [H|T]) -> fold(F, F(H, A), [T]).

foldr(_F, A, []) -> A;
foldr(F, A, [H|T]) -> F(H, foldr(F, A, T)).

_F means F is unnecessary since first part of function not use F.

[H|T] is unique characteristic of erlang. H is the first element. T is the rest. [H|T] = [1,2,3,4,5]. H is 1. T is [2,3,4,5]

We can run this function:foldl(fun (H, A) -> H + A end, 0 , [1,2,3,4,5]).

MakeFile

Sample makefile from a erlang homework:

all:
	erlc *erl
	erl -eval "lexgrm:start(), halt()" -noshell -detached

run: all
	erl

clean:
	rm -f *beam grm.erl lex.erl