Kernel.SpecialForms — Elixir v1.16.3

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# View Source Kernel.SpecialForms (Elixir v1.16.3)

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

The `Kernel.SpecialForms` module consists solely of macros that can be invoked anywhere in Elixir code without the use of the `Kernel.SpecialForms.` prefix. This is possible because they all have been automatically imported, in the same fashion as the functions and macros from the `Kernel` module.

These building blocks are defined in this module. Some of these special forms are lexical (such as `alias/2` and `case/2`). The macros `{}/1` and `<<>>/1` are also special forms used to define tuple and binary data structures respectively.

This module also documents macros that return information about Elixir's compilation environment, such as (`__ENV__/0`, `__MODULE__/0`, `__DIR__/0`, `__STACKTRACE__/0`, and `__CALLER__/0`).

Additionally, it documents two special forms, `__block__/1` and `__aliases__/1`, 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

## Functions

%struct{}

Matches on or builds a struct.

%{}

Creates a map.

&expr

Capture operator. Captures or creates an anonymous function.

left . right

Dot operator. Defines a remote call, a call to an anonymous function, or an alias.

__aliases__(args)

Internal special form to hold aliases information.

__block__(args)

Internal special form for block expressions.

__CALLER__

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

__DIR__

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

__ENV__

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

__MODULE__

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

__STACKTRACE__

Returns the stacktrace for the currently handled exception.

left :: right

Type operator. Used by types and bitstrings to specify types.

<<args>>

Defines a new bitstring.

left = right

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

alias(module, opts)

`alias/2` is used to set up aliases, often useful with modules' names.

case(condition, clauses)

Matches the given expression against the given clauses.

cond(clauses)

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

fn(clauses)

Defines an anonymous function.

for(args)

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

import(module, opts)

Imports functions and macros from other modules.

quote(opts, block)

Gets the representation of any expression.

receive(args)

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

require(module, opts)

Requires a module in order to use its macros.

super(args)

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

try(args)

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

unquote(expr)

Unquotes the given expression inside a quoted expression.

unquote_splicing(expr)

Unquotes the given list expanding its arguments.

with(args)

Combine matching clauses.

^var

Pin operator. Accesses an already bound variable in match clauses.

{args}

Creates a tuple.

# Functions

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# %struct{}

View Source (macro)

Matches on or builds 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}

The struct fields can be given when building the struct:

    %User{age: 31}
    #=> %{__struct__: User, name: "john", age: 31}

Or also on pattern matching to extract values out:

    %User{age: age} = user

An update operation specific for structs is also available:

    %User{user | age: 28}

The advantage of structs is that they validate 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"}

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.

## Pattern matching on struct names

Besides allowing pattern matching on struct fields, such as:

    %User{age: age} = user

Structs also allow pattern matching on the struct name:

    %struct_name{} = user
    struct_name #=> User

You can also assign the struct name to `_` when you want to check if something is a struct but you are not interested in its name:

    %_{} = user

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# %{}

View Source (macro)

Creates a map.

See the `Map` module for more information about maps, their syntax, and ways to access and manipulate them.

## AST representation

Regardless of whether `=>` or the keyword syntax is used, key-value pairs in 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}]}

Link to this macro

# &expr

View Source (macro)

Capture operator. 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

See also `Function.capture/3`.

## 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`.

We can partially apply a remote function with placeholder:

    iex> take_five = &Enum.take(&1, 5)
    iex> take_five.(1..10)
    [1, 2, 3, 4, 5]

Another example while using an imported or local function:

    iex> first_elem = &elem(&1, 0)
    iex> first_elem.({0, 1})
    0

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, 3])
    [1, 2, 3]

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)

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# left . right

View Source (macro)

Dot operator. Defines a remote call, a call to an anonymous function, 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` module, passing `"FOO"` as argument.

The dot may be used to invoke anonymous functions too:

    iex> (fn n -> n end).(7)
    7

in which case there is a function on the left hand side.

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 with an uppercase letter, 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."+"(1, 2)
    3

Wrapping the function name in single- or double-quotes is always a remote call. Therefore `Kernel."Foo"` will attempt to call the function "Foo" and not return the alias `Kernel.Foo`. This is done by design as module names are more strict than function names.

When the dot is used to invoke an anonymous function there is only one operand, but it is still written using a postfix notation:

    iex> negate = fn n -> -n end
    iex> negate.(7)
    -7

## 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"]}

Note that 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 list of arguments as third. In this case, the arguments are the alias `String` and the atom `:downcase`. The second argument in a remote call is always an atom.

In the case of calls to anonymous functions, the inner tuple with the dot special form has only one argument, reflecting the fact that the operator is unary:

    iex> quote do
    ...>   negate.(0)
    ...> end
    {{:., [], [{:negate, [], __MODULE__}]}, [], [0]}

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__/1` 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."FUNCTION_NAME"`, `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]}

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# __aliases__(args)

View Source (macro)

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 hold 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.

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# __block__(args)

View Source (macro)

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]}

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# __CALLER__

View Source (macro)

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.

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# __DIR__

View Source (macro)

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.

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# __ENV__

View Source (macro)

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.

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# __MODULE__

View Source (macro)

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

Although the module can be accessed in the `__ENV__/0`, this macro is a convenient shortcut.

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# __STACKTRACE__

View Source (since 1.7.0) (macro)

Returns the stacktrace for the currently handled exception.

It is available only in the `catch` and `rescue` clauses of `try/1` expressions.

To retrieve the stacktrace of the current process, use `Process.info(self(), :current_stacktrace)` instead.

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# left :: right

View Source (macro)

Type operator. 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 Typespecs page and `<<>>/1` for more information on typespecs and bitstrings respectively.

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# <<args>>

View Source (macro)

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 which expands to integers:

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

You can use one of `utf8` (the default), `utf16`, and `utf32` to control how the string is encoded:

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

Which is equivalent to writing:

    iex> <<?f::utf16, ?o::utf16, ?o::utf16>>
    <<0, 102, 0, 111, 0, 111>>

At runtime, binaries need to be explicitly tagged as `binary`:

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

Otherwise we get an `ArgumentError` when constructing the binary:

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

## 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`).

Type | Default Unit
--- | ---
`integer` | 1 bit
`float` | 1 bit
`binary` | 8 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 16, 32, or 64, corresponding to IEEE 754 binary16, 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"}

The size can be a variable or any valid guard expression:

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

The size can access prior variables defined in the binary itself:

    iex> <<name_size::size(8), name::binary-size(name_size), " the ", species::binary>> = <<5, "Frank the Walrus">>
    iex> {name, species}
    {"Frank", "Walrus"}

However, it cannot access variables defined in the match outside of the binary/bitstring:

    {name_size, <<name::binary-size(name_size), _rest::binary>>} = {5, <<"Frank the Walrus">>}
    ** (CompileError): undefined variable "name_size" in bitstring segment

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.

Modifier | Relevant Type(s)
--- | ---
`signed` | `integer`
`unsigned` (default) | `integer`
`little` | `integer`, `float`, `utf16`, `utf32`
`big` (default) | `integer`, `float`, `utf16`, `utf32`
`native` | `integer`, `float`, `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 ImageType do
      @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 the "Constructing and matching binaries" chapter of the Erlang's Efficiency Guide.

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# left = right

View Source (macro)

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

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# alias(module, opts)

View Source (macro)

`alias/2` is used to set up aliases, often useful with modules' names.

## Examples

`alias/2` can be used to set up 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

Note 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

We can also alias multiple modules in one line:

    alias Foo.{Bar, Baz, Biz}

Is the same as:

    alias Foo.Bar
    alias Foo.Baz
    alias Foo.Biz

## 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`.

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# case(condition, clauses)

View Source (macro)

Matches the given expression against the given clauses.

## Examples

    case File.read(file) do
      {:ok, contents} when is_binary(contents) ->
        String.split(contents, "\n")

      {:error, _reason} ->
        Logger.warning "could not find #{file}, assuming empty..."
        []
    end

In the example above, we match the result of `File.read/1` 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)"

## Variable handling

Note that variables bound in a clause do not leak to the outer context:

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

    value
    #=> unbound variable value

Variables in the outer context cannot be overridden either:

    value = 7

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

    value
    #=> 7

In the example above, `value` is going to be `7` regardless of the value of `lucky?`. The variable `value` bound in the clause and the variable `value` bound in the outer context are two entirely separate variables.

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"

## Using guards to match against multiple values

While it is not possible to match against multiple patterns in a single clause, it's possible to match against multiple values by using guards:

    case data do
      value when value in [:one, :two] ->
        "#{value} has been matched"

      :three ->
        "three has been matched"
    end

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# cond(clauses)

View Source (macro)

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"

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# fn(clauses)

View Source (macro)

Defines an anonymous function.

See `Function` for more information.

## Examples

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

Anonymous functions can also have multiple clauses. All clauses should expect the same number of arguments:

    iex> negate = fn
    ...>   true -> false
    ...>   false -> true
    ...> end
    iex> negate.(false)
    true

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# for(args)

View Source (macro)

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. `for` uses the `<-` operator to extract values from the enumerable on its right side and match them against the pattern on the left. We call them generators:

    # 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]

Filters must evaluate to truthy values (everything but `nil` and `false`). If a filter is falsy, then the current value is discarded.

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.

Variable assignments inside filters must still return a truthy value, otherwise values are discarded. Let's see an example. Imagine you have a keyword list where the key is a programming language and the value is its direct parent. Then let's try to compute the grandparent of each language. You could try this:

    iex> languages = [elixir: :erlang, erlang: :prolog, prolog: nil]
    iex> for {language, parent} <- languages, grandparent = languages[parent], do: {language, grandparent}
    [elixir: :prolog]

Given the grandparents of Erlang and Prolog were nil, those values were filtered out. If you don't want this behaviour, a simple option is to move the filter inside the do-block:

    iex> languages = [elixir: :erlang, erlang: :prolog, prolog: nil]
    iex> for {language, parent} <- languages do
    ...>   grandparent = languages[parent]
    ...>   {language, grandparent}
    ...> end
    [elixir: :prolog, erlang: nil, prolog: nil]

However, such option is not always available, as you may have further filters. An alternative is to convert the filter into a generator by wrapping the right side of `=` in a list:

    iex> languages = [elixir: :erlang, erlang: :prolog, prolog: nil]
    iex> for {language, parent} <- languages, grandparent <- [languages[parent]], do: {language, grandparent}
    [elixir: :prolog, erlang: nil, prolog: nil]

## The `:into` and `:uniq` options

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(), into: IO.stream() do
      String.upcase(line)
    end

Similarly, `uniq: true` can also be given to comprehensions to guarantee the results are only added to the collection if they were not returned before. For example:

    iex> for x <- [1, 1, 2, 3], uniq: true, do: x * 2
    [2, 4, 6]

    iex> for <<x <- "abcabc">>, uniq: true, into: "", do: <<x - 32>>
    "ABC"

## The `:reduce` option

While the `:into` option allows us to customize the comprehension behaviour to a given data type, such as putting all of the values inside a map or inside a binary, it is not always enough.

For example, imagine that you have a binary with letters where you want to count how many times each lowercase letter happens, ignoring all uppercase ones. For instance, for the string `"AbCabCABc"`, we want to return the map `%{"a" => 1, "b" => 2, "c" => 1}`.

If we were to use `:into`, we would need a data type that computes the frequency of each element it holds. While there is no such data type in Elixir, you could implement one yourself.

A simpler option would be to use comprehensions for the mapping and filtering of letters, and then we invoke `Enum.reduce/3` to build a map, for example:

    iex> letters = for <<x <- "AbCabCABc">>, x in ?a..?z, do: <<x>>
    iex> Enum.reduce(letters, %{}, fn x, acc -> Map.update(acc, x, 1, & &1 + 1) end)
    %{"a" => 1, "b" => 2, "c" => 1}

While the above is straight-forward, it has the downside of traversing the data at least twice. If you are expecting long strings as inputs, this can be quite expensive.

Luckily, comprehensions also support the `:reduce` option, which would allow us to fuse both steps above into a single step:

    iex> for <<x <- "AbCabCABc">>, x in ?a..?z, reduce: %{} do
    ...>   acc -> Map.update(acc, <<x>>, 1, & &1 + 1)
    ...> end
    %{"a" => 1, "b" => 2, "c" => 1}

When the `:reduce` key is given, its value is used as the initial accumulator and the `do` block must be changed to use `->` clauses, where the left side of `->` receives the accumulated value of the previous iteration and the expression on the right side must return the new accumulator value. Once there are no more elements, the final accumulated value is returned. If there are no elements at all, then the initial accumulator value is returned.

Link to this macro

# import(module, opts)

View Source (macro)

Imports functions and macros from other modules.

`import/2` allows one to easily access functions or macros from other 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 an underscore (which are usually callbacks):

    import List

A developer can filter to import only functions, macros, or sigils (which can be functions or macros) via the `:only` option:

    import List, only: :functions
    import List, only: :macros
    import Kernel, only: :sigils

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]

Importing the same module again will erase the previous imports, except when the `except` option is used, which is always exclusive on a previously declared `import/2`. If there is no previous import, then it applies to all functions and macros in the module. 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 note 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.

Link to this macro

# quote(opts, block)

View Source (macro)

Gets the representation of any expression.

## Examples

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

## Elixir's AST (Abstract Syntax Tree)

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).

Besides the tuple described above, Elixir has a few literals that are also part of its AST. Those literals return themselves when quoted. They are:

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

Any other value, such as a map or a four-element tuple, must be escaped (`Macro.escape/1`) before being introduced into an AST.

## Options

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

- `:context` - sets the resolution context.

- `:generated` - marks the given chunk as generated so it does not emit warnings. It is also useful to avoid dialyzer reporting errors when macros generate unused clauses.

- `:file` - sets the quoted expressions to have the given file.

- `:line` - sets the quoted expressions to have the given line.

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

- `:unquote` - when `false`, disables unquoting. This means any `unquote` call will be kept as is in the AST, instead of replaced by the `unquote` arguments. For example:

      iex> quote do
      ...>   unquote("hello")
      ...> end
      "hello"

      iex> quote unquote: false do
      ...>   unquote("hello")
      ...> end
      {:unquote, [], ["hello"]}

## 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). In practice, there is no reason to define such 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 `squared(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 prevent 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"

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

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

    require Hygiene
    Hygiene.write()
    Hygiene.read()
    ** (CompileError) undefined variable "a" (context Hygiene)

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

    require Hygiene
    ContextHygiene.write()
    ContextHygiene.read()
    #=> 1

The contexts of a variable is identified by the third element of the tuple. The default context is `nil` and `quote` assigns another context to all variables within:

    quote(do: var)
    #=> {:var, [], Elixir}

In case of variables returned by macros, there may also be a `:counter` key in the metadata, which is used to further refine its contexts and guarantee isolation between macro invocations as seen in the previous example.

## 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()
    #=> %{}

Note 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 `Hygiene.return_length/0` returns `3` even though the `Kernel.length/1` function is not imported. In fact, even if `return_length/0` 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/0` will still return `3` as result.

Elixir is smart enough to delay the resolution to the latest possible moment. 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. `location: :keep` affects only definitions inside the quote.

> #### `location: :keep` and unquote

> Do not use `location: :keep` if the function definition also `unquote`s some of the macro arguments. If you do so, Elixir will store the file definition of the current location but the unquoted arguments may contain line information of the macro caller, leading to erroneous stacktraces.

## Binding and unquote fragments

Elixir quote/unquote mechanisms provide 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.

Link to this macro

# receive(args)

View Source (macro)

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, number, name} when is_integer(number) ->
        name
      name when is_atom(name) ->
        name
      _ ->
        IO.puts(:stderr, "Unexpected message received")
    end

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

    receive do
      {:selector, number, name} when is_integer(number) ->
        name
      name when is_atom(name) ->
        name
      _ ->
        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 any expression evaluating to one of the allowed values:

- `:infinity` - the process should wait indefinitely for a matching message, this is the same as not using the after clause

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

- positive integer smaller than or equal to `4_294_967_295` (`0xFFFFFFFF` in hexadecimal notation) - it should be possible to represent the timeout value as an unsigned 32-bit integer.

## Variable handling

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

Link to this macro

# require(module, opts)

View Source (macro)

Requires a module in order to use its macros.

## Examples

Public functions in modules are globally available, but in order to use macros, you need to opt-in by requiring the module they are defined in.

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.

Link to this macro

# super(args)

View Source (macro)

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

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

Link to this macro

# try(args)

View Source (macro)

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 #{inspect(value)}")
    else
      value ->
        IO.puts("Success! The result was #{inspect(value)}")
    after
      IO.puts("This is printed regardless if it failed or succeeded")
    end

The `rescue` clause is used to handle exceptions while the `catch` clause can be used to catch thrown values and exits. 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 (similar to the `case` special form).

Calls inside `try/1` are not tail recursive since the VM needs to keep the stacktrace in case an exception happens. To retrieve the stacktrace, access `__STACKTRACE__/0` inside the `rescue` or `catch` clause.

## `rescue` clauses

Besides relying on pattern matching, `rescue` clauses provide some conveniences around exceptions that allow one to rescue an exception by its name. All the following formats are valid patterns in `rescue` clauses:

    # Rescue a single exception without binding the exception
    # to a variable
    try do
      UndefinedModule.undefined_function
    rescue
      UndefinedFunctionError -> nil
    end

    # Rescue any of the given exception without binding
    try do
      UndefinedModule.undefined_function
    rescue
      [UndefinedFunctionError, ArgumentError] -> nil
    end

    # Rescue and bind the exception to the variable "x"
    try do
      UndefinedModule.undefined_function
    rescue
      x in [UndefinedFunctionError] -> nil
    end

    # Rescue all kinds of exceptions and bind the rescued exception
    # to the variable "x"
    try do
      UndefinedModule.undefined_function
    rescue
      x -> nil
    end

### Erlang errors

Erlang errors are transformed into Elixir ones when rescuing:

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

The most common Erlang errors will be transformed into their Elixir counterpart. Those which are not will be transformed into the more generic `ErlangError`:

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

In fact, `ErlangError` can be used to rescue any error that is not a proper Elixir error. 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
    #=> :ok

## `catch` clauses

The `catch` clause can be used to catch thrown values, exits, and errors.

### Catching thrown values

`catch` can be used to catch values thrown by `Kernel.throw/1`:

    try do
      throw(:some_value)
    catch
      thrown_value ->
        IO.puts("A value was thrown: #{inspect(thrown_value)}")
    end

### Catching values of any kind

The `catch` clause also supports catching exits and errors. To do that, it allows matching on both the kind of the caught value as well as the value itself:

    try do
      exit(:shutdown)
    catch
      :exit, value ->
        IO.puts("Exited with value #{inspect(value)}")
    end

    try do
      exit(:shutdown)
    catch
      kind, value when kind in [:exit, :throw] ->
        IO.puts("Caught exit or throw with value #{inspect(value)}")
    end

The `catch` clause also supports `:error` alongside `:exit` and `:throw` as in Erlang, although this is commonly avoided in favor of `raise`/`rescue` control mechanisms. One reason for this is that when catching `:error`, the error is not automatically transformed into an Elixir error:

    try do
      :erlang.error(:badarg)
    catch
      :error, :badarg -> :ok
    end
    #=> :ok

## `after` clauses

An `after` clause allows you to define cleanup logic that will be invoked both when the block of code passed to `try/1` succeeds and also when an error is raised. Note that the process will exit as usual 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, and so on) 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

Although `after` clauses are invoked whether or not there was an error, they do not modify the return value. All of the following examples return `:return_me`:

    try do
      :return_me
    after
      IO.puts("I will be printed")
      :not_returned
    end

    try do
      raise "boom"
    rescue
      _ -> :return_me
    after
      IO.puts("I will be printed")
      :not_returned
    end

## `else` clauses

`else` clauses allow the result of the body passed to `try/1` 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 then 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 cannot 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.

Only the result of the tried expression falls down to the `else` clause. If the `try` ends up in the `rescue` or `catch` clauses, their result will not fall down to `else`:

    try do
      throw(:catch_this)
    catch
      :throw, :catch_this ->
        :it_was_caught
    else
      # :it_was_caught will not fall down to this "else" clause.
      other ->
        {:else, other}
    end

## 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

Link to this macro

# unquote(expr)

View Source (macro)

Unquotes the given expression inside a quoted expression.

This function expects a valid Elixir AST, also known as quoted expression, as argument. If you would like to `unquote` any value, such as a map or a four-element tuple, you should call `Macro.escape/1` before unquoting.

## Examples

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

    value =
      quote do
        13
      end

    quote do
      sum(1, value, 3)
    end

Which the argument for the `:sum` function call is not the expected result:

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

For this, we use `unquote`:

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

If you want to unquote a value that is not a quoted expression, such as a map, you need to call `Macro.escape/1` before:

    iex> value = %{foo: :bar}
    iex> quote do
    ...>   process_map(unquote(Macro.escape(value)))
    ...> end
    {:process_map, [], [{:%{}, [], [foo: :bar]}]}

If you forget to escape it, Elixir will raise an error when compiling the code.

Link to this macro

# unquote_splicing(expr)

View Source (macro)

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]}

Link to this macro

# with(args)

View Source (macro)

Combine matching clauses.

One of the ways to understand with is to show which code patterns it improves. Imagine you have a map where the fields `width` and `height` are optional and you want to compute its area, as `{:ok, area}` or return `:error`. We could implement this function as:

    def area(opts) do
      case Map.fetch(opts, :width) do
        {:ok, width} ->
          case Map.fetch(opts, :height) do
            {:ok, height} -> {:ok, width * height}
            :error -> :error
          end

        :error ->
          :error
      end
    end

when called as `area(%{width: 10, height: 15})`, it should return `{:ok, 150}`. If any of the fields are missing, it returns `:error`.

While the code above works, it is quite verbose. Using `with`, we could rewrite it as:

    def area(opts) do
      with {:ok, width} <- Map.fetch(opts, :width),
           {:ok, height} <- Map.fetch(opts, :height) do
        {:ok, width * height}
      end
    end

Instead of defining nested `case`s with clauses, we use `with` alongside the `PATTERN <- EXPRESSION` operator to match expressions on its right side against the pattern on the left. Consider `<-` as a sibling to `=`, except that, while `=` raises in case of not matches, `<-` will simply abort the `with` chain and return the non-matched value.

Let's give it a try on IEx:

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

As in `for/1`, variables bound inside `with/1` won't be accessible outside of `with/1`.

Expressions without `<-` may also be used in clauses. For instance, you can perform regular matches with the `=` operator:

    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}
    ...> end
    {:ok, 300}
    iex> width
    nil

The behaviour of any expression in a clause is the same as if it was written outside of `with`. For example, `=` will raise a `MatchError` instead of returning the non-matched value:

    with :foo = :bar, do: :ok
    ** (MatchError) no match of right hand side value: :bar

As with any other function or macro call in Elixir, explicit parens can also be used around the arguments before the `do`-`end` block:

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

The choice between parens and no parens is a matter of preference.

## Else clauses

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}
    ...>
    ...>   _other_error ->
    ...>     :unexpected_error
    ...> end
    {:error, :wrong_data}

The `else` block works like a `case` clause: it can have multiple clauses, and the first match will be used. Variables bound inside `with` (such as `width` in this example) are not available in the `else` block.

If an `else` block is used and there are no matching clauses, a `WithClauseError` exception is raised.

### Beware!

Keep in mind that, one of potential drawback of `with` is that all failure clauses are flattened into a single `else` block. For example, take this code that checks if a given path points to an Elixir file and that it exists before creating a backup copy:

    with ".ex" <- Path.extname(path),
         true <- File.exists?(path) do
      backup_path = path <> ".backup"
      File.cp!(path, backup_path)
      {:ok, backup_path}
    else
      binary when is_binary(binary) ->
        {:error, :invalid_extension}

      false ->
        {:error, :missing_file}
    end

Note how we are having to reconstruct the result types of `Path.extname/1` and `File.exists?/1` to build error messages. In this case, it is better to refactor the code so each `<-` already return the desired format in case of errors, like this:

    with :ok <- validate_extension(path),
         :ok <- validate_exists(path) do
      backup_path = path <> ".backup"
      File.cp!(path, backup_path)
      {:ok, backup_path}
    end

    defp validate_extension(path) do
      if Path.extname(path) == ".ex", do: :ok, else: {:error, :invalid_extension}
    end

    defp validate_exists(path) do
      if File.exists?(path), do: :ok, else: {:error, :missing_file}
    end

Note how the code above is better organized and clearer once we make sure each `<-` in `with` returns a normalized format.

Link to this macro

# ^var

View Source (macro)

Pin operator. Accesses an already bound variable in match clauses.

## 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

Link to this macro

# {args}

View Source (macro)

Creates a tuple.

More information about the tuple data type and about functions to manipulate tuples can be found in the `Tuple` module; some functions for working with tuples are also available in `Kernel` (such as `Kernel.elem/2` or `Kernel.tuple_size/1`).

## AST representation

Only two-element tuples are considered literals in Elixir and return themselves when quoted. Therefore, all other tuples are represented in the AST as calls to the `:{}` special form.

    iex> quote do
    ...>   {1, 2}
    ...> end
    {1, 2}

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