Elixir v1.4.5 Kernel View Source
Provides the default macros and functions Elixir imports into your environment.
These macros and functions can be skipped or cherry-picked via the
import/2
macro. For instance, if you want to tell Elixir not to
import the if/2
macro, you can do:
import Kernel, except: [if: 2]
Elixir also has special forms that are always imported and
cannot be skipped. These are described in Kernel.SpecialForms
.
Some of the functions described in this module are inlined by
the Elixir compiler into their Erlang counterparts in the :erlang
module. Those functions are called BIFs (built-in internal functions)
in Erlang-land and they exhibit interesting properties, as some of
them are allowed in guards and others are used for compiler
optimizations.
Most of the inlined functions can be seen in effect when capturing the function:
iex> &Kernel.is_atom/1
&:erlang.is_atom/1
Those functions will be explicitly marked in their docs as “inlined by the compiler”.
Link to this section Summary
Functions
Boolean not
Returns true
if the two items are not equal
Returns true
if the two items are not exactly equal
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
Arithmetic multiplication
Arithmetic unary plus
Arithmetic addition
Concatenates a proper list and a term, returning a list
Arithmetic unary minus
Arithmetic subtraction
Removes the first occurrence of an item on the left list for each item on the right
Returns a range with the specified start and end
Arithmetic division
Returns true
if left is less than right
Returns true
if left is less than or equal to right
Concatenates two binaries
Returns true
if the two items are equal
Returns true
if the two items are exactly equal
Matches the term on the left against the regular expression or string on the right
Returns true
if left is more than right
Returns true
if left is more than or equal to right
Reads and writes attributes of the current module
Returns an integer or float which is the arithmetical absolute value of number
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
Boolean and
Invokes the given fun
with the list of arguments args
Invokes the given fun
from module
with the list of arguments args
Extracts the part of the binary starting at start
with length length
.
Binaries are zero-indexed
Returns the binding for the given context as a keyword list
Returns an integer which is the size in bits of bitstring
Returns the number of bytes needed to contain bitstring
Defines a function with the given name and body
Defines a function that delegates to another module
Defines an exception
Defines an implementation for the given protocol
Defines a macro with the given name and body
Defines a private macro with the given name and body
Defines a module given by name with the given contents
Makes the given functions in the current module overridable
Defines a private function with the given name and body
Defines a protocol
Defines a struct
Destructures two lists, assigning each term in the right one to the matching term in the left one
Performs an integer division
Gets the element at the zero-based index
in tuple
Stops the execution of the calling process with the given reason
Returns true
if module
is loaded and contains a
public function
with the given arity
, otherwise false
Gets a value and updates a nested data structure via the given path
Gets a value and updates a nested structure
Gets a value from a nested structure
Returns the head of a list. Raises ArgumentError
if the list is empty
Provides an if/2
macro
Checks if the element on the left-hand side is a member of the collection on the right-hand side
Inspects the given argument according to the Inspect
protocol.
The second argument is a keyword list with options to control
inspection
Returns true
if term
is an atom; otherwise returns false
Returns true
if term
is a binary; otherwise returns false
Returns true
if term
is a bitstring (including a binary); otherwise returns false
Returns true
if term
is either the atom true
or the atom false
(i.e.,
a boolean); otherwise returns false
Returns true
if term
is a floating point number; otherwise returns false
Returns true
if term
is a function; otherwise returns false
Returns true
if term
is a function that can be applied with arity
number of arguments;
otherwise returns false
Returns true
if term
is an integer; otherwise returns false
Returns true
if term
is a list with zero or more elements; otherwise returns false
Returns true
if term
is a map; otherwise returns false
Returns true
if term
is nil
, false
otherwise
Returns true
if term
is either an integer or a floating point number;
otherwise returns false
Returns true
if term
is a PID (process identifier); otherwise returns false
Returns true
if term
is a port identifier; otherwise returns false
Returns true
if term
is a reference; otherwise returns false
Returns true
if term
is a tuple; otherwise returns false
Returns the length of list
Returns true
if module
is loaded and contains a
public macro
with the given arity
, otherwise false
Returns an almost unique reference
Returns the size of a map
A convenience macro that checks if the right side (an expression) matches the left side (a pattern)
Returns the biggest of the two given terms according to Erlang’s term ordering. If the terms compare equal, the first one is returned
Returns the smallest of the two given terms according to Erlang’s term ordering. If the terms compare equal, the first one is returned
Returns an atom representing the name of the local node.
If the node is not alive, :nonode@nohost
is returned instead
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
Boolean not
Boolean or
Pops a key from the nested structure via the given path
Pops a key from the given nested structure
Inserts value
at the given zero-based index
in tuple
Puts a value in a nested structure via the given path
Puts a value in a nested structure
Raises an exception
Raises an exception
Computes the remainder of an integer division
Raises an exception preserving a previous stacktrace
Raises an exception preserving a previous stacktrace
Rounds a number to the nearest integer
Returns the PID (process identifier) of the calling process
Sends a message to the given dest
and returns the message
Handles the sigil ~C
Handles the sigil ~D
for dates
Handles the sigil ~N
for naive date times
Handles the sigil ~R
Handles the sigil ~S
Handles the sigil ~T
for times
Handles the sigil ~W
Handles the sigil ~c
Handles the sigil ~r
Handles the sigil ~s
Handles the sigil ~w
Spawns the given function and returns its PID
Spawns the given module and function passing the given args and returns its PID
Spawns the given function, links it to the current process and returns its PID
Spawns the given module and function passing the given args, links it to the current process and returns its PID
Spawns the given function, monitors it and returns its PID and monitoring reference
Spawns the given module and function passing the given args, monitors it and returns its PID and monitoring reference
Creates and updates structs
Similar to struct/2
but checks for key validity
A non-local return from a function
Returns the tail of a list. Raises ArgumentError
if the list is empty
Converts the argument to a charlist according to the List.Chars
protocol
Converts the argument to a string according to the
String.Chars
protocol
Returns the integer part of number
Returns the size of a tuple
Provides an unless
macro
Updates a nested structure via the given path
Updates a key in a nested structure
Uses the given module in the current context
When used inside quoting, marks that the given variable should not be hygienized
Pipe operator
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
Link to this section Functions
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 are not exactly equal.
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.
integer * integer :: integer
float * float :: float
integer * float :: float
float * integer :: float
Arithmetic multiplication.
Allowed in guard tests. Inlined by the compiler.
Examples
iex> 1 * 2
2
integer + integer :: integer
float + float :: float
integer + float :: float
float + integer :: float
Arithmetic addition.
Allowed in guard tests. Inlined by the compiler.
Examples
iex> 1 + 2
3
Concatenates a proper list and a term, returning a list.
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.
If the right
operand is not a proper list, it returns an improper list.
If the left
operand is not a proper list, it raises ArgumentError
.
Inlined by the compiler.
Examples
iex> [1] ++ [2, 3]
[1, 2, 3]
iex> 'foo' ++ 'bar'
'foobar'
# returns an improper list
iex> [1] ++ 2
[1 | 2]
# returns a proper list
iex> [1] ++ [2]
[1, 2]
# improper list on the right will return an improper list
iex> [1] ++ [2 | 3]
[1, 2 | 3]
-0 :: 0
-pos_integer :: neg_integer
-neg_integer :: pos_integer
-float :: float
Arithmetic unary minus.
Allowed in guard tests. Inlined by the compiler.
Examples
iex> -2
-2
integer - integer :: integer
float - float :: float
integer - float :: float
float - integer :: float
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.
Raises ArithmeticError
if right
is 0 or 0.0.
Allowed in guard tests. Inlined by the compiler.
Examples
1 / 2
#=> 0.5
-3.0 / 2.0
#=> -1.5
5 / 1
#=> 5.0
7 / 0
#=> ** (ArithmeticError) bad argument in arithmetic expression
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 stricter
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 exactly equal.
The items are only considered to be exactly equal if they
have the same value and are of the same type. For example,
1 == 1.0
returns true, but since they are of different
types, 1 === 1.0
returns false.
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]
binary_part(binary, pos_integer, integer) :: binary
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
rescue/catch/after
Function bodies support rescue
, catch
and after
as SpecialForms.try/1
does. The following two functions are equivalent:
def format(value) do
try do
format!(value)
catch
:exit, reason -> {:error, reason}
end
end
def format(value) do
format!(value)
catch
:exit, reason -> {:error, reason}
end
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 module to dispatch to.:as
- the function to call on the target given in:to
. This parameter is optional and defaults to the name being delegated (funs
).
Examples
defmodule MyList do
defdelegate reverse(list), to: :lists
defdelegate other_reverse(list), to: :lists, as: :reverse
end
MyList.reverse([1, 2, 3])
#=> [3, 2, 1]
MyList.other_reverse([1, 2, 3])
#=> [3, 2, 1]
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 toraise/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.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, we have two verbs for checking how many items there
are in a data structure: length
and size
. length
means the
information must be computed. For example, length(list)
needs to
traverse the whole list to calculate its length. On the other hand,
tuple_size(tuple)
and byte_size(binary)
do not depend on the
tuple and binary size as the size information is precomputed in
the data structure.
Although Elixir includes specific functions such as tuple_size
,
binary_size
and map_size
, sometimes we want to be able to
retrieve the size of a data structure regardless of its type.
In Elixir we can write polymorphic code, i.e. code that works
with different shapes/types, by using protocols. A size protocol
could be implemented as follows:
defprotocol Size do
@doc "Calculates the size (and not the length!) of a data structure"
def size(data)
end
Now that the protocol can be implemented for every data structure the protocol may have a compliant implementation for:
defimpl Size, for: Binary do
def size(binary), do: byte_size(binary)
end
defimpl Size, for: Map do
def size(map), do: map_size(map)
end
defimpl Size, for: Tuple do
def size(tuple), do: tuple_size(tuple)
end
Notice we didn’t implement it for lists as we don’t have the
size
information on lists, rather its value needs to be
computed with length
.
It is possible to implement protocols for all Elixir types:
- 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 Size
protocol
for those types as well:
defimpl Size, for: MapSet do
def size(map_set), do: MapSet.size(map_set)
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 Size do
def size(%User{}), do: 2 # two fields
end
end
If a protocol implementation is not found for a given type,
invoking the protocol will raise unless it is configured to
fallback to Any
. Conveniences for building implementations
on top of existing ones are also available, 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 Size do
@fallback_to_any true
def size(data)
end
The Size
protocol can now be implemented for Any
:
defimpl Size, for: Any do
def size(_), do: 0
end
Although the implementation above is arguably not a reasonable
one. For example, it makes no sense to say a PID or an Integer
have a size of 0. That’s one of the reasons why @fallback_to_any
is an opt-in behaviour. For the majority of protocols, raising
an error when a protocol is not implemented is the proper behaviour.
Types
Defining a protocol automatically defines a type named t
, which
can be used as follows:
@spec print_size(Size.t) :: :ok
def print_size(data) do
IO.puts(case Size.size(data) do
0 -> "data has no items"
1 -> "data has one item"
n -> "data has #{n} items"
end)
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 givenimpl_for/1
- receives a structure and returns the module that implements the protocol for the structure,nil
otherwiseimpl_for!/1
- same as above but raises an error if an implementation is not foundEnumerable.__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.
Protocol consolidation is applied by default to all Mix projects.
For applying consolidation manually, 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 simple 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.t
instead 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).
div(integer, neg_integer | pos_integer) :: integer
Performs an integer division.
Raises an ArithmeticError
exception if one of the arguments is not an
integer, or when the divisor
is 0
.
Allowed in guard tests. Inlined by the compiler.
div/2
performs truncated integer division. This means that
the result is always rounded towards zero.
If you want to perform floored integer division (rounding towards negative infinity),
use Integer.floor_div/2
instead.
Examples
div(5, 2)
#=> 2
div(6, -4)
#=> -1
div(-99, 2)
#=> -49
div(100, 0)
#=> ** (ArithmeticError) bad argument in arithmetic expression
Gets the element at the zero-based index
in tuple
.
It raises ArgumentError
when index is negative or it is out of range of the tuple elements.
Allowed in guard tests. Inlined by the compiler.
Examples
tuple = {:foo, :bar, 3}
elem(tuple, 1)
#=> :bar
elem({}, 0)
#=> ** (ArgumentError) argument error
elem({:foo, :bar}, 2)
#=> ** (ArgumentError) argument error
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.
function_exported?(module, atom, arity) :: boolean
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]
- accesses the keybar
infoo
; in casefoo
is nil,nil
is returnedfoo.bar
- accesses 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.
data
is a nested structure (ie. a map, keyword
list, or struct that implements the Access
behaviour).
The fun
argument receives the value of key
(or nil
if key
is not present) and must return a two-element tuple: the “get” value
(the retrieved value, which can be operated on before being returned)
and the new value to be stored under key
. The fun
may also
return :pop
, implying the current value shall be removed
from the structure and returned.
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
anonymous function defined above. See Access.all/0
,
Access.key/2
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.
hd(nonempty_maybe_improper_list(elem, any)) :: elem when elem: term
Returns the head of a list. Raises ArgumentError
if the list is empty.
Allowed in guard tests. Inlined by the compiler.
Examples
hd([1, 2, 3, 4])
#=> 1
hd([])
#=> ** (ArgumentError) argument error
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.
is_function(term, non_neg_integer) :: boolean
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
macro_exported?(module, atom, arity) :: boolean
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.
If module
is an Erlang module (as opposed to an Elixir module), this
function always returns false
.
Examples
iex> macro_exported?(Kernel, :use, 2)
true
iex> macro_exported?(:erlang, :abs, 1)
false
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()
[]
max(first, second) :: first | second when first: term, second: term
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
min(first, second) :: first | second when first: term, second: term
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.
put_elem(tuple, non_neg_integer, term) :: tuple
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.exception/1
callback expected by raise/2
.
For more information, see defexception/1
.
Examples
iex> raise(ArgumentError, message: "Sample")
** (ArgumentError) Sample
rem(integer, neg_integer | pos_integer) :: integer
Computes the remainder of an integer division.
rem/2
uses truncated division, which means that
the result will always have the sign of the dividend
.
Raises an ArithmeticError
exception if one of the arguments is not an
integer, or when the divisor
is 0
.
Allowed in guard tests. Inlined by the compiler.
Examples
iex> rem(5, 2)
1
iex> rem(6, -4)
2
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 as explained in raise/2
.
Examples
try do
raise "oops"
rescue
exception ->
stacktrace = System.stacktrace
reraise WrapperError, [exception: exception], stacktrace
end
round(float) :: integer
round(value) :: value when value: integer
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
iex> round(-9)
-9
Returns the PID (process identifier) of the calling process.
Allowed in guard clauses. Inlined by the compiler.
send(dest :: pid | port | atom | {atom, node}, msg) :: msg when msg: any
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 atomsc
: 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 atomsc
: 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.
The anonymous function receives 0 arguments, and may return any value.
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.
The anonymous function receives 0 arguments, and may return any value.
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
spawn_link(module, atom, list) :: pid
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.
The anonymous function receives 0 arguments, and may return any value.
Inlined by the compiler.
Examples
current = self()
spawn_monitor(fn -> send current, {self(), 1 + 2} end)
spawn_monitor(module, atom, list) :: {pid, reference}
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])
struct(module | struct, Enum.t) :: struct
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"}
struct!(module | struct, Enum.t) :: struct | no_return
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.
tl(nonempty_maybe_improper_list(elem, tail)) :: maybe_improper_list(elem, tail) | tail when elem: term, tail: term
Returns the tail of a list. Raises ArgumentError
if the list is empty.
Allowed in guard tests. Inlined by the compiler.
Examples
tl([1, 2, 3, :go])
#=> [2, 3, :go]
tl([])
#=> ** (ArgumentError) argument error
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"
trunc(value) :: value when value: integer
trunc(float) :: integer
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
iex> trunc(-5)
-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.
When calling:
use MyModule, some: :options
the __using__/1
macro from the MyModule
module is invoked with the second
argument passed to use
as its argument. Since __using__/1
is a macro, all
the usual macro rules apply, and its return value should be quoted code
that is then inserted where use/2
is called.
Examples
For example, in order to write test cases using the ExUnit
framework
provided with Elixir, a developer should use
the ExUnit.Case
module:
defmodule AssertionTest do
use ExUnit.Case, async: true
test "always pass" do
assert true
end
end
In this example, ExUnit.Case.__using__/1
is called with the keyword list
[async: true]
as its argument; use/2
translates to:
defmodule AssertionTest do
require ExUnit.Case
ExUnit.Case.__using__([async: true])
test "always pass" do
assert true
end
end
ExUnit.Case
will then define the __using__/1
macro:
defmodule ExUnit.Case do
defmacro __using__(opts) do
# do something with opts
quote do
# return some code to inject in the caller
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
, see the documentation
for Module
for more information) 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, avoid this:
defmodule MyModule do
defmacro __using__(_opts) do
quote do
import MyModule
end
end
end
In such cases, developers should instead import or alias the module
directly, so that they can 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
or are functions
meant to be overridden (see defoverridable/1
). Even in these cases,
defining functions should be seen as a “last resource”.
In case you want to provide some existing functionality to the user module,
please define it in a module which will be imported accordingly; for example,
ExUnit.Case
doesn’t define the test/3
macro in the module that calls
use ExUnit.Case
, but it defines ExUnit.Case.test/3
and just imports that
into the caller when used.
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.