Boolean not.

Receives any argument (not just booleans) and returns true if the argument is false or nil; returns false otherwise.

Not allowed in guard clauses.

Examples

iex> !Enum.empty?([])
false

iex> !List.first([])
true

Returns true if the two items are not equal.

This operator considers 1 and 1.0 to be equal. For match comparison, use !== instead.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 != 2
true

iex> 1 != 1.0
false

Returns true if the two items do not match.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 !== 2
true

iex> 1 !== 1.0
true

Provides a short-circuit operator that evaluates and returns the second expression only if the first one evaluates to true (i.e., it is neither nil nor false). Returns the first expression otherwise.

Not allowed in guard clauses.

Examples

iex> Enum.empty?([]) && Enum.empty?([])
true

iex> List.first([]) && true
nil

iex> Enum.empty?([]) && List.first([1])
1

iex> false && throw(:bad)
false

Note that, unlike and/2, this operator accepts any expression as the first argument, not only booleans.

Arithmetic multiplication.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 * 2
2

Arithmetic unary plus.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> +1
1

Arithmetic addition.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 + 2
3

Concatenates two lists.

The complexity of a ++ b is proportional to length(a), so avoid repeatedly appending to lists of arbitrary length, e.g. list ++ [item].

Instead, consider prepending via [item | rest] and then reversing.

Inlined by the compiler.

Examples

iex> [1] ++ [2, 3]
[1, 2, 3]

iex> 'foo' ++ 'bar'
'foobar'

Arithmetic unary minus.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> -2
-2

Arithmetic subtraction.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 - 2
-1

Removes the first occurrence of an item on the left list for each item on the right.

The complexity of a -- b is proportional to length(a) * length(b), meaning that it will be very slow if both a and b are long lists. In such cases, consider converting each list to a MapSet and using MapSet.difference/2.

Inlined by the compiler.

Examples

iex> [1, 2, 3] -- [1, 2]
[3]

iex> [1, 2, 3, 2, 1] -- [1, 2, 2]
[3, 1]

Returns a range with the specified start and end.

Both ends are included.

Examples

iex> 0 in 1..3
false

iex> 1 in 1..3
true

iex> 2 in 1..3
true

iex> 3 in 1..3
true

Arithmetic division.

The result is always a float. Use div/2 and rem/2 if you want an integer division or the remainder.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 / 2
0.5

iex> 2 / 1
2.0

Returns true if left is less than right.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 < 2
true

Returns true if left is less than or equal to right.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 <= 2
true

Concatenates two binaries.

Examples

iex> "foo" <> "bar"
"foobar"

The <> operator can also be used in pattern matching (and guard clauses) as long as the first part is a literal binary:

iex> "foo" <> x = "foobar"
iex> x
"bar"

x <> "bar" = "foobar" would have resulted in a CompileError exception.

Returns true if the two items are equal.

This operator considers 1 and 1.0 to be equal. For match semantics, use === instead.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 == 2
false

iex> 1 == 1.0
true

Returns true if the two items are match.

This operator gives the same semantics as the one existing in pattern matching, i.e., 1 and 1.0 are equal, but they do not match.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 === 2
false

iex> 1 === 1.0
false

Matches the term on the left against the regular expression or string on the right. Returns true if left matches right (if it’s a regular expression) or contains right (if it’s a string).

Examples

iex> "abcd" =~ ~r/c(d)/
true

iex> "abcd" =~ ~r/e/
false

iex> "abcd" =~ "bc"
true

iex> "abcd" =~ "ad"
false

iex> "abcd" =~ ""
true

Returns true if left is more than right.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 > 2
false

Returns true if left is more than or equal to right.

All terms in Elixir can be compared with each other.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> 1 >= 2
false

Reads and writes attributes of the current module.

The canonical example for attributes is annotating that a module implements the OTP behaviour called gen_server:

defmodule MyServer do
  @behaviour :gen_server
  # ... callbacks ...
end

By default Elixir supports all the module attributes supported by Erlang, but custom attributes can be used as well:

defmodule MyServer do
  @my_data 13
  IO.inspect @my_data #=> 13
end

Unlike Erlang, such attributes are not stored in the module by default since it is common in Elixir to use custom attributes to store temporary data that will be available at compile-time. Custom attributes may be configured to behave closer to Erlang by using Module.register_attribute/3.

Finally, notice that attributes can also be read inside functions:

defmodule MyServer do
  @my_data 11
  def first_data, do: @my_data
  @my_data 13
  def second_data, do: @my_data
end

MyServer.first_data #=> 11
MyServer.second_data #=> 13

It is important to note that reading an attribute takes a snapshot of its current value. In other words, the value is read at compilation time and not at runtime. Check the Module module for other functions to manipulate module attributes.

Returns an integer or float which is the arithmetical absolute value of number.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> abs(-3.33)
3.33

iex> abs(-3)
3

When used inside quoting, marks that the given alias should not be hygienized. This means the alias will be expanded when the macro is expanded.

Check Kernel.SpecialForms.quote/2 for more information.

Boolean and.

If the first argument is false, false is returned; otherwise, the second argument is returned.

Requires only the first argument to be a boolean since it short-circuits. If the first argument is not a boolean, an ArgumentError exception is raised.

Allowed in guard tests.

Examples

iex> true and false
false
iex> true and "yay!"
"yay!"

Invokes the given fun with the list of arguments args.

Inlined by the compiler.

Examples

iex> apply(fn x -> x * 2 end, [2])
4

Invokes the given fun from module with the list of arguments args.

Inlined by the compiler.

Examples

iex> apply(Enum, :reverse, [[1, 2, 3]])
[3, 2, 1]

Extracts the part of the binary starting at start with length length. Binaries are zero-indexed.

If start or length reference in any way outside the binary, an ArgumentError exception is raised.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> binary_part("foo", 1, 2)
"oo"

A negative length can be used to extract bytes that come before the byte at start:

iex> binary_part("Hello", 5, -3)
"llo"

Returns the binding for the given context as a keyword list.

In the returned result, keys are variable names and values are the corresponding variable values.

If the given context is nil (by default it is), the binding for the current context is returned.

Examples

iex> x = 1
iex> binding()
[x: 1]
iex> x = 2
iex> binding()
[x: 2]

iex> binding(:foo)
[]
iex> var!(x, :foo) = 1
1
iex> binding(:foo)
[x: 1]

Returns an integer which is the size in bits of bitstring.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> bit_size(<<433::16, 3::3>>)
19

iex> bit_size(<<1, 2, 3>>)
24

Returns the number of bytes needed to contain bitstring.

That is, if the number of bits in bitstring is not divisible by 8, the resulting number of bytes will be rounded up (by excess). This operation happens in constant time.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> byte_size(<<433::16, 3::3>>)
3

iex> byte_size(<<1, 2, 3>>)
3

Defines a function with the given name and body.

Examples

defmodule Foo do
  def bar, do: :baz
end

Foo.bar #=> :baz

A function that expects arguments can be defined as follows:

defmodule Foo do
  def sum(a, b) do
    a + b
  end
end

In the example above, a sum/2 function is defined; this function receives two arguments and returns their sum.

Function and variable names

Function and variable names have the following syntax: A lowercase ASCII letter or an underscore, followed by any number of lowercase or uppercase ASCII letters, numbers, or underscores. Optionally they can end in either an exclamation mark or a question mark.

For variables, any identifier starting with an underscore should indicate an unused variable. For example:

def foo(bar) do
  []
end
#=> warning: variable bar is unused

def foo(_bar) do
  []
end
#=> no warning

def foo(_bar) do
  _bar
end
#=> warning: the underscored variable "_bar" is used after being set

Defines a function that delegates to another module.

Functions defined with defdelegate/2 are public and can be invoked from outside the module they’re defined in (like if they were defined using def/2). When the desire is to delegate as private functions, import/2 should be used.

Delegation only works with functions; delegating macros is not supported.

Options

• :to - the expression to delegate to. Any expression is allowed and its results will be evaluated at runtime. Usually evaluates to the name of a module.

• :as - the function to call on the target given in :to. This parameter is optional and defaults to the name being delegated (funs).

• :append_first - if true, when delegated, the first argument passed to the delegated function will be relocated to the end of the arguments when dispatched to the target.

  The motivation behind this is because Elixir normalizes the “handle” as the first argument while some Erlang modules expect it as the last argument.

Examples

defmodule MyList do
  defdelegate reverse(list), to: :lists
  defdelegate other_reverse(list), to: :lists, as: :reverse
  defdelegate [reverse(list), map(list, callback)], to: :lists, append_first: true
end

MyList.reverse([1, 2, 3])
#=> [3, 2, 1]

MyList.other_reverse([1, 2, 3])
#=> [3, 2, 1]

MyList.map([1, 2, 3], &(&1 * 2))
#=> [2, 4, 6]

Defines an exception.

Exceptions are structs backed by a module that implements the Exception behaviour. The Exception behaviour requires two functions to be implemented:

• exception/1 - receives the arguments given to raise/2 and returns the exception struct. The default implementation accepts either a set of keyword arguments that is merged into the struct or a string to be used as the exception’s message.

• message/1 - receives the exception struct and must return its message. Most commonly exceptions have a message field which by default is accessed by this function. However, if an exception does not have a message field, this function must be explicitly implemented.

Since exceptions are structs, the API supported by defstruct/1 is also available in defexception/1.

Raising exceptions

The most common way to raise an exception is via raise/2:

defmodule MyAppError do
  defexception [:message]
end

value = [:hello]

raise MyAppError,
  message: "did not get what was expected, got: #{inspect value}"

In many cases it is more convenient to pass the expected value to raise/2 and generate the message in the exception/1 callback:

defmodule MyAppError do
  defexception [:message]

  def exception(value) do
    msg = "did not get what was expected, got: #{inspect value}"
    %MyAppError{message: msg}
  end
end

raise MyAppError, value

The example above shows the preferred strategy for customizing exception messages.

Defines an implementation for the given protocol.

See defprotocol/2 for more information and examples on protocols.

Inside an implementation, the name of the protocol can be accessed via @protocol and the current target as @for.

Defines a macro with the given name and body.

Examples

defmodule MyLogic do
  defmacro unless(expr, opts) do
    quote do
      if !unquote(expr), unquote(opts)
    end
  end
end

require MyLogic
MyLogic.unless false do
  IO.puts "It works"
end

Defines a private macro with the given name and body.

Private macros are only accessible from the same module in which they are defined.

Check defmacro/2 for more information.

Defines a module given by name with the given contents.

This macro defines a module with the given alias as its name and with the given contents. It returns a tuple with four elements:

• :module
• the module name
• the binary contents of the module
• the result of evaluating the contents block

Examples

iex> defmodule Foo do
...>   def bar, do: :baz
...> end
iex> Foo.bar
:baz

Nesting

Nesting a module inside another module affects the name of the nested module:

defmodule Foo do
  defmodule Bar do
  end
end

In the example above, two modules - Foo and Foo.Bar - are created. When nesting, Elixir automatically creates an alias to the inner module, allowing the second module Foo.Bar to be accessed as Bar in the same lexical scope where it’s defined (the Foo module).

If the Foo.Bar module is moved somewhere else, the references to Bar in the Foo module need to be updated to the fully-qualified name (Foo.Bar) or an alias has to be explicitly set in the Foo module with the help of Kernel.SpecialForms.alias/2.

defmodule Foo.Bar do
  # code
end

defmodule Foo do
  alias Foo.Bar
  # code here can refer to "Foo.Bar" as just "Bar"
end

Module names

A module name can be any atom, but Elixir provides a special syntax which is usually used for module names. What is called a module name is an uppercase ASCII letter followed by any number of lowercase or uppercase ASCII letters, numbers, or underscores. This identifier is equivalent to an atom prefixed by Elixir.. So in the defmodule Foo example Foo is equivalent to :"Elixir.Foo"

Dynamic names

Elixir module names can be dynamically generated. This is very useful when working with macros. For instance, one could write:

defmodule String.to_atom("Foo#{1}") do
  # contents ...
end

Elixir will accept any module name as long as the expression passed as the first argument to defmodule/2 evaluates to an atom. Note that, when a dynamic name is used, Elixir won’t nest the name under the current module nor automatically set up an alias.

Makes the given functions in the current module overridable.

An overridable function is lazily defined, allowing a developer to override it.

Example

defmodule DefaultMod do
  defmacro __using__(_opts) do
    quote do
      def test(x, y) do
        x + y
      end

      defoverridable [test: 2]
    end
  end
end

defmodule InheritMod do
  use DefaultMod

  def test(x, y) do
    x * y + super(x, y)
  end
end

As seen as in the example above, super can be used to call the default implementation.

Defines a private function with the given name and body.

Private functions are only accessible from within the module in which they are defined. Trying to access a private function from outside the module it’s defined in results in an UndefinedFunctionError exception.

Check def/2 for more information.

Examples

defmodule Foo do
  def bar do
    sum(1, 2)
  end

  defp sum(a, b), do: a + b
end

Foo.bar #=> 3
Foo.sum(1, 2) #=> ** (UndefinedFunctionError) undefined function Foo.sum/2

Defines a protocol.

A protocol specifies an API that should be defined by its implementations.

Examples

In Elixir, only false and nil are considered falsy values. Everything else evaluates to true in if/2 clauses. Depending on the application, it may be important to specify a blank? protocol that returns a boolean for other data types that should be considered “blank”. For instance, an empty list or an empty binary could be considered blank.

Such protocol could be implemented as follows:

defprotocol Blank do
  @doc "Returns `true` if `data` is considered blank/empty"
  def blank?(data)
end

Now that the protocol is defined it can be implemented. It needs to be implemented for each Elixir type; for example:

# Integers are never blank
defimpl Blank, for: Integer do
  def blank?(number), do: false
end

# The only blank list is the empty one
defimpl Blank, for: List do
  def blank?([]), do: true
  def blank?(_),  do: false
end

# The only blank atoms are "false" and "nil"
defimpl Blank, for: Atom do
  def blank?(false), do: true
  def blank?(nil),   do: true
  def blank?(_),     do: false
end

The implementation of the Blank protocol would need to be defined for all Elixir types. The available types are:

• Structs (see below)
• Tuple
• Atom
• List
• BitString
• Integer
• Float
• Function
• PID
• Map
• Port
• Reference
• Any (see below)

Protocols and Structs

The real benefit of protocols comes when mixed with structs. For instance, Elixir ships with many data types implemented as structs, like MapSet. We can implement the Blank protocol for those types as well:

defimpl Blank, for: MapSet do
  def blank?(enum_like), do: Enum.empty?(enum_like)
end

When implementing a protocol for a struct, the :for option can be omitted if the defimpl call is inside the module that defines the struct:

defmodule User do
  defstruct [:email, :name]

  defimpl Blank do
    def blank?(%User{}), do: false
  end
end

If a protocol is not found for a given type, it will fallback to Any. Protocols that are implemented for maps don’t work by default on structs; look at defstruct/1 for more information about deriving protocols.

Fallback to any

In some cases, it may be convenient to provide a default implementation for all types. This can be achieved by setting the @fallback_to_any attribute to true in the protocol definition:

defprotocol Blank do
  @fallback_to_any true
  def blank?(data)
end

The Blank protocol can now be implemented for Any:

defimpl Blank, for: Any do
  def blank?(_), do: true
end

One may wonder why such behaviour (fallback to any) is not the default one.

It is two-fold: first, the majority of protocols cannot implement an action in a generic way for all types; in fact, providing a default implementation may be harmful, because users may rely on the default implementation instead of providing a specialized one.

Second, falling back to Any adds an extra lookup to all types, which is unnecessary overhead unless an implementation for Any is required.

Types

Defining a protocol automatically defines a type named t, which can be used as follows:

@spec present?(Blank.t) :: boolean
def present?(blank) do
  not Blank.blank?(blank)
end

The @spec above expresses that all types allowed to implement the given protocol are valid argument types for the given function.

Reflection

Any protocol module contains three extra functions:

• __protocol__/1 - returns the protocol name when :name is given, and a keyword list with the protocol functions and their arities when :functions is given

• impl_for/1 - receives a structure and returns the module that implements the protocol for the structure, nil otherwise

• impl_for!/1 - same as above but raises an error if an implementation is not found

  Enumerable.__protocol__(:functions)
  #=> [count: 1, member?: 2, reduce: 3]
  
  Enumerable.impl_for([])
  #=> Enumerable.List
  
  Enumerable.impl_for(42)
  #=> nil

Consolidation

In order to cope with code loading in development, protocols in Elixir provide a slow implementation of protocol dispatching specific to development.

In order to speed up dispatching in production environments, where all implementations are known up-front, Elixir provides a feature called protocol consolidation. For this reason, all protocols are compiled with debug_info set to true, regardless of the option set by elixirc compiler. The debug info though may be removed after consolidation.

For more information on how to apply protocol consolidation to a given project, please check the functions in the Protocol module or the mix compile.protocols task.

Defines 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.

To define a struct, a developer must define both __struct__/0 and __struct__/1 functions. defstruct/1 is a convenience macro which defines such functions with some conveniences.

For more information about structs, please check Kernel.SpecialForms.%/2.

Examples

defmodule User do
  defstruct name: nil, age: nil
end

Struct fields are evaluated at compile-time, which allows them to be dynamic. In the example below, 10 + 11 is evaluated at compile-time and the age field is stored with value 21:

defmodule User do
  defstruct name: nil, age: 10 + 11
end

The fields argument is usually a keyword list with field names as atom keys and default values as corresponding values. defstruct/1 also supports a list of atoms as its argument: in that case, the atoms in the list will be used as the struct’s field names and they will all default to nil.

defmodule Post do
  defstruct [:title, :content, :author]
end

Deriving

Although structs are maps, by default structs do not implement any of the protocols implemented for maps. For example, attempting to use a protocol with the User struct leads to an error:

john = %User{name: "John"}
MyProtocol.call(john)
** (Protocol.UndefinedError) protocol MyProtocol not implemented for %User{...}

defstruct/1, however, allows protocol implementations to be derived. This can be done by defining a @derive attribute as a list before invoking defstruct/1:

defmodule User do
  @derive [MyProtocol]
  defstruct name: nil, age: 10 + 11
end

MyProtocol.call(john) #=> works

For each protocol in the @derive list, Elixir will assert there is an implementation of that protocol for any (regardless if fallback to any is true) and check if the any implementation defines a __deriving__/3 callback. If so, the callback is invoked, otherwise an implementation that simply points to the any implementation is automatically derived.

Enforcing keys

When building a struct, Elixir will automatically guarantee all keys belongs to the struct:

%User{name: "john", unknown: :key}
** (KeyError) key :unknown not found in: %User{age: 21, name: nil}

Elixir also allows developers to enforce certain keys must always be given when building the struct:

defmodule User do
  @enforce_keys [:name]
  defstruct name: nil, age: 10 + 11
end

Now trying to build a struct without the name key will fail:

%User{age: 21}
** (ArgumentError) the following keys must also be given when building struct User: [:name]

Keep in mind @enforce_keys is a simply a compile-time guarantee to aid developers when building structs. It is not enforced on updates and it does not provide any sort of value-validation.

Types

It is recommended to define types for structs. By convention such type is called t. To define a struct inside a type, the struct literal syntax is used:

defmodule User do
  defstruct name: "John", age: 25
  @type t :: %User{name: String.t, age: non_neg_integer}
end

It is recommended to only use the struct syntax when defining the struct’s type. When referring to another struct it’s better to use User.tinstead of %User{}.

The types of the struct fields that are not included in %User{} default to term.

Structs whose internal structure is private to the local module (pattern matching them or directly accessing their fields should not be allowed) should use the @opaque attribute. Structs whose internal structure is public should use @type.

Destructures two lists, assigning each term in the right one to the matching term in the left one.

Unlike pattern matching via =, if the sizes of the left and right lists don’t match, destructuring simply stops instead of raising an error.

Examples

iex> destructure([x, y, z], [1, 2, 3, 4, 5])
iex> {x, y, z}
{1, 2, 3}

In the example above, even though the right list has more entries than the left one, destructuring works fine. If the right list is smaller, the remaining items are simply set to nil:

iex> destructure([x, y, z], [1])
iex> {x, y, z}
{1, nil, nil}

The left-hand side supports any expression you would use on the left-hand side of a match:

x = 1
destructure([^x, y, z], [1, 2, 3])

The example above will only work if x matches the first value in the right list. Otherwise, it will raise a MatchError (like the = operator would do).

Performs an integer division.

Raises an ArithmeticError exception if one of the arguments is not an integer.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> div(5, 2)
2

Gets the element at the zero-based index in tuple.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> tuple = {:foo, :bar, 3}
iex> elem(tuple, 1)
:bar

Stops the execution of the calling process with the given reason.

Since evaluating this function causes the process to terminate, it has no return value.

Inlined by the compiler.

Examples

When a process reaches its end, by default it exits with reason :normal. You can also call exit/1 explicitly if you want to terminate a process but not signal any failure:

exit(:normal)

In case something goes wrong, you can also use exit/1 with a different reason:

exit(:seems_bad)

If the exit reason is not :normal, all the processes linked to the process that exited will crash (unless they are trapping exits).

OTP exits

Exits are used by the OTP to determine if a process exited abnormally or not. The following exits are considered “normal”:

• exit(:normal)
• exit(:shutdown)
• exit({:shutdown, term})

Exiting with any other reason is considered abnormal and treated as a crash. This means the default supervisor behaviour kicks in, error reports are emitted, etc.

This behaviour is relied on in many different places. For example, ExUnit uses exit(:shutdown) when exiting the test process to signal linked processes, supervision trees and so on to politely shutdown too.

CLI exits

Building on top of the exit signals mentioned above, if the process started by the command line exits with any of the three reasons above, its exit is considered normal and the Operating System process will exit with status 0.

It is, however, possible to customize the Operating System exit signal by invoking:

exit({:shutdown, integer})

This will cause the OS process to exit with the status given by integer while signaling all linked OTP processes to politely shutdown.

Any other exit reason will cause the OS process to exit with status 1 and linked OTP processes to crash.

Returns true if module is loaded and contains a public function with the given arity, otherwise false.

Note that this function does not load the module in case it is not loaded. Check Code.ensure_loaded/1 for more information.

Examples

iex> function_exported?(Enum, :member?, 2)
true

Gets a value and updates a nested data structure via the given path.

This is similar to get_and_update_in/3, except the path is extracted via a macro rather than passing a list. For example:

get_and_update_in(opts[:foo][:bar], &{&1, &1 + 1})

Is equivalent to:

get_and_update_in(opts, [:foo, :bar], &{&1, &1 + 1})

Note that in order for this macro to work, the complete path must always be visible by this macro. See the Paths section below.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> get_and_update_in(users["john"].age, &{&1, &1 + 1})
{27, %{"john" => %{age: 28}, "meg" => %{age: 23}}}

Paths

A path may start with a variable, local or remote call, and must be followed by one or more:

• foo[bar] - access the key bar in foo; in case foo is nil, nil is returned

• foo.bar - access a map/struct field; in case the field is not present, an error is raised

Here are some valid paths:

users["john"][:age]
users["john"].age
User.all["john"].age
all_users()["john"].age

Here are some invalid ones:

# Does a remote call after the initial value
users["john"].do_something(arg1, arg2)

# Does not access any key or field
users

Gets a value and updates a nested structure.

It expects a tuple to be returned, containing the value retrieved and the update one.

It uses the Access module to traverse the structures according to the given keys, unless the key is a function.

If a key is a function, the function will be invoked passing three arguments, the operation (:get_and_update), the data to be accessed, and a function to be invoked next.

This means get_and_update_in/3 can be extended to provide custom lookups. The downside is that functions cannot be stored as keys in the accessed data structures.

Examples

This function is useful when there is a need to retrieve the current value (or something calculated in function of the current value) and update it at the same time. For example, it could be used to increase the age of a user by one and return the previous age in one pass:

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> get_and_update_in(users, ["john", :age], &{&1, &1 + 1})
{27, %{"john" => %{age: 28}, "meg" => %{age: 23}}}

When one of the keys is a function, the function is invoked. In the example below, we use a function to get and increment all ages inside a list:

iex> users = [%{name: "john", age: 27}, %{name: "meg", age: 23}]
iex> all = fn :get_and_update, data, next ->
...>   Enum.map(data, next) |> :lists.unzip
...> end
iex> get_and_update_in(users, [all, :age], &{&1, &1 + 1})
{[27, 23], [%{name: "john", age: 28}, %{name: "meg", age: 24}]}

If the previous value before invoking the function is nil, the function will receive nil as a value and must handle it accordingly (be it by failing or providing a sane default).

The Access module ships with many convenience accessor functions, like the all function defined above. See Access.all/0, Access.key/1 and others as examples.

Gets a value from a nested structure.

Uses the Access module to traverse the structures according to the given keys, unless the key is a function.

If a key is a function, the function will be invoked passing three arguments, the operation (:get), the data to be accessed, and a function to be invoked next.

This means get_in/2 can be extended to provide custom lookups. The downside is that functions cannot be stored as keys in the accessed data structures.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> get_in(users, ["john", :age])
27

In case any of entries in the middle returns nil, nil will be returned as per the Access module:

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> get_in(users, ["unknown", :age])
nil

When one of the keys is a function, the function is invoked. In the example below, we use a function to get all the maps inside a list:

iex> users = [%{name: "john", age: 27}, %{name: "meg", age: 23}]
iex> all = fn :get, data, next -> Enum.map(data, next) end
iex> get_in(users, [all, :age])
[27, 23]

If the previous value before invoking the function is nil, the function will receive nil as a value and must handle it accordingly.

Returns the head of a list; raises ArgumentError if the list is empty.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> hd([1, 2, 3, 4])
1

Provides an if/2 macro.

This macro expects the first argument to be a condition and the second argument to be a keyword list.

One-liner examples

if(foo, do: bar)

In the example above, bar will be returned if foo evaluates to true (i.e., it is neither false nor nil). Otherwise, nil will be returned.

An else option can be given to specify the opposite:

if(foo, do: bar, else: baz)

Blocks examples

It’s also possible to pass a block to the if/2 macro. The first example above would be translated to:

if foo do
  bar
end

Note that do/end become delimiters. The second example would translate to:

if foo do
  bar
else
  baz
end

In order to compare more than two clauses, the cond/1 macro has to be used.

Checks if the element on the left-hand side is a member of the collection on the right-hand side.

Examples

iex> x = 1
iex> x in [1, 2, 3]
true

This operator (which is a macro) simply translates to a call to Enum.member?/2. The example above would translate to:

Enum.member?([1, 2, 3], x)

Guards

The in/2 operator can be used in guard clauses as long as the right-hand side is a range or a list. In such cases, Elixir will expand the operator to a valid guard expression. For example:

when x in [1, 2, 3]

translates to:

when x === 1 or x === 2 or x === 3

When using ranges:

when x in 1..3

translates to:

when x >= 1 and x <= 3

Inspects the given argument according to the Inspect protocol. The second argument is a keyword list with options to control inspection.

Options

inspect/2 accepts a list of options that are internally translated to an Inspect.Opts struct. Check the docs for Inspect.Opts to see the supported options.

Examples

iex> inspect(:foo)
":foo"

iex> inspect [1, 2, 3, 4, 5], limit: 3
"[1, 2, 3, ...]"

iex> inspect [1, 2, 3], pretty: true, width: 0
"[1,\n 2,\n 3]"

iex> inspect("olá" <> <<0>>)
"<<111, 108, 195, 161, 0>>"

iex> inspect("olá" <> <<0>>, binaries: :as_strings)
"\"olá\\0\""

iex> inspect("olá", binaries: :as_binaries)
"<<111, 108, 195, 161>>"

iex> inspect('bar')
"'bar'"

iex> inspect([0 | 'bar'])
"[0, 98, 97, 114]"

iex> inspect(100, base: :octal)
"0o144"

iex> inspect(100, base: :hex)
"0x64"

Note that the Inspect protocol does not necessarily return a valid representation of an Elixir term. In such cases, the inspected result must start with #. For example, inspecting a function will return:

inspect fn a, b -> a + b end
#=> #Function<...>

Returns true if term is an atom; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a binary; otherwise returns false.

A binary always contains a complete number of bytes.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> is_binary "foo"
true
iex> is_binary <<1::3>>
false

Returns true if term is a bitstring (including a binary); otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> is_bitstring "foo"
true
iex> is_bitstring <<1::3>>
true

Returns true if term is either the atom true or the atom false (i.e., a boolean); otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a floating point number; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a function; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a function that can be applied with arity number of arguments; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> is_function(fn(x) -> x * 2 end, 1)
true
iex> is_function(fn(x) -> x * 2 end, 2)
false

Returns true if term is an integer; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a list with zero or more elements; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a map; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is nil, false otherwise.

Allowed in guard clauses.

Examples

iex> is_nil(1)
false

iex> is_nil(nil)
true

Returns true if term is either an integer or a floating point number; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a pid (process identifier); otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a port identifier; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a reference; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns true if term is a tuple; otherwise returns false.

Allowed in guard tests. Inlined by the compiler.

Returns the length of list.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> length([1, 2, 3, 4, 5, 6, 7, 8, 9])
9

Returns true if module is loaded and contains a public macro with the given arity, otherwise false.

Note that this function does not load the module in case it is not loaded. Check Code.ensure_loaded/1 for more information.

Examples

iex> macro_exported?(Kernel, :use, 2)
true

Returns an almost unique reference.

The returned reference will re-occur after approximately 2^82 calls; therefore it is unique enough for practical purposes.

Inlined by the compiler.

Examples

make_ref() #=> #Reference<0.0.0.135>

Returns the size of a map.

The size of a map is the number of key-value pairs that the map contains.

This operation happens in constant time.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> map_size(%{a: "foo", b: "bar"})
2

A convenience macro that checks if the right side (an expression) matches the left side (a pattern).

Examples

iex> match?(1, 1)
true

iex> match?(1, 2)
false

iex> match?({1, _}, {1, 2})
true

iex> map = %{a: 1, b: 2}
iex> match?(%{a: _}, map)
true

iex> a = 1
iex> match?(^a, 1)
true

match?/2 is very useful when filtering of finding a value in an enumerable:

list = [{:a, 1}, {:b, 2}, {:a, 3}]
Enum.filter list, &match?({:a, _}, &1)
#=> [{:a, 1}, {:a, 3}]

Guard clauses can also be given to the match:

list = [{:a, 1}, {:b, 2}, {:a, 3}]
Enum.filter list, &match?({:a, x} when x < 2, &1)
#=> [{:a, 1}]

However, variables assigned in the match will not be available outside of the function call (unlike regular pattern matching with the = operator):

iex> match?(_x, 1)
true
iex> binding()
[]

Returns the biggest of the two given terms according to Erlang’s term ordering. If the terms compare equal, the first one is returned.

Inlined by the compiler.

Examples

iex> max(1, 2)
2
iex> max(:a, :b)
:b

Returns the smallest of the two given terms according to Erlang’s term ordering. If the terms compare equal, the first one is returned.

Inlined by the compiler.

Examples

iex> min(1, 2)
1
iex> min("foo", "bar")
"bar"

Returns an atom representing the name of the local node. If the node is not alive, :nonode@nohost is returned instead.

Allowed in guard tests. Inlined by the compiler.

Returns the node where the given argument is located. The argument can be a pid, a reference, or a port. If the local node is not alive, :nonode@nohost is returned.

Allowed in guard tests. Inlined by the compiler.

Boolean not.

arg must be a boolean; if it’s not, an ArgumentError exception is raised.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> not false
true

Boolean or.

If the first argument is true, true is returned; otherwise, the second argument is returned.

Requires only the first argument to be a boolean since it short-circuits. If the first argument is not a boolean, an ArgumentError exception is raised.

Allowed in guard tests.

Examples

iex> true or false
true
iex> false or 42
42

Pops a key from the nested structure via the given path.

This is similar to pop_in/2, except the path is extracted via a macro rather than passing a list. For example:

pop_in(opts[:foo][:bar])

Is equivalent to:

pop_in(opts, [:foo, :bar])

Note that in order for this macro to work, the complete path must always be visible by this macro. For more information about the supported path expressions, please check get_and_update_in/2 docs.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> pop_in(users["john"][:age])
{27, %{"john" => %{}, "meg" => %{age: 23}}}

iex> users = %{john: %{age: 27}, meg: %{age: 23}}
iex> pop_in(users.john[:age])
{27, %{john: %{}, meg: %{age: 23}}}

In case any entry returns nil, its key will be removed and the deletion will be considered a success.

Pops a key from the given nested structure.

Uses the Access protocol to traverse the structures according to the given keys, unless the key is a function. If the key is a function, it will be invoked as specified in get_and_update_in/3.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> pop_in(users, ["john", :age])
{27, %{"john" => %{}, "meg" => %{age: 23}}}

In case any entry returns nil, its key will be removed and the deletion will be considered a success.

Inserts value at the given zero-based index in tuple.

Inlined by the compiler.

Examples

iex> tuple = {:foo, :bar, 3}
iex> put_elem(tuple, 0, :baz)
{:baz, :bar, 3}

Puts a value in a nested structure via the given path.

This is similar to put_in/3, except the path is extracted via a macro rather than passing a list. For example:

put_in(opts[:foo][:bar], :baz)

Is equivalent to:

put_in(opts, [:foo, :bar], :baz)

Note that in order for this macro to work, the complete path must always be visible by this macro. For more information about the supported path expressions, please check get_and_update_in/2 docs.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> put_in(users["john"][:age], 28)
%{"john" => %{age: 28}, "meg" => %{age: 23}}

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> put_in(users["john"].age, 28)
%{"john" => %{age: 28}, "meg" => %{age: 23}}

Puts a value in a nested structure.

Uses the Access module to traverse the structures according to the given keys, unless the key is a function. If the key is a function, it will be invoked as specified in get_and_update_in/3.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> put_in(users, ["john", :age], 28)
%{"john" => %{age: 28}, "meg" => %{age: 23}}

In case any of entries in the middle returns nil, an error will be raised when trying to access it next.

Raises an exception.

If the argument msg is a binary, it raises a RuntimeError exception using the given argument as message.

If msg is an atom, it just calls raise/2 with the atom as the first argument and [] as the second argument.

If msg is anything else, raises an ArgumentError exception.

Examples

iex> raise "oops"
** (RuntimeError) oops

try do
  1 + :foo
rescue
  x in [ArithmeticError] ->
    IO.puts "that was expected"
    raise x
end

Raises an exception.

Calls the exception/1 function on the given argument (which has to be a module name like ArgumentError or RuntimeError) passing attrs as the attributes in order to retrieve the exception struct.

Any module that contains a call to the defexception/1 macro automatically implements the exception/1 callback expected by raise/2. See the docs for defexception/1 for more information.

Examples

iex> raise(ArgumentError, message: "Sample")
** (ArgumentError) Sample

Computes the remainder of an integer division.

Raises an ArithmeticError exception if one of the arguments is not an integer.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> rem(5, 2)
1

Raises an exception preserving a previous stacktrace.

Works like raise/1 but does not generate a new stacktrace.

Notice that System.stacktrace/0 returns the stacktrace of the last exception. That said, it is common to assign the stacktrace as the first expression inside a rescue clause as any other exception potentially raised (and rescued) between the rescue clause and the raise call may change the System.stacktrace/0 value.

Examples

try do
  raise "oops"
rescue
  exception ->
    stacktrace = System.stacktrace
    if Exception.message(exception) == "oops" do
      reraise exception, stacktrace
    end
end

Raises an exception preserving a previous stacktrace.

reraise/3 works like reraise/2, except it passes arguments to the exception/1 function like explained in raise/2.

Examples

try do
  raise "oops"
rescue
  exception ->
    stacktrace = System.stacktrace
    reraise WrapperError, [exception: exception], stacktrace
end

Rounds a number to the nearest integer.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> round(5.6)
6
iex> round(5.2)
5
iex> round(-9.9)
-10

Returns the pid (process identifier) of the calling process.

Allowed in guard clauses. Inlined by the compiler.

Sends a message to the given dest and returns the message.

dest may be a remote or local pid, a (local) port, a locally registered name, or a tuple {registered_name, node} for a registered name at another node.

Inlined by the compiler.

Examples

iex> send self(), :hello
:hello

Handles the sigil ~C.

It simply returns a charlist without escaping characters and without interpolations.

Examples

iex> ~C(foo)
'foo'

iex> ~C(f#{o}o)
'f\#{o}o'

Handles the sigil ~D for dates.

The lower case ~d variant does not exist as interpolation and escape characters are not useful for date sigils.

Examples

iex> ~D[2015-01-13]
~D[2015-01-13]

Handles the sigil ~N for naive date times.

The lower case ~n variant does not exist as interpolation and escape characters are not useful for datetime sigils.

Examples

iex> ~N[2015-01-13 13:00:07]
~N[2015-01-13 13:00:07]
iex> ~N[2015-01-13T13:00:07.001]
~N[2015-01-13 13:00:07.001]

Handles the sigil ~R.

It returns a regular expression pattern without escaping nor interpreting interpolations.

More information on regexes can be found in the Regex module.

Examples

iex> Regex.match?(~R(f#{1,3}o), "f#o")
true

Handles the sigil ~S.

It simply returns a string without escaping characters and without interpolations.

Examples

iex> ~S(foo)
"foo"

iex> ~S(f#{o}o)
"f\#{o}o"

Handles the sigil ~T for times.

The lower case ~t variant does not exist as interpolation and escape characters are not useful for time sigils.

Examples

iex> ~T[13:00:07]
~T[13:00:07]
iex> ~T[13:00:07.001]
~T[13:00:07.001]

Handles the sigil ~W.

It returns a list of “words” split by whitespace without escaping nor interpreting interpolations.

Modifiers

• s: words in the list are strings (default)
• a: words in the list are atoms
• c: words in the list are charlists

Examples

iex> ~W(foo #{bar} baz)
["foo", "\#{bar}", "baz"]

Handles the sigil ~c.

It returns a charlist as if it were a single quoted string, unescaping characters and replacing interpolations.

Examples

iex> ~c(foo)
'foo'

iex> ~c(f#{:o}o)
'foo'

iex> ~c(f\#{:o}o)
'f\#{:o}o'

Handles the sigil ~r.

It returns a regular expression pattern, unescaping characters and replacing interpolations.

More information on regexes can be found in the Regex module.

Examples

iex> Regex.match?(~r(foo), "foo")
true

iex> Regex.match?(~r/abc/, "abc")
true

Handles the sigil ~s.

It returns a string as if it was a double quoted string, unescaping characters and replacing interpolations.

Examples

iex> ~s(foo)
"foo"

iex> ~s(f#{:o}o)
"foo"

iex> ~s(f\#{:o}o)
"f\#{:o}o"

Handles the sigil ~w.

It returns a list of “words” split by whitespace. Character unescaping and interpolation happens for each word.

Modifiers

• s: words in the list are strings (default)
• a: words in the list are atoms
• c: words in the list are charlists

Examples

iex> ~w(foo #{:bar} baz)
["foo", "bar", "baz"]

iex> ~w(foo #{" bar baz "})
["foo", "bar", "baz"]

iex> ~w(--source test/enum_test.exs)
["--source", "test/enum_test.exs"]

iex> ~w(foo bar baz)a
[:foo, :bar, :baz]

Spawns the given function and returns its pid.

Check the Process and Node modules for other functions to handle processes, including spawning functions in nodes.

Inlined by the compiler.

Examples

current = self()
child   = spawn(fn -> send current, {self(), 1 + 2} end)

receive do
  {^child, 3} -> IO.puts "Received 3 back"
end

Spawns the given module and function passing the given args and returns its pid.

Check the Process and Node modules for other functions to handle processes, including spawning functions in nodes.

Inlined by the compiler.

Examples

spawn(SomeModule, :function, [1, 2, 3])

Spawns the given function, links it to the current process and returns its pid.

Check the Process and Node modules for other functions to handle processes, including spawning functions in nodes.

Inlined by the compiler.

Examples

current = self()
child   = spawn_link(fn -> send current, {self(), 1 + 2} end)

receive do
  {^child, 3} -> IO.puts "Received 3 back"
end

Spawns the given module and function passing the given args, links it to the current process and returns its pid.

Check the Process and Node modules for other functions to handle processes, including spawning functions in nodes.

Inlined by the compiler.

Examples

spawn_link(SomeModule, :function, [1, 2, 3])

Spawns the given function, monitors it and returns its pid and monitoring reference.

Check the Process and Node modules for other functions to handle processes, including spawning functions in nodes.

Inlined by the compiler.

Examples

current = self()
spawn_monitor(fn -> send current, {self(), 1 + 2} end)

Spawns the given module and function passing the given args, monitors it and returns its pid and monitoring reference.

Check the Process and Node modules for other functions to handle processes, including spawning functions in nodes.

Inlined by the compiler.

Examples

spawn_monitor(SomeModule, :function, [1, 2, 3])

Creates and updates structs.

The struct argument may be an atom (which defines defstruct) or a struct itself. The second argument is any Enumerable that emits two-element tuples (key-value pairs) during enumeration.

Keys in the Enumerable that don’t exist in the struct are automatically discarded. Note that keys must be atoms, as only atoms are allowed when defining a struct.

This function is useful for dynamically creating and updating structs, as well as for converting maps to structs; in the latter case, just inserting the appropriate :__struct__ field into the map may not be enough and struct/2 should be used instead.

Examples

defmodule User do
  defstruct name: "john"
end

struct(User)
#=> %User{name: "john"}

opts = [name: "meg"]
user = struct(User, opts)
#=> %User{name: "meg"}

struct(user, unknown: "value")
#=> %User{name: "meg"}

struct(User, %{name: "meg"})
#=> %User{name: "meg"}

# String keys are ignored
struct(User, %{"name" => "meg"})
#=> %User{name: "john"}

Similar to struct/2 but checks for key validity.

The function struct!/2 emulates the compile time behaviour of structs. This means that:

• when building a struct, as in struct!(SomeStruct, key: :value), it is equivalent to %SomeStruct{key: :value} and therefore this function will check if every given key-value belongs to the struct. If the struct is enforcing any key via @enforce_keys, those will be enforced as well;

• when updating a struct, as in struct!(%SomeStruct{}, key: :value), it is equivalent to %SomeStruct{struct | key: :value} and therefore this function will check if every given key-value belongs to the struct. However, updating structs does not enforce keys, as keys are enforced only when building;

A non-local return from a function. Check Kernel.SpecialForms.try/1 for more information.

Inlined by the compiler.

Returns the tail of a list. Raises ArgumentError if the list is empty.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> tl([1, 2, 3, :go])
[2, 3, :go]

Converts the argument to a charlist according to the List.Chars protocol.

Examples

iex> to_charlist(:foo)
'foo'

Converts the argument to a string according to the String.Chars protocol.

This is the function invoked when there is string interpolation.

Examples

iex> to_string(:foo)
"foo"

Returns the integer part of number.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> trunc(5.4)
5
iex> trunc(5.99)
5

Returns the size of a tuple.

This operation happens in constant time.

Allowed in guard tests. Inlined by the compiler.

Examples

iex> tuple_size {:a, :b, :c}
3

Provides an unless macro.

This macro evaluates and returns the do block passed in as the second argument unless clause evaluates to true. Otherwise, it returns the value of the else block if present or nil if not.

See also if/2.

Examples

iex> unless(Enum.empty?([]), do: "Hello")
nil

iex> unless(Enum.empty?([1, 2, 3]), do: "Hello")
"Hello"

iex> unless Enum.sum([2, 2]) == 5 do
...>   "Math still works"
...> else
...>   "Math is broken"
...> end
"Math still works"

Updates a nested structure via the given path.

This is similar to update_in/3, except the path is extracted via a macro rather than passing a list. For example:

update_in(opts[:foo][:bar], &(&1 + 1))

Is equivalent to:

update_in(opts, [:foo, :bar], &(&1 + 1))

Note that in order for this macro to work, the complete path must always be visible by this macro. For more information about the supported path expressions, please check get_and_update_in/2 docs.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> update_in(users["john"][:age], &(&1 + 1))
%{"john" => %{age: 28}, "meg" => %{age: 23}}

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> update_in(users["john"].age, &(&1 + 1))
%{"john" => %{age: 28}, "meg" => %{age: 23}}

Updates a key in a nested structure.

Uses the Access module to traverse the structures according to the given keys, unless the key is a function. If the key is a function, it will be invoked as specified in get_and_update_in/3.

Examples

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> update_in(users, ["john", :age], &(&1 + 1))
%{"john" => %{age: 28}, "meg" => %{age: 23}}

In case any of entries in the middle returns nil, an error will be raised when trying to access it next.

Uses the given module in the current context.

Examples

For example, in order to write tests using the ExUnit framework, a developer should use the ExUnit.Case module:

defmodule AssertionTest do
  use ExUnit.Case, async: true

  test "always pass" do
    assert true
  end
end

By calling use/2, a hook called __using__/1 will be invoked in ExUnit.Case which will then do the proper setup.

Simply put, use/2 translates to:

defmodule AssertionTest do
  require ExUnit.Case
  ExUnit.Case.__using__([async: true])

  test "always pass" do
    assert true
  end
end

Where __using__/1 is just a regular macro that can be defined in any module:

defmodule MyModule do
  defmacro __using__(opts) do
    quote do
      # code that will run in the module that uses MyModule
    end
  end
end

Best practices

__using__/1 is typically used when there is a need to set some state (via module attributes) or callbacks (like @before_compile) into the caller.

__using__/1 may also be used to alias, require or import functionality from different modules:

defmodule MyModule do
  defmacro __using__(opts) do
    quote do
      import MyModule.Foo
      import MyModule.Bar
      import MyModule.Baz

      alias MyModule.Repo
    end
  end
end

However, do not provide __using__/1 if all it does is to import, alias or require the module itself. For example, do not:

defmodule MyModule do
  defmacro __using__(opts) do
    quote do
      import MyModule
    end
  end
end

In such cases, developers must just import or alias the module directly, allowing developers to customize those as they wish, without the indirection behind use/2.

Finally, developers should also avoid defining functions inside the __using__/1 callback, unless those functions are the default implementation of a previously defined @callback. In case you want to provide some existing functionality to the user module, please define it in a module which will be imported accordingly.

When used inside quoting, marks that the given variable should not be hygienized.

The argument can be either a variable unquoted or in standard tuple form {name, meta, context}.

Check Kernel.SpecialForms.quote/2 for more information.

Pipe operator.

This operator introduces the expression on the left-hand side as the first argument to the function call on the right-hand side.

Examples

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

The example above is the same as calling List.flatten([1, [2], 3]).

The |> operator is mostly useful when there is a desire to execute a series of operations resembling a pipeline:

iex> [1, [2], 3] |> List.flatten |> Enum.map(fn x -> x * 2 end)
[2, 4, 6]

In the example above, the list [1, [2], 3] is passed as the first argument to the List.flatten/1 function, then the flattened list is passed as the first argument to the Enum.map/2 function which doubles each element of the list.

In other words, the expression above simply translates to:

Enum.map(List.flatten([1, [2], 3]), fn x -> x * 2 end)

Pitfalls

There are two common pitfalls when using the pipe operator.

The first one is related to operator precedence. For example, the following expression:

String.graphemes "Hello" |> Enum.reverse

Translates to:

String.graphemes("Hello" |> Enum.reverse)

which results in an error as the Enumerable protocol is not defined for binaries. Adding explicit parentheses resolves the ambiguity:

String.graphemes("Hello") |> Enum.reverse

Or, even better:

"Hello" |> String.graphemes |> Enum.reverse

The second pitfall is that the |> operator works on calls. For example, when you write:

"Hello" |> some_function()

Elixir sees the right-hand side is a function call and pipes to it. This means that, if you want to pipe to an anonymous or captured function, it must also be explicitly called.

Given the anonymous function:

fun = fn x -> IO.puts(x) end
fun.("Hello")

This won’t work as it will rather try to invoke the local function fun:

"Hello" |> fun()

This works:

"Hello" |> fun.()

As you can see, the |> operator retains the same semantics as when the pipe is not used since both require the fun.(...) notation.

Provides a short-circuit operator that evaluates and returns the second expression only if the first one does not evaluate to true (i.e., it is either nil or false). Returns the first expression otherwise.

Not allowed in guard clauses.

Examples

iex> Enum.empty?([1]) || Enum.empty?([1])
false

iex> List.first([]) || true
true

iex> Enum.empty?([1]) || 1
1

iex> Enum.empty?([]) || throw(:bad)
true

Note that, unlike or/2, this operator accepts any expression as the first argument, not only booleans.