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399 lines
9.8 KiB
Markdown
399 lines
9.8 KiB
Markdown
---
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language: elixir
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contributors:
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- ["Joao Marques", "http://github.com/mrshankly"]
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filename: learnelixir.ex
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---
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Elixir is a modern functional language built on top of the Erlang VM.
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It's fully compatible with Erlang, but features a more standard syntax
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and many more features.
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```elixir
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# Single line comments start with a number symbol.
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# There's no multi-line comment,
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# but you can stack multiple comments.
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# To use the elixir shell use the `iex` command.
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# Compile your modules with the `elixirc` command.
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# Both should be in your path if you installed elixir correctly.
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## ---------------------------
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## -- Basic types
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## ---------------------------
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# There are numbers
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3 # integer
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0x1F # integer
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3.0 # float
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# Atoms, that are literals, a constant with name. They start with `:`.
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:hello # atom
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# Tuples that are stored contiguously in memory.
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{1,2,3} # tuple
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# We can access a tuple element with the `elem` function:
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elem({1, 2, 3}, 0) #=> 1
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# Lists that are implemented as linked lists.
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[1,2,3] # list
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# We can access the head and tail of a list as follows:
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[head | tail] = [1,2,3]
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head #=> 1
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tail #=> [2,3]
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# In elixir, just like in Erlang, the `=` denotes pattern matching and
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# not an assignment.
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#
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# This means that the left-hand side (pattern) is matched against a
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# right-hand side.
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#
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# This is how the above example of accessing the head and tail of a list works.
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# A pattern match will error when the sides don't match, in this example
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# the tuples have different sizes.
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# {a, b, c} = {1, 2} #=> ** (MatchError) no match of right hand side value: {1,2}
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# There's also binaries
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<<1,2,3>> # binary
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# Strings and char lists
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"hello" # string
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'hello' # char list
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# Multi-line strings
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"""
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I'm a multi-line
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string.
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"""
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#=> "I'm a multi-line\nstring.\n"
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# Strings are all encoded in UTF-8:
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"héllò" #=> "héllò"
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# Strings are really just binaries, and char lists are just lists.
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<<?a, ?b, ?c>> #=> "abc"
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[?a, ?b, ?c] #=> 'abc'
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# `?a` in elixir returns the ASCII integer for the letter `a`
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?a #=> 97
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# To concatenate lists use `++`, for binaries use `<>`
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[1,2,3] ++ [4,5] #=> [1,2,3,4,5]
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'hello ' ++ 'world' #=> 'hello world'
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<<1,2,3>> <> <<4,5>> #=> <<1,2,3,4,5>>
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"hello " <> "world" #=> "hello world"
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## ---------------------------
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## -- Operators
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## ---------------------------
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# Some math
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1 + 1 #=> 2
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10 - 5 #=> 5
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5 * 2 #=> 10
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10 / 2 #=> 5.0
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# In elixir the operator `/` always returns a float.
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# To do integer division use `div`
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div(10, 2) #=> 5
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# To get the division remainder use `rem`
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rem(10, 3) #=> 1
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# There's also boolean operators: `or`, `and` and `not`.
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# These operators expect a boolean as their first argument.
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true and true #=> true
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false or true #=> true
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# 1 and true #=> ** (ArgumentError) argument error
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# Elixir also provides `||`, `&&` and `!` which accept arguments of any type.
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# All values except `false` and `nil` will evaluate to true.
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1 || true #=> 1
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false && 1 #=> false
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nil && 20 #=> nil
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!true #=> false
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# For comparisons we have: `==`, `!=`, `===`, `!==`, `<=`, `>=`, `<` and `>`
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1 == 1 #=> true
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1 != 1 #=> false
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1 < 2 #=> true
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# `===` and `!==` are more strict when comparing integers and floats:
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1 == 1.0 #=> true
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1 === 1.0 #=> false
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# We can also compare two different data types:
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1 < :hello #=> true
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# The overall sorting order is defined below:
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# number < atom < reference < functions < port < pid < tuple < list < bit string
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# To quote Joe Armstrong on this: "The actual order is not important,
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# but that a total ordering is well defined is important."
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## ---------------------------
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## -- Control Flow
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## ---------------------------
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# `if` expression
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if false do
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"This will never be seen"
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else
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"This will"
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end
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# There's also `unless`
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unless true do
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"This will never be seen"
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else
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"This will"
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end
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# Remember pattern matching? Many control-flow structures in elixir rely on it.
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# `case` allows us to compare a value against many patterns:
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case {:one, :two} do
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{:four, :five} ->
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"This won't match"
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{:one, x} ->
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"This will match and assign `x` to `:two`"
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_ ->
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"This will match any value"
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end
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# It's common practice to assign a value to `_` if we don't need it.
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# For example, if only the head of a list matters to us:
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[head | _] = [1,2,3]
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head #=> 1
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# For better readability we can do the following:
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[head | _tail] = [:a, :b, :c]
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head #=> :a
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# `cond` lets us check for many conditions at the same time.
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# Use `cond` instead of nesting many `if` expressions.
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cond do
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1 + 1 == 3 ->
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"I will never be seen"
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2 * 5 == 12 ->
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"Me neither"
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1 + 2 == 3 ->
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"But I will"
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end
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# It is common to see a last condition equal to `true`, which will always match.
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cond do
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1 + 1 == 3 ->
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"I will never be seen"
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2 * 5 == 12 ->
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"Me neither"
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true ->
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"But I will (this is essentially an else)"
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end
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# `try/catch` is used to catch values that are thrown, it also supports an
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# `after` clause that is invoked whether or not a value is caught.
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try do
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throw(:hello)
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catch
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message -> "Got #{message}."
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after
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IO.puts("I'm the after clause.")
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end
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#=> I'm the after clause
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# "Got :hello"
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## ---------------------------
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## -- Modules and Functions
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## ---------------------------
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# Anonymous functions (notice the dot)
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square = fn(x) -> x * x end
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square.(5) #=> 25
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# They also accept many clauses and guards.
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# Guards let you fine tune pattern matching,
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# they are indicated by the `when` keyword:
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f = fn
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x, y when x > 0 -> x + y
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x, y -> x * y
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end
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f.(1, 3) #=> 4
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f.(-1, 3) #=> -3
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# Elixir also provides many built-in functions.
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# These are available in the current scope.
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is_number(10) #=> true
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is_list("hello") #=> false
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elem({1,2,3}, 0) #=> 1
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# You can group several functions into a module. Inside a module use `def`
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# to define your functions.
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defmodule Math do
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def sum(a, b) do
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a + b
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end
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def square(x) do
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x * x
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end
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end
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Math.sum(1, 2) #=> 3
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Math.square(3) #=> 9
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# To compile our simple Math module save it as `math.ex` and use `elixirc`
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# in your terminal: elixirc math.ex
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# Inside a module we can define functions with `def` and private functions with `defp`.
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# A function defined with `def` is available to be invoked from other modules,
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# a private function can only be invoked locally.
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defmodule PrivateMath do
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def sum(a, b) do
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do_sum(a, b)
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end
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defp do_sum(a, b) do
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a + b
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end
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end
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PrivateMath.sum(1, 2) #=> 3
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# PrivateMath.do_sum(1, 2) #=> ** (UndefinedFunctionError)
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# Function declarations also support guards and multiple clauses:
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defmodule Geometry do
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def area({:rectangle, w, h}) do
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w * h
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end
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def area({:circle, r}) when is_number(r) do
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3.14 * r * r
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end
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end
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Geometry.area({:rectangle, 2, 3}) #=> 6
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Geometry.area({:circle, 3}) #=> 28.25999999999999801048
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# Geometry.area({:circle, "not_a_number"})
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#=> ** (FunctionClauseError) no function clause matching in Geometry.area/1
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# Due to immutability, recursion is a big part of elixir
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defmodule Recursion do
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def sum_list([head | tail], acc) do
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sum_list(tail, acc + head)
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end
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def sum_list([], acc) do
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acc
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end
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end
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Recursion.sum_list([1,2,3], 0) #=> 6
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# Elixir modules support attributes, there are built-in attributes and you
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# may also add custom attributes.
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defmodule MyMod do
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@moduledoc """
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This is a built-in attribute on a example module.
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"""
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@my_data 100 # This is a custom attribute.
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IO.inspect(@my_data) #=> 100
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end
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## ---------------------------
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## -- Records and Exceptions
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## ---------------------------
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# Records are basically structures that allow you to associate a name with
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# a particular value.
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defrecord Person, name: nil, age: 0, height: 0
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joe_info = Person.new(name: "Joe", age: 30, height: 180)
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#=> Person[name: "Joe", age: 30, height: 180]
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# Access the value of name
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joe_info.name #=> "Joe"
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# Update the value of age
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joe_info = joe_info.age(31) #=> Person[name: "Joe", age: 31, height: 180]
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# The `try` block with the `rescue` keyword is used to handle exceptions
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try do
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raise "some error"
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rescue
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RuntimeError -> "rescued a runtime error"
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_error -> "this will rescue any error"
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end
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# All exceptions have a message
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try do
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raise "some error"
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rescue
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x in [RuntimeError] ->
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x.message
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end
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## ---------------------------
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## -- Concurrency
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## ---------------------------
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# Elixir relies on the actor model for concurrency. All we need to write
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# concurrent programs in elixir are three primitives: spawning processes,
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# sending messages and receiving messages.
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# To start a new process we use the `spawn` function, which takes a function
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# as argument.
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f = fn -> 2 * 2 end #=> #Function<erl_eval.20.80484245>
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spawn(f) #=> #PID<0.40.0>
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# `spawn` returns a pid (process identifier), you can use this pid to send
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# messages to the process. To do message passing we use the `send` operator.
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# For all of this to be useful we need to be able to receive messages. This is
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# achived with the `receive` mechanism:
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defmodule Geometry do
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def area_loop do
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receive do
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{:rectangle, w, h} ->
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IO.puts("Area = #{w * h}")
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area_loop()
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{:circle, r} ->
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IO.puts("Area = #{3.14 * r * r}")
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area_loop()
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end
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end
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end
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# Compile the module and create a process that evaluates `area_loop` in the shell
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pid = spawn(fn -> Geometry.area_loop() end) #=> #PID<0.40.0>
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# Send a message to `pid` that will match a pattern in the receive statement
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send pid, {:rectangle, 2, 3}
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#=> Area = 6
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# {:rectangle,2,3}
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send pid, {:circle, 2}
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#=> Area = 12.56000000000000049738
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# {:circle,2}
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# The shell is also a process, you can use `self` to get the current pid
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self() #=> #PID<0.27.0>
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```
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## References
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* [Getting started guide](http://elixir-lang.org/getting_started/1.html) from [elixir webpage](http://elixir-lang.org)
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* [Elixir Documentation](http://elixir-lang.org/docs/master/)
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* ["Learn You Some Erlang for Great Good!"](http://learnyousomeerlang.com/) by Fred Hebert
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* "Programming Erlang: Software for a Concurrent World" by Joe Armstrong
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