Elixir v1.3.1 Kernel.SpecialForms

Special forms are the basic building blocks of Elixir, and therefore cannot be overridden by the developer.

We define them in this module. Some of these forms are lexical (like alias/2, case/2, etc). The macros {} and <<>> are also special forms used to define tuple and binary data structures respectively.

This module also documents Elixir’s pseudo variables (__ENV__, __MODULE__, __DIR__ and __CALLER__). Pseudo variables return information about Elixir’s compilation environment and can only be read, never assigned to.

Finally, it also documents two special forms, __block__ and __aliases__, which are not intended to be called directly by the developer but they appear in quoted contents since they are essential in Elixir’s constructs.

Summary

Macros

%

Creates a struct

%{}

Creates a map

Captures or creates an anonymous function

Defines a remote call or an alias

Used by types and bitstrings to specify types

Defines a new bitstring

Matches the value on the right against the pattern on the left

Accesses an already bound variable in match clauses. Also known as the pin operator

Returns the current calling environment as a Macro.Env struct

Returns the absolute path of the directory of the current file as a binary

Returns the current environment information as a Macro.Env struct

Returns the current module name as an atom or nil otherwise

Internal special form to hold aliases information

Internal special form for block expressions

alias/2 is used to setup aliases, often useful with modules names

Matches the given expression against the given clauses

Evaluates the expression corresponding to the first clause that evaluates to a truthy value

Defines an anonymous function

Comprehensions allow you to quickly build a data structure from an enumerable or a bitstring

Imports functions and macros from other modules

Gets the representation of any expression

Checks if there is a message matching the given clauses in the current process mailbox

Requires a given module to be compiled and loaded

Calls the overridden function when overriding it with Kernel.defoverridable/1

Evaluates the given expressions and handles any error, exit or throw that may have happened

Unquotes the given expression from inside a macro

Unquotes the given list expanding its arguments. Similar to unquote/1

Used to combine matching clauses

Creates a tuple

Macros

Creates a struct.

A struct is a tagged map that allows developers to provide default values for keys, tags to be used in polymorphic dispatches and compile time assertions.

Structs are usually defined with the Kernel.defstruct/1 macro:

defmodule User do
  defstruct name: "john", age: 27
end

Now a struct can be created as follows:

%User{}

Underneath a struct is just a map with a :__struct__ key pointing to the User module:

%User{} == %{__struct__: User, name: "john", age: 27}

A struct also validates that the given keys are part of the defined struct. The example below will fail because there is no key :full_name in the User struct:

%User{full_name: "john doe"}

An update operation specific for structs is also available:

%User{user | age: 28}

The syntax above will guarantee the given keys are valid at compilation time and it will guarantee at runtime the given argument is a struct, failing with BadStructError otherwise.

Although structs are maps, by default structs do not implement any of the protocols implemented for maps. Check Kernel.defprotocol/2 for more information on how structs can be used with protocols for polymorphic dispatch. Also see Kernel.struct/2 and Kernel.struct!/2 for examples on how to create and update structs dynamically.

%{}

Creates a map.

Maps are key-value stores where keys are compared using the match operator (===). Maps can be created with the %{} special form where keys are associated via =>:

%{1 => 2}

Maps also support the keyword notation, as other special forms, as long as they are at the end of the argument list:

%{hello: :world, with: :keywords}
%{:hello => :world, with: :keywords}

If a map has duplicated keys, the last key will always have higher precedence:

iex> %{a: :b, a: :c}
%{a: :c}

Conveniences for manipulating maps can be found in the Map module.

Access syntax

Besides the access functions available in the Map module, like Map.get/3 and Map.fetch/2, a map can be accessed using the . operator:

iex> map = %{a: :b}
iex> map.a
:b

Note that the . operator expects the key :a to exist in the map. If not, an ArgumentError is raised.

Update syntax

Maps also support an update syntax:

iex> map = %{:a => :b}
iex> %{map | :a => :c}
%{:a => :c}

Notice the update syntax requires the given keys to exist. Trying to update a key that does not exist will raise an KeyError.

AST representation

Regardless if => or the keywords syntax is used, Maps are always represented internally as a list of two-element tuples for simplicity:

iex> quote do
...>   %{"a" => :b, c: :d}
...> end
{:%{}, [], [{"a", :b}, {:c, :d}]}
&(expr)

Captures or creates an anonymous function.

Capture

The capture operator is most commonly used to capture a function with given name and arity from a module:

iex> fun = &Kernel.is_atom/1
iex> fun.(:atom)
true
iex> fun.("string")
false

In the example above, we captured Kernel.is_atom/1 as an anonymous function and then invoked it.

The capture operator can also be used to capture local functions, including private ones, and imported functions by omitting the module name:

&local_function/1

Anonymous functions

The capture operator can also be used to partially apply functions, where &1, &2 and so on can be used as value placeholders. For example:

iex> double = &(&1 * 2)
iex> double.(2)
4

In other words, &(&1 * 2) is equivalent to fn x -> x * 2 end. Another example using a local function:

iex> fun = &is_atom(&1)
iex> fun.(:atom)
true

The & operator can be used with more complex expressions:

iex> fun = &(&1 + &2 + &3)
iex> fun.(1, 2, 3)
6

As well as with lists and tuples:

iex> fun = &{&1, &2}
iex> fun.(1, 2)
{1, 2}

iex> fun = &[&1 | &2]
iex> fun.(1, 2)
[1 | 2]

The only restrictions when creating anonymous functions is that at least one placeholder must be present, i.e. it must contain at least &1, and that block expressions are not supported:

# No placeholder, fails to compile.
&(:foo)

# Block expression, fails to compile.
&(&1; &2)
left . right

Defines a remote call or an alias.

The dot (.) in Elixir can be used for remote calls:

iex> String.downcase("FOO")
"foo"

In this example above, we have used . to invoke downcase in the String alias, passing “FOO” as argument. We can also use the dot for creating aliases:

iex> Hello.World
Hello.World

This time, we have joined two aliases, defining the final alias Hello.World.

Syntax

The right side of . may be a word starting in upcase, which represents an alias, a word starting with lowercase or underscore, any valid language operator or any name wrapped in single- or double-quotes. Those are all valid examples:

iex> Kernel.Sample
Kernel.Sample

iex> Kernel.length([1, 2, 3])
3

iex> Kernel.+(1, 2)
3

iex> Kernel."length"([1, 2, 3])
3

iex> Kernel.'+'(1, 2)
3

Note that Kernel."HELLO" will be treated as a remote call and not an alias. This choice was done so every time single- or double-quotes are used, we have a remote call regardless of the quote contents. This decision is also reflected in the quoted expressions discussed below.

Quoted expression

When . is used, the quoted expression may take two distinct forms. When the right side starts with a lowercase letter (or underscore):

iex> quote do
...>   String.downcase("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}

Notice we have an inner tuple, containing the atom :. representing the dot as first element:

{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}

This tuple follows the general quoted expression structure in Elixir, with the name as first argument, some keyword list as metadata as second, and the number of arguments as third. In this case, the arguments is the alias String and the atom :downcase. The second argument is always an atom:

iex> quote do
...>   String."downcase"("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}

The tuple containing :. is wrapped in another tuple, which actually represents the function call, and has "FOO" as argument.

When the right side is an alias (i.e. starts with uppercase), we get instead:

iex> quote do
...>   Hello.World
...> end
{:__aliases__, [alias: false], [:Hello, :World]}

We go into more details about aliases in the __aliases__ special form documentation.

Unquoting

We can also use unquote to generate a remote call in a quoted expression:

iex> x = :downcase
iex> quote do
...>   String.unquote(x)("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}

Similar to Kernel."HELLO", unquote(x) will always generate a remote call, independent of the value of x. To generate an alias via the quoted expression, one needs to rely on Module.concat/2:

iex> x = Sample
iex> quote do
...>   Module.concat(String, unquote(x))
...> end
{{:., [], [{:__aliases__, [alias: false], [:Module]}, :concat]}, [],
 [{:__aliases__, [alias: false], [:String]}, Sample]}
left :: right

Used by types and bitstrings to specify types.

This operator is used in two distinct occasions in Elixir. It is used in typespecs to specify the type of a variable, function or of a type itself:

@type number :: integer | float
@spec add(number, number) :: number

It may also be used in bit strings to specify the type of a given bit segment:

<<int::integer-little, rest::bits>> = bits

Read the documentation on the Typespec page and <<>>/1 for more information on typespecs and bitstrings respectively.

<<args>>

Defines a new bitstring.

Examples

iex> <<1, 2, 3>>
<<1, 2, 3>>

Types

A bitstring is made of many segments and each segment has a type. There are 9 types used in bitstrings:

  • integer
  • float
  • bits (alias for bitstring)
  • bitstring
  • binary
  • bytes (alias for binary)
  • utf8
  • utf16
  • utf32

When no type is specified, the default is integer:

iex> <<1, 2, 3>>
<<1, 2, 3>>

Elixir also accepts by default the segment to be a literal string or a literal charlist, which are by default expanded to integers:

iex> <<0, "foo">>
<<0, 102, 111, 111>>

Variables or any other type need to be explicitly tagged:

iex> rest = "oo"
iex> <<102, rest>>
** (ArgumentError) argument error

We can solve this by explicitly tagging it as a binary:

iex> rest = "oo"
iex> <<102, rest::binary>>
"foo"

The utf8, utf16, and utf32 types are for Unicode codepoints. They can also be applied to literal strings and charlists:

iex> <<"foo"::utf16>>
<<0, 102, 0, 111, 0, 111>>
iex> <<"foo"::utf32>>
<<0, 0, 0, 102, 0, 0, 0, 111, 0, 0, 0, 111>>

Options

Many options can be given by using - as separator. Order is arbitrary, so the following are all equivalent:

<<102::integer-native, rest::binary>>
<<102::native-integer, rest::binary>>
<<102::unsigned-big-integer, rest::binary>>
<<102::unsigned-big-integer-size(8), rest::binary>>
<<102::unsigned-big-integer-8, rest::binary>>
<<102::8-integer-big-unsigned, rest::binary>>
<<102, rest::binary>>

Unit and Size

The length of the match is equal to the unit (a number of bits) times the size (the number of repeated segments of length unit).

TypeDefault Unit
integer1 bit
float1 bit
binary8 bits

Sizes for types are a bit more nuanced. The default size for integers is 8.

For floats, it is 64. For floats, size * unit must result in 32 or 64, corresponding to IEEE 754 binary32 and binary64, respectively.

For binaries, the default is the size of the binary. Only the last binary in a match can use the default size. All others must have their size specified explicitly, even if the match is unambiguous. For example:

iex> <<name::binary-size(5), " the ", species::binary>> = <<"Frank the Walrus">>
"Frank the Walrus"
iex> {name, species}
{"Frank", "Walrus"}

Failing to specify the size for the non-last causes compilation to fail:

<<name::binary, " the ", species::binary>> = <<"Frank the Walrus">>
** (CompileError): a binary field without size is only allowed at the end of a binary pattern

Shortcut Syntax

Size and unit can also be specified using a syntax shortcut when passing integer values:

iex> x = 1
iex> <<x::8>> == <<x::size(8)>>
true
iex> <<x::8 * 4>> == <<x::size(8)-unit(4)>>
true

This syntax reflects the fact the effective size is given by multiplying the size by the unit.

Modifiers

Some types have associated modifiers to clear up ambiguity in byte representation.

ModifierRelevant Type(s)
signedinteger
unsigned (default)integer
littleinteger, utf16, utf32
big (default)integer, utf16, utf32
nativeinteger, utf16, utf32

Sign

Integers can be signed or unsigned, defaulting to unsigned.

iex> <<int::integer>> = <<-100>>
<<156>>
iex> int
156
iex> <<int::integer-signed>> = <<-100>>
<<156>>
iex> int
-100

signed and unsigned are only used for matching binaries (see below) and are only used for integers.

iex> <<-100::signed, _rest::binary>> = <<-100, "foo">>
<<156, 102, 111, 111>>

Endianness

Elixir has three options for endianness: big, little, and native. The default is big:

iex> <<number::little-integer-size(16)>> = <<0, 1>>
<<0, 1>>
iex> number
256
iex> <<number::big-integer-size(16)>> = <<0, 1>>
<<0, 1>>
iex> number
1

native is determined by the VM at startup and will depend on the host operating system.

Binary/Bitstring Matching

Binary matching is a powerful feature in Elixir that is useful for extracting information from binaries as well as pattern matching.

Binary matching can be used by itself to extract information from binaries:

iex> <<"Hello, ", place::binary>> = "Hello, World"
"Hello, World"
iex> place
"World"

Or as a part of function definitions to pattern match:

defmodule ImageTyper
  @png_signature <<137::size(8), 80::size(8), 78::size(8), 71::size(8),
                13::size(8), 10::size(8), 26::size(8), 10::size(8)>>
  @jpg_signature <<255::size(8), 216::size(8)>>

  def type(<<@png_signature, rest::binary>>), do: :png
  def type(<<@jpg_signature, rest::binary>>), do: :jpg
  def type(_), do :unknown
end

Performance & Optimizations

The Erlang compiler can provide a number of optimizations on binary creation and matching. To see optimization output, set the bin_opt_info compiler option:

ERL_COMPILER_OPTIONS=bin_opt_info mix compile

To learn more about specific optimizations and performance considerations, check out Erlang’s Efficiency Guide on handling binaries.

left = right

Matches the value on the right against the pattern on the left.

^var

Accesses an already bound variable in match clauses. Also known as the pin operator.

Examples

Elixir allows variables to be rebound via static single assignment:

iex> x = 1
iex> x = x + 1
iex> x
2

However, in some situations, it is useful to match against an existing value, instead of rebinding. This can be done with the ^ special form, colloquially known as the pin operator:

iex> x = 1
iex> ^x = List.first([1])
iex> ^x = List.first([2])
** (MatchError) no match of right hand side value: 2

Note that ^x always refers to the value of x prior to the match. The following example will match:

iex> x = 0
iex> {x, ^x} = {1, 0}
iex> x
1
__CALLER__

Returns the current calling environment as a Macro.Env struct.

In the environment you can access the filename, line numbers, set up aliases, the function and others.

__DIR__

Returns the absolute path of the directory of the current file as a binary.

Although the directory can be accessed as Path.dirname(__ENV__.file), this macro is a convenient shortcut.

__ENV__

Returns the current environment information as a Macro.Env struct.

In the environment you can access the current filename, line numbers, set up aliases, the current function and others.

__MODULE__

Returns the current module name as an atom or nil otherwise.

Although the module can be accessed in the __ENV__, this macro is a convenient shortcut.

__aliases__(args)

Internal special form to hold aliases information.

It is usually compiled to an atom:

iex> quote do
...>   Foo.Bar
...> end
{:__aliases__, [alias: false], [:Foo, :Bar]}

Elixir represents Foo.Bar as __aliases__ so calls can be unambiguously identified by the operator :.. For example:

iex> quote do
...>   Foo.bar
...> end
{{:., [], [{:__aliases__, [alias: false], [:Foo]}, :bar]}, [], []}

Whenever an expression iterator sees a :. as the tuple key, it can be sure that it represents a call and the second argument in the list is an atom.

On the other hand, aliases holds some properties:

  1. The head element of aliases can be any term that must expand to an atom at compilation time.

  2. The tail elements of aliases are guaranteed to always be atoms.

  3. When the head element of aliases is the atom :Elixir, no expansion happens.
__block__(args)

Internal special form for block expressions.

This is the special form used whenever we have a block of expressions in Elixir. This special form is private and should not be invoked directly:

iex> quote do
...>   1
...>   2
...>   3
...> end
{:__block__, [], [1, 2, 3]}
alias(module, opts)

alias/2 is used to setup aliases, often useful with modules names.

Examples

alias/2 can be used to setup an alias for any module:

defmodule Math do
  alias MyKeyword, as: Keyword
end

In the example above, we have set up MyKeyword to be aliased as Keyword. So now, any reference to Keyword will be automatically replaced by MyKeyword.

In case one wants to access the original Keyword, it can be done by accessing Elixir:

Keyword.values   #=> uses MyKeyword.values
Elixir.Keyword.values #=> uses Keyword.values

Notice that calling alias without the as: option automatically sets an alias based on the last part of the module. For example:

alias Foo.Bar.Baz

Is the same as:

alias Foo.Bar.Baz, as: Baz

Lexical scope

import/2, require/2 and alias/2 are called directives and all have lexical scope. This means you can set up aliases inside specific functions and it won’t affect the overall scope.

Warnings

If you alias a module and you don’t use the alias, Elixir is going to issue a warning implying the alias is not being used.

In case the alias is generated automatically by a macro, Elixir won’t emit any warnings though, since the alias was not explicitly defined.

Both warning behaviours could be changed by explicitly setting the :warn option to true or false.

case(condition, clauses)

Matches the given expression against the given clauses.

Examples

case thing do
  {:selector, i, value} when is_integer(i) ->
    value
  value ->
    value
end

In the example above, we match thing against each clause “head” and execute the clause “body” corresponding to the first clause that matches.

If no clause matches, an error is raised. For this reason, it may be necessary to add a final catch-all clause (like _) which will always match.

x = 10

case x do
  0 ->
    "This clause won't match"
  _ ->
    "This clause would match any value (x = #{x})"
end
#=> "This clause would match any value (x = 10)"

Variables handling

Notice that variables bound in a clause “head” do not leak to the outer context:

case data do
  {:ok, value} -> value
  :error -> nil
end

value #=> unbound variable value

However, variables explicitly bound in the clause “body” are accessible from the outer context:

value = 7

case lucky? do
  false -> value = 13
  true  -> true
end

value #=> 7 or 13

In the example above, value is going to be 7 or 13 depending on the value of lucky?. In case value has no previous value before case, clauses that do not explicitly bind a value have the variable bound to nil.

If you want to pattern match against an existing variable, you need to use the ^/1 operator:

x = 1

case 10 do
  ^x -> "Won't match"
  _  -> "Will match"
end
#=> "Will match"
cond(clauses)

Evaluates the expression corresponding to the first clause that evaluates to a truthy value.

cond do
  hd([1, 2, 3]) ->
    "1 is considered as true"
end
#=> "1 is considered as true"

Raises an error if all conditions evaluate to nil or false. For this reason, it may be necessary to add a final always-truthy condition (anything non-false and non-nil), which will always match.

Examples

cond do
  1 + 1 == 1 ->
    "This will never match"
  2 * 2 != 4 ->
    "Nor this"
  true ->
    "This will"
end
#=> "This will"
fn [clauses] end

Defines an anonymous function.

Examples

iex> add = fn a, b -> a + b end
iex> add.(1, 2)
3
for(args)

Comprehensions allow you to quickly build a data structure from an enumerable or a bitstring.

Let’s start with an example:

iex> for n <- [1, 2, 3, 4], do: n * 2
[2, 4, 6, 8]

A comprehension accepts many generators and filters. Enumerable generators are defined using <-:

# A list generator:
iex> for n <- [1, 2, 3, 4], do: n * 2
[2, 4, 6, 8]

# A comprehension with two generators
iex> for x <- [1, 2], y <- [2, 3], do: x*y
[2, 3, 4, 6]

Filters can also be given:

# A comprehension with a generator and a filter
iex> for n <- [1, 2, 3, 4, 5, 6], rem(n, 2) == 0, do: n
[2, 4, 6]

Note generators can also be used to filter as it removes any value that doesn’t match the pattern on the left side of <-:

iex> users = [user: "john", admin: "meg", guest: "barbara"]
iex> for {type, name} when type != :guest <- users do
...>   String.upcase(name)
...> end
["JOHN", "MEG"]

Bitstring generators are also supported and are very useful when you need to organize bitstring streams:

iex> pixels = <<213, 45, 132, 64, 76, 32, 76, 0, 0, 234, 32, 15>>
iex> for <<r::8, g::8, b::8 <- pixels >>, do: {r, g, b}
[{213, 45, 132}, {64, 76, 32}, {76, 0, 0}, {234, 32, 15}]

Variable assignments inside the comprehension, be it in generators, filters or inside the block, are not reflected outside of the comprehension.

Into

In the examples above, the result returned by the comprehension was always a list. The returned result can be configured by passing an :into option, that accepts any structure as long as it implements the Collectable protocol.

For example, we can use bitstring generators with the :into option to easily remove all spaces in a string:

iex> for <<c <- " hello world ">>, c != ?\s, into: "", do: <<c>>
"helloworld"

The IO module provides streams, that are both Enumerable and Collectable, here is an upcase echo server using comprehensions:

for line <- IO.stream(:stdio, :line), into: IO.stream(:stdio, :line) do
  String.upcase(line)
end
import(module, opts)

Imports functions and macros from other modules.

import/2 allows one to easily access functions or macros from others modules without using the qualified name.

Examples

If you are using several functions from a given module, you can import those functions and reference them as local functions, for example:

iex> import List
iex> flatten([1, [2], 3])
[1, 2, 3]

Selector

By default, Elixir imports functions and macros from the given module, except the ones starting with underscore (which are usually callbacks):

import List

A developer can filter to import only macros or functions via the only option:

import List, only: :functions
import List, only: :macros

Alternatively, Elixir allows a developer to pass pairs of name/arities to :only or :except as a fine grained control on what to import (or not):

import List, only: [flatten: 1]
import String, except: [split: 2]

Notice that calling except for a previously declared import/2 simply filters the previously imported elements. For example:

import List, only: [flatten: 1, keyfind: 4]
import List, except: [flatten: 1]

After the two import calls above, only List.keyfind/4 will be imported.

Underscore functions

By default functions starting with _ are not imported. If you really want to import a function starting with _ you must explicitly include it in the :only selector.

import File.Stream, only: [__build__: 3]

Lexical scope

It is important to notice that import/2 is lexical. This means you can import specific macros inside specific functions:

defmodule Math do
  def some_function do
    # 1) Disable "if/2" from Kernel
    import Kernel, except: [if: 2]

    # 2) Require the new "if/2" macro from MyMacros
    import MyMacros

    # 3) Use the new macro
    if do_something, it_works
  end
end

In the example above, we imported macros from MyMacros, replacing the original if/2 implementation by our own within that specific function. All other functions in that module will still be able to use the original one.

Warnings

If you import a module and you don’t use any of the imported functions or macros from this module, Elixir is going to issue a warning implying the import is not being used.

In case the import is generated automatically by a macro, Elixir won’t emit any warnings though, since the import was not explicitly defined.

Both warning behaviours could be changed by explicitly setting the :warn option to true or false.

Ambiguous function/macro names

If two modules A and B are imported and they both contain a foo function with an arity of 1, an error is only emitted if an ambiguous call to foo/1 is actually made; that is, the errors are emitted lazily, not eagerly.

quote(opts, block)

Gets the representation of any expression.

Examples

iex> quote do
...>   sum(1, 2, 3)
...> end
{:sum, [], [1, 2, 3]}

Explanation

Any Elixir code can be represented using Elixir data structures. The building block of Elixir macros is a tuple with three elements, for example:

{:sum, [], [1, 2, 3]}

The tuple above represents a function call to sum passing 1, 2 and 3 as arguments. The tuple elements are:

  • The first element of the tuple is always an atom or another tuple in the same representation.

  • The second element of the tuple represents metadata.

  • The third element of the tuple are the arguments for the function call. The third argument may be an atom, which is usually a variable (or a local call).

Options

  • :unquote - when false, disables unquoting. Useful when you have a quote inside another quote and want to control what quote is able to unquote.

  • :location - when set to :keep, keeps the current line and file from quote. Read the Stacktrace information section below for more information.

  • :generated - marks the given chunk as generated so it does not emit warnings. Currently it only works on special forms (for example, you can annotate a case but not an if).

  • :context - sets the resolution context.

  • :bind_quoted - passes a binding to the macro. Whenever a binding is given, unquote/1 is automatically disabled.

Quote literals

Besides the tuple described above, Elixir has a few literals that when quoted return themselves. They are:

:sum         #=> Atoms
1            #=> Integers
2.0          #=> Floats
[1, 2]       #=> Lists
"strings"    #=> Strings
{key, value} #=> Tuples with two elements

Quote and macros

quote/2 is commonly used with macros for code generation. As an exercise, let’s define a macro that multiplies a number by itself (squared). Note there is no reason to define such as a macro (and it would actually be seen as a bad practice), but it is simple enough that it allows us to focus on the important aspects of quotes and macros:

defmodule Math do
  defmacro squared(x) do
    quote do
      unquote(x) * unquote(x)
    end
  end
end

We can invoke it as:

import Math
IO.puts "Got #{squared(5)}"

At first, there is nothing in this example that actually reveals it is a macro. But what is happening is that, at compilation time, squared(5) becomes 5 * 5. The argument 5 is duplicated in the produced code, we can see this behaviour in practice though because our macro actually has a bug:

import Math
my_number = fn ->
  IO.puts "Returning 5"
  5
end
IO.puts "Got #{squared(my_number.())}"

The example above will print:

Returning 5
Returning 5
Got 25

Notice how “Returning 5” was printed twice, instead of just once. This is because a macro receives an expression and not a value (which is what we would expect in a regular function). This means that:

squared(my_number.())

Actually expands to:

my_number.() * my_number.()

Which invokes the function twice, explaining why we get the printed value twice! In the majority of the cases, this is actually unexpected behaviour, and that’s why one of the first things you need to keep in mind when it comes to macros is to not unquote the same value more than once.

Let’s fix our macro:

defmodule Math do
  defmacro squared(x) do
    quote do
      x = unquote(x)
      x * x
    end
  end
end

Now invoking square(my_number.()) as before will print the value just once.

In fact, this pattern is so common that most of the times you will want to use the bind_quoted option with quote/2:

defmodule Math do
  defmacro squared(x) do
    quote bind_quoted: [x: x] do
      x * x
    end
  end
end

:bind_quoted will translate to the same code as the example above. :bind_quoted can be used in many cases and is seen as good practice, not only because it helps us from running into common mistakes but also because it allows us to leverage other tools exposed by macros, such as unquote fragments discussed in some sections below.

Before we finish this brief introduction, you will notice that, even though we defined a variable x inside our quote:

quote do
  x = unquote(x)
  x * x
end

When we call:

import Math
squared(5)
x #=> ** (CompileError) undefined variable x or undefined function x/0

We can see that x did not leak to the user context. This happens because Elixir macros are hygienic, a topic we will discuss at length in the next sections as well.

Hygiene in variables

Consider the following example:

defmodule Hygiene do
  defmacro no_interference do
    quote do
      a = 1
    end
  end
end

require Hygiene

a = 10
Hygiene.no_interference
a #=> 10

In the example above, a returns 10 even if the macro is apparently setting it to 1 because variables defined in the macro do not affect the context the macro is executed in. If you want to set or get a variable in the caller’s context, you can do it with the help of the var! macro:

defmodule NoHygiene do
  defmacro interference do
    quote do
      var!(a) = 1
    end
  end
end

require NoHygiene

a = 10
NoHygiene.interference
a #=> 1

Note that you cannot even access variables defined in the same module unless you explicitly give it a context:

defmodule Hygiene do
  defmacro write do
    quote do
      a = 1
    end
  end

  defmacro read do
    quote do
      a
    end
  end
end

Hygiene.write
Hygiene.read
#=> ** (RuntimeError) undefined variable a or undefined function a/0

For such, you can explicitly pass the current module scope as argument:

defmodule ContextHygiene do
  defmacro write do
    quote do
      var!(a, ContextHygiene) = 1
    end
  end

  defmacro read do
    quote do
      var!(a, ContextHygiene)
    end
  end
end

ContextHygiene.write
ContextHygiene.read
#=> 1

Hygiene in aliases

Aliases inside quote are hygienic by default. Consider the following example:

defmodule Hygiene do
  alias Map, as: M

  defmacro no_interference do
    quote do
      M.new
    end
  end
end

require Hygiene
Hygiene.no_interference #=> %{}

Notice that, even though the alias M is not available in the context the macro is expanded, the code above works because M still expands to Map.

Similarly, even if we defined an alias with the same name before invoking a macro, it won’t affect the macro’s result:

defmodule Hygiene do
  alias Map, as: M

  defmacro no_interference do
    quote do
      M.new
    end
  end
end

require Hygiene
alias SomethingElse, as: M
Hygiene.no_interference #=> %{}

In some cases, you want to access an alias or a module defined in the caller. For such, you can use the alias! macro:

defmodule Hygiene do
  # This will expand to Elixir.Nested.hello
  defmacro no_interference do
    quote do
      Nested.hello
    end
  end

  # This will expand to Nested.hello for
  # whatever is Nested in the caller
  defmacro interference do
    quote do
      alias!(Nested).hello
    end
  end
end

defmodule Parent do
  defmodule Nested do
    def hello, do: "world"
  end

  require Hygiene
  Hygiene.no_interference
  #=> ** (UndefinedFunctionError) ...

  Hygiene.interference
  #=> "world"
end

Hygiene in imports

Similar to aliases, imports in Elixir are hygienic. Consider the following code:

defmodule Hygiene do
  defmacrop get_length do
    quote do
      length([1, 2, 3])
    end
  end

  def return_length do
    import Kernel, except: [length: 1]
    get_length
  end
end

Hygiene.return_length #=> 3

Notice how return_length returns 3 even though the length/1 function is not imported. In fact, even if return_length imported a function with the same name and arity from another module, it wouldn’t affect the function result:

def return_length do
  import String, only: [length: 1]
  get_length
end

Calling this new return_length will still return 3 as result.

Elixir is smart enough to delay the resolution to the latest moment possible. So, if you call length([1, 2, 3]) inside quote, but no length/1 function is available, it is then expanded in the caller:

defmodule Lazy do
  defmacrop get_length do
    import Kernel, except: [length: 1]

    quote do
      length("hello")
    end
  end

  def return_length do
    import Kernel, except: [length: 1]
    import String, only: [length: 1]
    get_length
  end
end

Lazy.return_length #=> 5

Stacktrace information

When defining functions via macros, developers have the option of choosing if runtime errors will be reported from the caller or from inside the quote. Let’s see an example:

# adder.ex
defmodule Adder do
  @doc "Defines a function that adds two numbers"
  defmacro defadd do
    quote location: :keep do
      def add(a, b), do: a + b
    end
  end
end

# sample.ex
defmodule Sample do
  import Adder
  defadd
end

require Sample
Sample.add(:one, :two)
#=> ** (ArithmeticError) bad argument in arithmetic expression
#=>     adder.ex:5: Sample.add/2

When using location: :keep and invalid arguments are given to Sample.add/2, the stacktrace information will point to the file and line inside the quote. Without location: :keep, the error is reported to where defadd was invoked. Note location: :keep affects only definitions inside the quote.

Binding and unquote fragments

Elixir quote/unquote mechanisms provides a functionality called unquote fragments. Unquote fragments provide an easy way to generate functions on the fly. Consider this example:

kv = [foo: 1, bar: 2]
Enum.each kv, fn {k, v} ->
  def unquote(k)(), do: unquote(v)
end

In the example above, we have generated the functions foo/0 and bar/0 dynamically. Now, imagine that, we want to convert this functionality into a macro:

defmacro defkv(kv) do
  Enum.map kv, fn {k, v} ->
    quote do
      def unquote(k)(), do: unquote(v)
    end
  end
end

We can invoke this macro as:

defkv [foo: 1, bar: 2]

However, we can’t invoke it as follows:

kv = [foo: 1, bar: 2]
defkv kv

This is because the macro is expecting its arguments to be a keyword list at compilation time. Since in the example above we are passing the representation of the variable kv, our code fails.

This is actually a common pitfall when developing macros. We are assuming a particular shape in the macro. We can work around it by unquoting the variable inside the quoted expression:

defmacro defkv(kv) do
  quote do
    Enum.each unquote(kv), fn {k, v} ->
      def unquote(k)(), do: unquote(v)
    end
  end
end

If you try to run our new macro, you will notice it won’t even compile, complaining that the variables k and v do not exist. This is because of the ambiguity: unquote(k) can either be an unquote fragment, as previously, or a regular unquote as in unquote(kv).

One solution to this problem is to disable unquoting in the macro, however, doing that would make it impossible to inject the kv representation into the tree. That’s when the :bind_quoted option comes to the rescue (again!). By using :bind_quoted, we can automatically disable unquoting while still injecting the desired variables into the tree:

defmacro defkv(kv) do
  quote bind_quoted: [kv: kv] do
    Enum.each kv, fn {k, v} ->
      def unquote(k)(), do: unquote(v)
    end
  end
end

In fact, the :bind_quoted option is recommended every time one desires to inject a value into the quote.

receive(args)

Checks if there is a message matching the given clauses in the current process mailbox.

In case there is no such message, the current process hangs until a message arrives or waits until a given timeout value.

Examples

receive do
  {:selector, i, value} when is_integer(i) ->
    value
  value when is_atom(value) ->
    value
  _ ->
    IO.puts :stderr, "Unexpected message received"
end

An optional after clause can be given in case the message was not received after the specified timeout period:

receive do
  {:selector, i, value} when is_integer(i) ->
    value
  value when is_atom(value) ->
    value
  _ ->
    IO.puts :stderr, "Unexpected message received"
after
  5000 ->
    IO.puts :stderr, "No message in 5 seconds"
end

The after clause can be specified even if there are no match clauses. The timeout value given to after can be a variable; two special values are allowed:

  • :infinity - the process should wait indefinitely for a matching message, this is the same as not using a timeout

  • 0 - if there is no matching message in the mailbox, the timeout will occur immediately

Variables handling

The receive/1 special form handles variables exactly as the case/2 special macro. For more information, check the docs for case/2.

require(module, opts)

Requires a given module to be compiled and loaded.

Examples

Notice that usually modules should not be required before usage, the only exception is if you want to use the macros from a module. In such cases, you need to explicitly require them.

Let’s suppose you created your own if/2 implementation in the module MyMacros. If you want to invoke it, you need to first explicitly require the MyMacros:

defmodule Math do
  require MyMacros
  MyMacros.if do_something, it_works
end

An attempt to call a macro that was not loaded will raise an error.

Alias shortcut

require/2 also accepts as: as an option so it automatically sets up an alias. Please check alias/2 for more information.

super(args)

Calls the overridden function when overriding it with Kernel.defoverridable/1.

See Kernel.defoverridable/1 for more information and documentation.

try(args)

Evaluates the given expressions and handles any error, exit or throw that may have happened.

Examples

try do
  do_something_that_may_fail(some_arg)
rescue
  ArgumentError ->
    IO.puts "Invalid argument given"
catch
  value ->
    IO.puts "caught #{value}"
else
  value ->
    IO.puts "Success! The result was #{value}"
after
  IO.puts "This is printed regardless if it failed or succeed"
end

The rescue clause is used to handle exceptions, while the catch clause can be used to catch thrown values. The else clause can be used to control flow based on the result of the expression. Catch, rescue and else clauses work based on pattern matching.

Note that calls inside try/1 are not tail recursive since the VM needs to keep the stacktrace in case an exception happens.

Rescue clauses

Besides relying on pattern matching, rescue clauses provides some conveniences around exceptions that allows one to rescue an exception by its name. All the following formats are valid rescue expressions:

try do
  UndefinedModule.undefined_function
rescue
  UndefinedFunctionError -> nil
end

try do
  UndefinedModule.undefined_function
rescue
  [UndefinedFunctionError] -> nil
end

# rescue and bind to x
try do
  UndefinedModule.undefined_function
rescue
  x in [UndefinedFunctionError] -> nil
end

# rescue all and bind to x
try do
  UndefinedModule.undefined_function
rescue
  x -> nil
end

Erlang errors

Erlang errors are transformed into Elixir ones during rescue:

try do
  :erlang.error(:badarg)
rescue
  ArgumentError -> :ok
end

The most common Erlang errors will be transformed into their Elixir counter-part. Those which are not will be transformed into ErlangError:

try do
  :erlang.error(:unknown)
rescue
  ErlangError -> :ok
end

In fact, ErlangError can be used to rescue any error that is not an Elixir error proper. For example, it can be used to rescue the earlier :badarg error too, prior to transformation:

try do
  :erlang.error(:badarg)
rescue
  ErlangError -> :ok
end

Catching throws and exits

The catch clause can be used to catch throws values and exits.

try do
  exit(:shutdown)
catch
  :exit, :shutdown -> IO.puts "Exited with shutdown reason"
end

try do
  throw(:sample)
catch
  :throw, :sample ->
    IO.puts "sample thrown"
end

catch values also support :error, as in Erlang, although it is commonly avoided in favor of raise/rescue control mechanisms.

After clauses

An after clause allows you to define cleanup logic that will be invoked both when the tried block of code succeeds and also when an error is raised. Note that the process will exit as usually when receiving an exit signal that causes it to exit abruptly and so the after clause is not guaranteed to be executed. Luckily, most resources in Elixir (such as open files, ETS tables, ports, sockets, etc.) are linked to or monitor the owning process and will automatically clean themselves up if that process exits.

File.write!("tmp/story.txt", "Hello, World")
try do
  do_something_with("tmp/story.txt")
after
  File.rm("tmp/story.txt")
end

Else clauses

Else clauses allow the result of the expression to be pattern matched on:

x = 2
try do
  1 / x
rescue
  ArithmeticError ->
    :infinity
else
  y when y < 1 and y > -1 ->
    :small
  _ ->
    :large
end

If an else clause is not present and no exceptions are raised, the result of the expression will be returned:

x = 1
^x =
  try do
    1 / x
  rescue
    ArithmeticError ->
      :infinity
  end

However when an else clause is present but the result of the expression does not match any of the patterns an exception will be raised. This exception will not be caught by a catch or rescue in the same try:

x = 1
try do
  try do
    1 / x
  rescue
    # The TryClauseError can not be rescued here:
    TryClauseError ->
      :error_a
  else
    0 ->
      :small
  end
rescue
  # The TryClauseError is rescued here:
  TryClauseError ->
    :error_b
end

Similarly an exception inside an else clause is not caught or rescued inside the same try:

try do
  try do
    nil
  catch
    # The exit(1) call below can not be caught here:
    :exit, _ ->
      :exit_a
  else
    _ ->
      exit(1)
  end
catch
  # The exit is caught here:
  :exit, _ ->
    :exit_b
end

This means the VM no longer needs to keep the stacktrace once inside an else clause and so tail recursion is possible when using a try with a tail call as the final call inside an else clause. The same is true for rescue and catch clauses.

Variable handling

Since an expression inside try may not have been evaluated due to an exception, any variable created inside try cannot be accessed externally. For instance:

try do
  x = 1
  do_something_that_may_fail(same_arg)
  :ok
catch
  _, _ -> :failed
end

x #=> unbound variable "x"

In the example above, x cannot be accessed since it was defined inside the try clause. A common practice to address this issue is to return the variables defined inside try:

x =
  try do
    x = 1
    do_something_that_may_fail(same_arg)
    x
  catch
    _, _ -> :failed
  end
unquote(expr)

Unquotes the given expression from inside a macro.

Examples

Imagine the situation you have a variable value and you want to inject it inside some quote. The first attempt would be:

value = 13
quote do
  sum(1, value, 3)
end

Which would then return:

{:sum, [], [1, {:value, [], quoted}, 3]}

Which is not the expected result. For this, we use unquote:

iex> value = 13
iex> quote do
...>   sum(1, unquote(value), 3)
...> end
{:sum, [], [1, 13, 3]}
unquote_splicing(expr)

Unquotes the given list expanding its arguments. Similar to unquote/1.

Examples

iex> values = [2, 3, 4]
iex> quote do
...>   sum(1, unquote_splicing(values), 5)
...> end
{:sum, [], [1, 2, 3, 4, 5]}
with(args)

Used to combine matching clauses.

Let’s start with an example:

iex> opts = %{width: 10, height: 15}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...>      {:ok, height} <- Map.fetch(opts, :height),
...>   do: {:ok, width * height}
{:ok, 150}

If all clauses match, the do block is executed, returning its result. Otherwise the chain is aborted and the non-matched value is returned:

iex> opts = %{width: 10}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...>      {:ok, height} <- Map.fetch(opts, :height),
...>   do: {:ok, width * height}
:error

Guards can be used in patterns as well:

iex> users = %{"melany" => "guest", "bob" => :admin}
iex> with {:ok, role} when not is_binary(role) <- Map.fetch(users, "bob"),
...>   do: {:ok, to_string(role)}
{:ok, "admin"}

As in for/1, variables bound inside with/1 won’t leak; “bare expressions” may also be inserted between the clauses:

iex> width = nil
iex> opts = %{width: 10, height: 15}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...>      double_width = width * 2,
...>      {:ok, height} <- Map.fetch(opts, :height),
...>   do: {:ok, double_width * height}
{:ok, 300}
iex> width
nil

An else option can be given to modify what is being returned from with in the case of a failed match:

iex> opts = %{width: 10}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...>      {:ok, height} <- Map.fetch(opts, :height) do
...>   {:ok, width * height}
...> else
...>   :error ->
...>     {:error, :wrong_data}
...> end
{:error, :wrong_data}

If there is no matching else condition, then a WithClauseError exception is raised.

{args}

Creates a tuple.

Only two item tuples are considered literals in Elixir. Therefore all other tuples are represented in the AST as a call to the special form :{}.

Conveniences for manipulating tuples can be found in the Tuple module. Some functions for working with tuples are also available in Kernel, namely Kernel.elem/2, Kernel.put_elem/3 and Kernel.tuple_size/1.

Examples

iex> {1, 2, 3}
{1, 2, 3}

iex> quote do
...>   {1, 2, 3}
...> end
{:{}, [], [1, 2, 3]}