Day 1: Starting Out and Modules
With Erlang you can try out the language using the Erlang shell by typing erl, one great thing about the Erlang shell is that it has a built-in line editor based on Emacs, allowing you to use something like CTRL-A and CTRL-E to move your cursor to the beginning and the end of a line.
Pressing CTRL-G and then h shows yet another power or Erlang shell, turns out the Erlang shell is a buncle of shell instances, each running different jobs you can type something like j to list all the jobs.
Erlang Basics Erlang is a small language having few built-in types upon which a few more syntactic elements have been added. One thing unique about Erlang is that expressions have to be terminated with a period followed by whitespace (line break, a space etc.).
1. Numbers
1> 2 + 15.
17
2> 5 / 2.
2.5
To get an integer-to-integer division, you have to use div, and to have the modulo operator, use rem (remainder), same story in Elixir.
1> 5 div 2.
2
2> 5 rem 2.
1
2. Invariable Variables Variables in Erlang are slightly different there is no assignment but pattern matching The first thing these commands tell us is that you can assign a value to a variable exactly once; then you can ‘pretend’ to assign a value to a variable if it’s the same value it already has. If it’s different, Erlang will complain.
1> One.
* 1: variable 'One' is unbound
2> One = 1.
1
Match operator in action.
1> 47 = 45 + 2.
47
2> 47 = 45 + 3.
** exception error: no match of right hand side value 48
3. Atoms Atoms are literals, constants with their own name for value. They are synonymous to symbols in Ruby, they are lower case thats is why variables in Erlang are capitalized.
1> atom.
atom
2> atoms_rule.
atoms_rule
3> atoms_rule@erlang.
atoms_rule@erlang
4> 'Atoms can be cheated!'.
'Atoms can be cheated!'
5> atom = 'atom'.
atom
4. Boolean Algrebra and Comparison Operators Erlang allows for comparison of values using boolean, this is done as follows:
1> true and false.
false
2> false or true.
true
3> true xor false.
true
4> not false.
true
5> not (true and true).
false
In Erlnag the boolean operators and and or will always evaluate arguments on both sides of the operator. If you want to have the short-circuit operators (which will only evaluate the right-side argument if it needs to), use andalso and orelse.
To test for equality and inequality is where Erlang uses a different set of symbols compared to what you may be used to.
6> 5 =:= 5.
true
7> 1 =:= 0.
false
8> 1 =/= 0.
true
9> 5 =:= 5.0.
false
10> 5 == 5.0.
true
11> 5 /= 5.0.
false
Instead of == and != that is common in many languages, Erlang uses =:= and =/= to test for equality and inequality respectively.
Another tripping point is =< (less than or equal to). which is a little confusing if you may ask me.
12> 1 < 2.
true
13> 1 < 1.
false
14> 1 >= 1.
true
15> 1 =< 1.
true
Erlang has no such things as boolean true and false. The terms true and false are atoms
5. Tuples A tuple is a way to organize data. It groups together many terms when you know how many there are. In Erlang, a tuple is written in the form {Element1, Element2, …, ElementN}.
1> X = 10, Y = 4.
4
2> Point = {X,Y}.
{10,4}
A tuple which contains an atom with one element following it is called a tagged tuple. Any element of a tuple can be of any type, even another tuple:
12> {point, {X,Y}}.
{point,{4,5}}
6. Lists Lists are a key component of functionalm programming, helping solve a multitude of problems, in Erlang they are among the most used data structures. You can put anything in a list from Numbers, atoms, tuples, other lists. The basic notation of a list is [Element1, Element2, …, ElementN].
1> [1, 2, 3, {numbers,[4,5,6]}, 5.34, atom].
[1,2,3,{numbers,[4,5,6]},5.34,atom]
In Erlang Strings are lists:
1> [97, 98, 99].
"abc"
2> [233].
"é"
Erlang will print lists of numbers as numbers only when at least one of them could not also represent a letter! There is no such thing as a real string in Erlang!
lists are joined using the ++ operator. The opposite of ++ is -- which removes elements from a list:
The first element of a list is named the Head, and the rest of the list is named the Tail. We will use two built-in functions (BIF) to get them.
11> hd([1,2,3,4]).
1
12> tl([1,2,3,4]).
[2,3,4]
7. List Comprehensions
List comprehensions allows for building or modifying lists. They also make programs short and easy to understand compared to other ways of manipulating lists. It’s based off the idea of set notation.
1> [2*N || N <- [1,2,3,4]].
[2,4,6,8]
2> [X || X <- [1,2,3,4,5,6,7,8,9,10], X rem 2 =:= 0].
[2,4,6,8,10]
3> RestaurantMenu = [{steak, 5.99}, {beer, 3.99}, {poutine, 3.50}, {kitten, 20.99}, {water, 0.00}].
[{steak,5.99},
{beer,3.99},
{poutine,3.5},
{kitten,20.99},
{water,0.0}]
4> [{Item, Price*1.07} || {Item, Price} <- RestaurantMenu, Price >= 3, Price =< 10].
[{steak,6.409300000000001},{beer,4.2693},{poutine,3.745}]
6> Weather = [{toronto, rain}, {montreal, storms}, {london, fog},
6> {paris, sun}, {boston, fog}, {vancouver, snow}].
[{toronto,rain},
{montreal,storms},
{london,fog},
{paris,sun},
{boston,fog},
{vancouver,snow}]
7> FoggyPlaces = [X || {X, fog} <- Weather].
[london,boston]
7. Binary Data
Erlang makes it easy to work with binary data using pattern matching to a great success.
1> Color = 16#F09A29.
15768105
2> Pixel = <<Color:24>>.
<<240,154,41>>
This basically says “Put the binary values of #F09A29 on 24 bits of space (Red on 8 bits, Green on 8 bits and Blue also on 8 bits) in the variable Pixel.” The value can later be taken to be written to a file.
Binary comprehensions are to bit syntax what list comprehensions are to lists: a way to make code short and concise.
1> [ X || <<X>> <= <<1,2,3,4,5>>, X rem 2 == 0].
[2,4]
The only change in syntax from regular list comprehensions is the <- which became <= and using binaries («») instead of lists ([]). Modules Modules are a bunch of functions regrouped in a single file, under a single name. Additionally, all functions in Erlang must be defined in modules. Below is an example of how a module is written in Erlang.
-module(what_the_if).
-export([heh_fine/0,oh_god/1, help_me/1]).
heh_fine() ->
if 1 =:= 1 ->
works
end,
if 1 =:= 2; 1 =:= 1 ->
works
end,
if 1 =:= 1, 1 =:= 1 ->
fail
end.
% what_the_if:oh_god(2)
oh_god(N) ->
if N =:= 2 -> might_succeed;
true -> always_does %% This is Erlang's if's 'else!'
end.
% what_the_if:help_me(dog).
help_me(Animal) ->
Talk = if Animal == cat -> "meow";
Animal == beef -> "mooo";
Animal == dog -> "bark";
Animal == tree -> "bark";
true -> "ghfghsafghdsgha"
end,
{Animal, "says " ++ Talk ++ "!"}.
References
For more information on this topic you can checkout the following amazing posts: