Elixir v1.4.5 Kernel.SpecialForms View Source
Special forms are the basic building blocks of Elixir, and therefore cannot be overridden by the developer.
We define them in this module. Some of these forms are lexical (like
alias/2
, case/2
, etc). The macros {}
and <<>>
are also special
forms used to define tuple and binary data structures respectively.
This module also documents macros that return information about Elixir’s
compilation environment, such as (__ENV__/0
, __MODULE__/0
, __DIR__/0
and __CALLER__/0
).
Finally, it also documents two special forms, __block__/1
and
__aliases__/1
, which are not intended to be called directly by the
developer but they appear in quoted contents since they are essential
in Elixir’s constructs.
Link to this section Summary
Functions
Creates a struct
Creates a map
Captures or creates an anonymous function
Defines a remote call or an alias
Used by types and bitstrings to specify types
Defines a new bitstring
Matches the value on the right against the pattern on the left
Accesses an already bound variable in match clauses. Also known as the pin operator
Returns the current calling environment as a Macro.Env
struct
Returns the absolute path of the directory of the current file as a binary
Returns the current module name as an atom or nil
otherwise
Internal special form to hold aliases information
Internal special form for block expressions
alias/2
is used to setup aliases, often useful with modules names
Matches the given expression against the given clauses
Evaluates the expression corresponding to the first clause that evaluates to a truthy value
Defines an anonymous function
Comprehensions allow you to quickly build a data structure from an enumerable or a bitstring
Imports functions and macros from other modules
Gets the representation of any expression
Checks if there is a message matching the given clauses in the current process mailbox
Requires a given module to be compiled and loaded
Calls the overridden function when overriding it with Kernel.defoverridable/1
Evaluates the given expressions and handles any error, exit or throw that may have happened
Unquotes the given expression from inside a macro
Unquotes the given list expanding its arguments. Similar
to unquote/1
Used to combine matching clauses
Creates a tuple
Link to this section Functions
Creates a struct.
A struct is a tagged map that allows developers to provide default values for keys, tags to be used in polymorphic dispatches and compile time assertions.
Structs are usually defined with the Kernel.defstruct/1
macro:
defmodule User do
defstruct name: "john", age: 27
end
Now a struct can be created as follows:
%User{}
Underneath a struct is just a map with a :__struct__
key
pointing to the User
module:
%User{} == %{__struct__: User, name: "john", age: 27}
A struct also validates that the given keys are part of the defined
struct. The example below will fail because there is no key
:full_name
in the User
struct:
%User{full_name: "john doe"}
An update operation specific for structs is also available:
%User{user | age: 28}
The syntax above will guarantee the given keys are valid at
compilation time and it will guarantee at runtime the given
argument is a struct, failing with BadStructError
otherwise.
Although structs are maps, by default structs do not implement
any of the protocols implemented for maps. Check
Kernel.defprotocol/2
for more information on how structs
can be used with protocols for polymorphic dispatch. Also
see Kernel.struct/2
and Kernel.struct!/2
for examples on
how to create and update structs dynamically.
Creates a map.
See the Map
module for more information about maps, their syntax, and ways to
access and manipulate them.
AST representation
Regardless of whether =>
or the keyword syntax is used, key-value pairs in
maps are always represented internally as a list of two-element tuples for
simplicity:
iex> quote do
...> %{"a" => :b, c: :d}
...> end
{:%{}, [], [{"a", :b}, {:c, :d}]}
Captures or creates an anonymous function.
Capture
The capture operator is most commonly used to capture a function with given name and arity from a module:
iex> fun = &Kernel.is_atom/1
iex> fun.(:atom)
true
iex> fun.("string")
false
In the example above, we captured Kernel.is_atom/1
as an
anonymous function and then invoked it.
The capture operator can also be used to capture local functions, including private ones, and imported functions by omitting the module name:
&local_function/1
Anonymous functions
The capture operator can also be used to partially apply
functions, where &1
, &2
and so on can be used as value
placeholders. For example:
iex> double = &(&1 * 2)
iex> double.(2)
4
In other words, &(&1 * 2)
is equivalent to fn x -> x * 2 end
.
Another example using a local function:
iex> fun = &is_atom(&1)
iex> fun.(:atom)
true
The &
operator can be used with more complex expressions:
iex> fun = &(&1 + &2 + &3)
iex> fun.(1, 2, 3)
6
As well as with lists and tuples:
iex> fun = &{&1, &2}
iex> fun.(1, 2)
{1, 2}
iex> fun = &[&1 | &2]
iex> fun.(1, 2)
[1 | 2]
The only restrictions when creating anonymous functions is that at
least one placeholder must be present, i.e. it must contain at least
&1
, and that block expressions are not supported:
# No placeholder, fails to compile.
&(:foo)
# Block expression, fails to compile.
&(&1; &2)
Defines a remote call or an alias.
The dot (.
) in Elixir can be used for remote calls:
iex> String.downcase("FOO")
"foo"
In this example above, we have used .
to invoke downcase
in the
String
alias, passing “FOO” as argument. We can also use the dot
for creating aliases:
iex> Hello.World
Hello.World
This time, we have joined two aliases, defining the final alias
Hello.World
.
Syntax
The right side of .
may be a word starting in upcase, which represents
an alias, a word starting with lowercase or underscore, any valid language
operator or any name wrapped in single- or double-quotes. Those are all valid
examples:
iex> Kernel.Sample
Kernel.Sample
iex> Kernel.length([1, 2, 3])
3
iex> Kernel.+(1, 2)
3
iex> Kernel."length"([1, 2, 3])
3
iex> Kernel.'+'(1, 2)
3
Note that Kernel."FUNCTION_NAME"
will be treated as a remote call and not an alias.
This choice was done so every time single- or double-quotes are used, we have
a remote call regardless of the quote contents. This decision is also reflected
in the quoted expressions discussed below.
Quoted expression
When .
is used, the quoted expression may take two distinct
forms. When the right side starts with a lowercase letter (or
underscore):
iex> quote do
...> String.downcase("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}
Notice we have an inner tuple, containing the atom :.
representing
the dot as first element:
{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}
This tuple follows the general quoted expression structure in Elixir,
with the name as first argument, some keyword list as metadata as second,
and the number of arguments as third. In this case, the arguments is the
alias String
and the atom :downcase
. The second argument is always
an atom:
iex> quote do
...> String."downcase"("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}
The tuple containing :.
is wrapped in another tuple, which actually
represents the function call, and has "FOO"
as argument.
When the right side is an alias (i.e. starts with uppercase), we get instead:
iex> quote do
...> Hello.World
...> end
{:__aliases__, [alias: false], [:Hello, :World]}
We go into more details about aliases in the __aliases__/1
special form
documentation.
Unquoting
We can also use unquote to generate a remote call in a quoted expression:
iex> x = :downcase
iex> quote do
...> String.unquote(x)("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}
Similar to Kernel."FUNCTION_NAME"
, unquote(x)
will always generate a remote call,
independent of the value of x
. To generate an alias via the quoted expression,
one needs to rely on Module.concat/2
:
iex> x = Sample
iex> quote do
...> Module.concat(String, unquote(x))
...> end
{{:., [], [{:__aliases__, [alias: false], [:Module]}, :concat]}, [],
[{:__aliases__, [alias: false], [:String]}, Sample]}
Used by types and bitstrings to specify types.
This operator is used in two distinct occasions in Elixir. It is used in typespecs to specify the type of a variable, function or of a type itself:
@type number :: integer | float
@spec add(number, number) :: number
It may also be used in bit strings to specify the type of a given bit segment:
<<int::integer-little, rest::bits>> = bits
Read the documentation on the Typespec
page and
<<>>/1
for more information on typespecs and
bitstrings respectively.
Defines a new bitstring.
Examples
iex> <<1, 2, 3>>
<<1, 2, 3>>
Types
A bitstring is made of many segments and each segment has a type. There are 9 types used in bitstrings:
integer
float
bits
(alias forbitstring
)bitstring
binary
bytes
(alias forbinary
)utf8
utf16
utf32
When no type is specified, the default is integer
:
iex> <<1, 2, 3>>
<<1, 2, 3>>
Elixir also accepts by default the segment to be a literal string or a literal charlist, which are by default expanded to integers:
iex> <<0, "foo">>
<<0, 102, 111, 111>>
Variables or any other type need to be explicitly tagged:
iex> rest = "oo"
iex> <<102, rest>>
** (ArgumentError) argument error
We can solve this by explicitly tagging it as binary
:
iex> rest = "oo"
iex> <<102, rest::binary>>
"foo"
The utf8
, utf16
, and utf32
types are for Unicode codepoints. They
can also be applied to literal strings and charlists:
iex> <<"foo"::utf16>>
<<0, 102, 0, 111, 0, 111>>
iex> <<"foo"::utf32>>
<<0, 0, 0, 102, 0, 0, 0, 111, 0, 0, 0, 111>>
Options
Many options can be given by using -
as separator. Order is
arbitrary, so the following are all equivalent:
<<102::integer-native, rest::binary>>
<<102::native-integer, rest::binary>>
<<102::unsigned-big-integer, rest::binary>>
<<102::unsigned-big-integer-size(8), rest::binary>>
<<102::unsigned-big-integer-8, rest::binary>>
<<102::8-integer-big-unsigned, rest::binary>>
<<102, rest::binary>>
Unit and Size
The length of the match is equal to the unit
(a number of bits) times the
size
(the number of repeated segments of length unit
).
Type | Default Unit |
---|---|
integer | 1 bit |
float | 1 bit |
binary | 8 bits |
Sizes for types are a bit more nuanced. The default size for integers is 8.
For floats, it is 64. For floats, size * unit
must result in 32 or 64,
corresponding to IEEE 754
binary32 and binary64, respectively.
For binaries, the default is the size of the binary. Only the last binary in a match can use the default size. All others must have their size specified explicitly, even if the match is unambiguous. For example:
iex> <<name::binary-size(5), " the ", species::binary>> = <<"Frank the Walrus">>
"Frank the Walrus"
iex> {name, species}
{"Frank", "Walrus"}
Failing to specify the size for the non-last causes compilation to fail:
<<name::binary, " the ", species::binary>> = <<"Frank the Walrus">>
** (CompileError): a binary field without size is only allowed at the end of a binary pattern
Shortcut Syntax
Size and unit can also be specified using a syntax shortcut when passing integer values:
iex> x = 1
iex> <<x::8>> == <<x::size(8)>>
true
iex> <<x::8*4>> == <<x::size(8)-unit(4)>>
true
This syntax reflects the fact the effective size is given by multiplying the size by the unit.
Modifiers
Some types have associated modifiers to clear up ambiguity in byte representation.
Modifier | Relevant Type(s) |
---|---|
signed | integer |
unsigned (default) | integer |
little | integer , float , utf16 , utf32 |
big (default) | integer , float , utf16 , utf32 |
native | integer , utf16 , utf32 |
Sign
Integers can be signed
or unsigned
, defaulting to unsigned
.
iex> <<int::integer>> = <<-100>>
<<156>>
iex> int
156
iex> <<int::integer-signed>> = <<-100>>
<<156>>
iex> int
-100
signed
and unsigned
are only used for matching binaries (see below) and
are only used for integers.
iex> <<-100::signed, _rest::binary>> = <<-100, "foo">>
<<156, 102, 111, 111>>
Endianness
Elixir has three options for endianness: big
, little
, and native
.
The default is big
:
iex> <<number::little-integer-size(16)>> = <<0, 1>>
<<0, 1>>
iex> number
256
iex> <<number::big-integer-size(16)>> = <<0, 1>>
<<0, 1>>
iex> number
1
native
is determined by the VM at startup and will depend on the
host operating system.
Binary/Bitstring Matching
Binary matching is a powerful feature in Elixir that is useful for extracting information from binaries as well as pattern matching.
Binary matching can be used by itself to extract information from binaries:
iex> <<"Hello, ", place::binary>> = "Hello, World"
"Hello, World"
iex> place
"World"
Or as a part of function definitions to pattern match:
defmodule ImageTyper
@png_signature <<137::size(8), 80::size(8), 78::size(8), 71::size(8),
13::size(8), 10::size(8), 26::size(8), 10::size(8)>>
@jpg_signature <<255::size(8), 216::size(8)>>
def type(<<@png_signature, rest::binary>>), do: :png
def type(<<@jpg_signature, rest::binary>>), do: :jpg
def type(_), do :unknown
end
Performance & Optimizations
The Erlang compiler can provide a number of optimizations on binary creation
and matching. To see optimization output, set the bin_opt_info
compiler
option:
ERL_COMPILER_OPTIONS=bin_opt_info mix compile
To learn more about specific optimizations and performance considerations, check out Erlang’s Efficiency Guide on handling binaries.
Matches the value on the right against the pattern on the left.
Accesses an already bound variable in match clauses. Also known as the pin operator.
Examples
Elixir allows variables to be rebound via static single assignment:
iex> x = 1
iex> x = x + 1
iex> x
2
However, in some situations, it is useful to match against an existing
value, instead of rebinding. This can be done with the ^
special form,
colloquially known as the pin operator:
iex> x = 1
iex> ^x = List.first([1])
iex> ^x = List.first([2])
** (MatchError) no match of right hand side value: 2
Note that ^x
always refers to the value of x
prior to the match. The
following example will match:
iex> x = 0
iex> {x, ^x} = {1, 0}
iex> x
1
Returns the current calling environment as a Macro.Env
struct.
In the environment you can access the filename, line numbers, set up aliases, the function and others.
Returns the absolute path of the directory of the current file as a binary.
Although the directory can be accessed as Path.dirname(__ENV__.file)
,
this macro is a convenient shortcut.
Returns the current environment information as a Macro.Env
struct.
In the environment you can access the current filename, line numbers, set up aliases, the current function and others.
Returns the current module name as an atom or nil
otherwise.
Although the module can be accessed in the __ENV__/0
, this macro
is a convenient shortcut.
Internal special form to hold aliases information.
It is usually compiled to an atom:
iex> quote do
...> Foo.Bar
...> end
{:__aliases__, [alias: false], [:Foo, :Bar]}
Elixir represents Foo.Bar
as __aliases__
so calls can be
unambiguously identified by the operator :.
. For example:
iex> quote do
...> Foo.bar
...> end
{{:., [], [{:__aliases__, [alias: false], [:Foo]}, :bar]}, [], []}
Whenever an expression iterator sees a :.
as the tuple key,
it can be sure that it represents a call and the second argument
in the list is an atom.
On the other hand, aliases holds some properties:
The head element of aliases can be any term that must expand to an atom at compilation time.
The tail elements of aliases are guaranteed to always be atoms.
When the head element of aliases is the atom
:Elixir
, no expansion happens.
Internal special form for block expressions.
This is the special form used whenever we have a block of expressions in Elixir. This special form is private and should not be invoked directly:
iex> quote do
...> 1
...> 2
...> 3
...> end
{:__block__, [], [1, 2, 3]}
alias/2
is used to setup aliases, often useful with modules names.
Examples
alias/2
can be used to setup an alias for any module:
defmodule Math do
alias MyKeyword, as: Keyword
end
In the example above, we have set up MyKeyword
to be aliased
as Keyword
. So now, any reference to Keyword
will be
automatically replaced by MyKeyword
.
In case one wants to access the original Keyword
, it can be done
by accessing Elixir
:
Keyword.values #=> uses MyKeyword.values
Elixir.Keyword.values #=> uses Keyword.values
Notice that calling alias
without the as:
option automatically
sets an alias based on the last part of the module. For example:
alias Foo.Bar.Baz
Is the same as:
alias Foo.Bar.Baz, as: Baz
We can also alias multiple modules in one line:
alias Foo.{Bar, Baz, Biz}
Is the same as:
alias Foo.Bar
alias Foo.Baz
alias Foo.Biz
Lexical scope
import/2
, require/2
and alias/2
are called directives and all
have lexical scope. This means you can set up aliases inside
specific functions and it won’t affect the overall scope.
Warnings
If you alias a module and you don’t use the alias, Elixir is going to issue a warning implying the alias is not being used.
In case the alias is generated automatically by a macro, Elixir won’t emit any warnings though, since the alias was not explicitly defined.
Both warning behaviours could be changed by explicitly
setting the :warn
option to true
or false
.
Matches the given expression against the given clauses.
Examples
case thing do
{:selector, i, value} when is_integer(i) ->
value
value ->
value
end
In the example above, we match thing
against each clause “head”
and execute the clause “body” corresponding to the first clause
that matches.
If no clause matches, an error is raised.
For this reason, it may be necessary to add a final catch-all clause (like _
)
which will always match.
x = 10
case x do
0 ->
"This clause won't match"
_ ->
"This clause would match any value (x = #{x})"
end
#=> "This clause would match any value (x = 10)"
Variables handling
Notice that variables bound in a clause “head” do not leak to the outer context:
case data do
{:ok, value} -> value
:error -> nil
end
value #=> unbound variable value
However, variables explicitly bound in the clause “body” are accessible from the outer context:
value = 7
case lucky? do
false -> value = 13
true -> true
end
value #=> 7 or 13
In the example above, value is going to be 7
or 13
depending on
the value of lucky?
. In case value
has no previous value before
case, clauses that do not explicitly bind a value have the variable
bound to nil
.
If you want to pattern match against an existing variable,
you need to use the ^/1
operator:
x = 1
case 10 do
^x -> "Won't match"
_ -> "Will match"
end
#=> "Will match"
Evaluates the expression corresponding to the first clause that evaluates to a truthy value.
cond do
hd([1, 2, 3]) ->
"1 is considered as true"
end
#=> "1 is considered as true"
Raises an error if all conditions evaluate to nil
or false
.
For this reason, it may be necessary to add a final always-truthy condition
(anything non-false
and non-nil
), which will always match.
Examples
cond do
1 + 1 == 1 ->
"This will never match"
2 * 2 != 4 ->
"Nor this"
true ->
"This will"
end
#=> "This will"
Comprehensions allow you to quickly build a data structure from an enumerable or a bitstring.
Let’s start with an example:
iex> for n <- [1, 2, 3, 4], do: n * 2
[2, 4, 6, 8]
A comprehension accepts many generators and filters. Enumerable
generators are defined using <-
:
# A list generator:
iex> for n <- [1, 2, 3, 4], do: n * 2
[2, 4, 6, 8]
# A comprehension with two generators
iex> for x <- [1, 2], y <- [2, 3], do: x * y
[2, 3, 4, 6]
Filters can also be given:
# A comprehension with a generator and a filter
iex> for n <- [1, 2, 3, 4, 5, 6], rem(n, 2) == 0, do: n
[2, 4, 6]
Note generators can also be used to filter as it removes any value
that doesn’t match the pattern on the left side of <-
:
iex> users = [user: "john", admin: "meg", guest: "barbara"]
iex> for {type, name} when type != :guest <- users do
...> String.upcase(name)
...> end
["JOHN", "MEG"]
Bitstring generators are also supported and are very useful when you need to organize bitstring streams:
iex> pixels = <<213, 45, 132, 64, 76, 32, 76, 0, 0, 234, 32, 15>>
iex> for <<r::8, g::8, b::8 <- pixels>>, do: {r, g, b}
[{213, 45, 132}, {64, 76, 32}, {76, 0, 0}, {234, 32, 15}]
Variable assignments inside the comprehension, be it in generators, filters or inside the block, are not reflected outside of the comprehension.
Into
In the examples above, the result returned by the comprehension was
always a list. The returned result can be configured by passing an
:into
option, that accepts any structure as long as it implements
the Collectable
protocol.
For example, we can use bitstring generators with the :into
option
to easily remove all spaces in a string:
iex> for <<c <- " hello world ">>, c != ?\s, into: "", do: <<c>>
"helloworld"
The IO
module provides streams, that are both Enumerable
and
Collectable
, here is an upcase echo server using comprehensions:
for line <- IO.stream(:stdio, :line), into: IO.stream(:stdio, :line) do
String.upcase(line)
end
Imports functions and macros from other modules.
import/2
allows one to easily access functions or macros from
others modules without using the qualified name.
Examples
If you are using several functions from a given module, you can import those functions and reference them as local functions, for example:
iex> import List
iex> flatten([1, [2], 3])
[1, 2, 3]
Selector
By default, Elixir imports functions and macros from the given module, except the ones starting with underscore (which are usually callbacks):
import List
A developer can filter to import only macros or functions via the only option:
import List, only: :functions
import List, only: :macros
Alternatively, Elixir allows a developer to pass pairs of
name/arities to :only
or :except
as a fine grained control
on what to import (or not):
import List, only: [flatten: 1]
import String, except: [split: 2]
Notice that calling except
for a previously declared import/2
simply filters the previously imported elements. For example:
import List, only: [flatten: 1, keyfind: 4]
import List, except: [flatten: 1]
After the two import calls above, only List.keyfind/4
will be
imported.
Underscore functions
By default functions starting with _
are not imported. If you really want
to import a function starting with _
you must explicitly include it in the
:only
selector.
import File.Stream, only: [__build__: 3]
Lexical scope
It is important to notice that import/2
is lexical. This means you
can import specific macros inside specific functions:
defmodule Math do
def some_function do
# 1) Disable "if/2" from Kernel
import Kernel, except: [if: 2]
# 2) Require the new "if/2" macro from MyMacros
import MyMacros
# 3) Use the new macro
if do_something, it_works
end
end
In the example above, we imported macros from MyMacros
,
replacing the original if/2
implementation by our own
within that specific function. All other functions in that
module will still be able to use the original one.
Warnings
If you import a module and you don’t use any of the imported functions or macros from this module, Elixir is going to issue a warning implying the import is not being used.
In case the import is generated automatically by a macro, Elixir won’t emit any warnings though, since the import was not explicitly defined.
Both warning behaviours could be changed by explicitly
setting the :warn
option to true
or false
.
Ambiguous function/macro names
If two modules A
and B
are imported and they both contain
a foo
function with an arity of 1
, an error is only emitted
if an ambiguous call to foo/1
is actually made; that is, the
errors are emitted lazily, not eagerly.
Gets the representation of any expression.
Examples
iex> quote do
...> sum(1, 2, 3)
...> end
{:sum, [], [1, 2, 3]}
Explanation
Any Elixir code can be represented using Elixir data structures. The building block of Elixir macros is a tuple with three elements, for example:
{:sum, [], [1, 2, 3]}
The tuple above represents a function call to sum
passing 1, 2 and
3 as arguments. The tuple elements are:
The first element of the tuple is always an atom or another tuple in the same representation.
The second element of the tuple represents metadata.
The third element of the tuple are the arguments for the function call. The third argument may be an atom, which is usually a variable (or a local call).
Options
:unquote
- whenfalse
, disables unquoting. Useful when you have a quote inside another quote and want to control what quote is able to unquote.:location
- when set to:keep
, keeps the current line and file from quote. Read the Stacktrace information section below for more information.:generated
- marks the given chunk as generated so it does not emit warnings. Currently it only works on special forms (for example, you can annotate acase
but not anif
).:context
- sets the resolution context.:bind_quoted
- passes a binding to the macro. Whenever a binding is given,unquote/1
is automatically disabled.
Quote literals
Besides the tuple described above, Elixir has a few literals that when quoted return themselves. They are:
:sum #=> Atoms
1 #=> Integers
2.0 #=> Floats
[1, 2] #=> Lists
"strings" #=> Strings
{key, value} #=> Tuples with two elements
Quote and macros
quote/2
is commonly used with macros for code generation. As an exercise,
let’s define a macro that multiplies a number by itself (squared). Note
there is no reason to define such as a macro (and it would actually be
seen as a bad practice), but it is simple enough that it allows us to focus
on the important aspects of quotes and macros:
defmodule Math do
defmacro squared(x) do
quote do
unquote(x) * unquote(x)
end
end
end
We can invoke it as:
import Math
IO.puts "Got #{squared(5)}"
At first, there is nothing in this example that actually reveals it is a
macro. But what is happening is that, at compilation time, squared(5)
becomes 5 * 5
. The argument 5
is duplicated in the produced code, we
can see this behaviour in practice though because our macro actually has
a bug:
import Math
my_number = fn ->
IO.puts "Returning 5"
5
end
IO.puts "Got #{squared(my_number.())}"
The example above will print:
Returning 5
Returning 5
Got 25
Notice how “Returning 5” was printed twice, instead of just once. This is because a macro receives an expression and not a value (which is what we would expect in a regular function). This means that:
squared(my_number.())
Actually expands to:
my_number.() * my_number.()
Which invokes the function twice, explaining why we get the printed value twice! In the majority of the cases, this is actually unexpected behaviour, and that’s why one of the first things you need to keep in mind when it comes to macros is to not unquote the same value more than once.
Let’s fix our macro:
defmodule Math do
defmacro squared(x) do
quote do
x = unquote(x)
x * x
end
end
end
Now invoking square(my_number.())
as before will print the value just
once.
In fact, this pattern is so common that most of the times you will want
to use the bind_quoted
option with quote/2
:
defmodule Math do
defmacro squared(x) do
quote bind_quoted: [x: x] do
x * x
end
end
end
:bind_quoted
will translate to the same code as the example above.
:bind_quoted
can be used in many cases and is seen as good practice,
not only because it helps us from running into common mistakes but also
because it allows us to leverage other tools exposed by macros, such as
unquote fragments discussed in some sections below.
Before we finish this brief introduction, you will notice that, even though
we defined a variable x
inside our quote:
quote do
x = unquote(x)
x * x
end
When we call:
import Math
squared(5)
x #=> ** (CompileError) undefined variable x or undefined function x/0
We can see that x
did not leak to the user context. This happens
because Elixir macros are hygienic, a topic we will discuss at length
in the next sections as well.
Hygiene in variables
Consider the following example:
defmodule Hygiene do
defmacro no_interference do
quote do
a = 1
end
end
end
require Hygiene
a = 10
Hygiene.no_interference
a #=> 10
In the example above, a
returns 10 even if the macro
is apparently setting it to 1 because variables defined
in the macro do not affect the context the macro is executed in.
If you want to set or get a variable in the caller’s context, you
can do it with the help of the var!
macro:
defmodule NoHygiene do
defmacro interference do
quote do
var!(a) = 1
end
end
end
require NoHygiene
a = 10
NoHygiene.interference
a #=> 1
Note that you cannot even access variables defined in the same module unless you explicitly give it a context:
defmodule Hygiene do
defmacro write do
quote do
a = 1
end
end
defmacro read do
quote do
a
end
end
end
Hygiene.write
Hygiene.read
#=> ** (RuntimeError) undefined variable a or undefined function a/0
For such, you can explicitly pass the current module scope as argument:
defmodule ContextHygiene do
defmacro write do
quote do
var!(a, ContextHygiene) = 1
end
end
defmacro read do
quote do
var!(a, ContextHygiene)
end
end
end
ContextHygiene.write
ContextHygiene.read
#=> 1
Hygiene in aliases
Aliases inside quote are hygienic by default. Consider the following example:
defmodule Hygiene do
alias Map, as: M
defmacro no_interference do
quote do
M.new
end
end
end
require Hygiene
Hygiene.no_interference #=> %{}
Notice that, even though the alias M
is not available
in the context the macro is expanded, the code above works
because M
still expands to Map
.
Similarly, even if we defined an alias with the same name before invoking a macro, it won’t affect the macro’s result:
defmodule Hygiene do
alias Map, as: M
defmacro no_interference do
quote do
M.new
end
end
end
require Hygiene
alias SomethingElse, as: M
Hygiene.no_interference #=> %{}
In some cases, you want to access an alias or a module defined
in the caller. For such, you can use the alias!
macro:
defmodule Hygiene do
# This will expand to Elixir.Nested.hello
defmacro no_interference do
quote do
Nested.hello
end
end
# This will expand to Nested.hello for
# whatever is Nested in the caller
defmacro interference do
quote do
alias!(Nested).hello
end
end
end
defmodule Parent do
defmodule Nested do
def hello, do: "world"
end
require Hygiene
Hygiene.no_interference
#=> ** (UndefinedFunctionError) ...
Hygiene.interference
#=> "world"
end
Hygiene in imports
Similar to aliases, imports in Elixir are hygienic. Consider the following code:
defmodule Hygiene do
defmacrop get_length do
quote do
length([1, 2, 3])
end
end
def return_length do
import Kernel, except: [length: 1]
get_length
end
end
Hygiene.return_length #=> 3
Notice how Hygiene.return_length/0
returns 3
even though the Kernel.length/1
function is not imported. In fact, even if return_length/0
imported a function with the same name and arity from another
module, it wouldn’t affect the function result:
def return_length do
import String, only: [length: 1]
get_length
end
Calling this new return_length/0
will still return 3
as result.
Elixir is smart enough to delay the resolution to the latest
possible moment. So, if you call length([1, 2, 3])
inside quote,
but no length/1
function is available, it is then expanded in
the caller:
defmodule Lazy do
defmacrop get_length do
import Kernel, except: [length: 1]
quote do
length("hello")
end
end
def return_length do
import Kernel, except: [length: 1]
import String, only: [length: 1]
get_length
end
end
Lazy.return_length #=> 5
Stacktrace information
When defining functions via macros, developers have the option of choosing if runtime errors will be reported from the caller or from inside the quote. Let’s see an example:
# adder.ex
defmodule Adder do
@doc "Defines a function that adds two numbers"
defmacro defadd do
quote location: :keep do
def add(a, b), do: a + b
end
end
end
# sample.ex
defmodule Sample do
import Adder
defadd
end
require Sample
Sample.add(:one, :two)
#=> ** (ArithmeticError) bad argument in arithmetic expression
#=> adder.ex:5: Sample.add/2
When using location: :keep
and invalid arguments are given to
Sample.add/2
, the stacktrace information will point to the file
and line inside the quote. Without location: :keep
, the error is
reported to where defadd
was invoked. Note location: :keep
affects
only definitions inside the quote.
Binding and unquote fragments
Elixir quote/unquote mechanisms provides a functionality called unquote fragments. Unquote fragments provide an easy way to generate functions on the fly. Consider this example:
kv = [foo: 1, bar: 2]
Enum.each kv, fn {k, v} ->
def unquote(k)(), do: unquote(v)
end
In the example above, we have generated the functions foo/0
and
bar/0
dynamically. Now, imagine that, we want to convert this
functionality into a macro:
defmacro defkv(kv) do
Enum.map kv, fn {k, v} ->
quote do
def unquote(k)(), do: unquote(v)
end
end
end
We can invoke this macro as:
defkv [foo: 1, bar: 2]
However, we can’t invoke it as follows:
kv = [foo: 1, bar: 2]
defkv kv
This is because the macro is expecting its arguments to be a
keyword list at compilation time. Since in the example above
we are passing the representation of the variable kv
, our
code fails.
This is actually a common pitfall when developing macros. We are assuming a particular shape in the macro. We can work around it by unquoting the variable inside the quoted expression:
defmacro defkv(kv) do
quote do
Enum.each unquote(kv), fn {k, v} ->
def unquote(k)(), do: unquote(v)
end
end
end
If you try to run our new macro, you will notice it won’t
even compile, complaining that the variables k
and v
do not exist. This is because of the ambiguity: unquote(k)
can either be an unquote fragment, as previously, or a regular
unquote as in unquote(kv)
.
One solution to this problem is to disable unquoting in the
macro, however, doing that would make it impossible to inject the
kv
representation into the tree. That’s when the :bind_quoted
option comes to the rescue (again!). By using :bind_quoted
, we
can automatically disable unquoting while still injecting the
desired variables into the tree:
defmacro defkv(kv) do
quote bind_quoted: [kv: kv] do
Enum.each kv, fn {k, v} ->
def unquote(k)(), do: unquote(v)
end
end
end
In fact, the :bind_quoted
option is recommended every time
one desires to inject a value into the quote.
Checks if there is a message matching the given clauses in the current process mailbox.
In case there is no such message, the current process hangs until a message arrives or waits until a given timeout value.
Examples
receive do
{:selector, i, value} when is_integer(i) ->
value
value when is_atom(value) ->
value
_ ->
IO.puts :stderr, "Unexpected message received"
end
An optional after
clause can be given in case the message was not
received after the given timeout period, specified in milliseconds:
receive do
{:selector, i, value} when is_integer(i) ->
value
value when is_atom(value) ->
value
_ ->
IO.puts :stderr, "Unexpected message received"
after
5000 ->
IO.puts :stderr, "No message in 5 seconds"
end
The after
clause can be specified even if there are no match clauses.
The timeout value given to after
can be a variable; two special
values are allowed:
:infinity
- the process should wait indefinitely for a matching message, this is the same as not using a timeout0
- if there is no matching message in the mailbox, the timeout will occur immediately
Variables handling
The receive/1
special form handles variables exactly as the case/2
special macro. For more information, check the docs for case/2
.
Requires a given module to be compiled and loaded.
Examples
Notice that usually modules should not be required before usage, the only exception is if you want to use the macros from a module. In such cases, you need to explicitly require them.
Let’s suppose you created your own if/2
implementation in the module
MyMacros
. If you want to invoke it, you need to first explicitly
require the MyMacros
:
defmodule Math do
require MyMacros
MyMacros.if do_something, it_works
end
An attempt to call a macro that was not loaded will raise an error.
Alias shortcut
require/2
also accepts as:
as an option so it automatically sets
up an alias. Please check alias/2
for more information.
Calls the overridden function when overriding it with Kernel.defoverridable/1
.
See Kernel.defoverridable/1
for more information and documentation.
Evaluates the given expressions and handles any error, exit or throw that may have happened.
Examples
try do
do_something_that_may_fail(some_arg)
rescue
ArgumentError ->
IO.puts "Invalid argument given"
catch
value ->
IO.puts "caught #{value}"
else
value ->
IO.puts "Success! The result was #{value}"
after
IO.puts "This is printed regardless if it failed or succeed"
end
The rescue clause is used to handle exceptions, while the catch clause can be used to catch thrown values. The else clause can be used to control flow based on the result of the expression. Catch, rescue and else clauses work based on pattern matching.
Note that calls inside try/1
are not tail recursive since the VM
needs to keep the stacktrace in case an exception happens.
Rescue clauses
Besides relying on pattern matching, rescue clauses provides some conveniences around exceptions that allows one to rescue an exception by its name. All the following formats are valid rescue expressions:
try do
UndefinedModule.undefined_function
rescue
UndefinedFunctionError -> nil
end
try do
UndefinedModule.undefined_function
rescue
[UndefinedFunctionError] -> nil
end
# rescue and bind to x
try do
UndefinedModule.undefined_function
rescue
x in [UndefinedFunctionError] -> nil
end
# rescue all and bind to x
try do
UndefinedModule.undefined_function
rescue
x -> nil
end
Erlang errors
Erlang errors are transformed into Elixir ones during rescue:
try do
:erlang.error(:badarg)
rescue
ArgumentError -> :ok
end
The most common Erlang errors will be transformed into their
Elixir counter-part. Those which are not will be transformed
into ErlangError
:
try do
:erlang.error(:unknown)
rescue
ErlangError -> :ok
end
In fact, ErlangError
can be used to rescue any error that is
not an Elixir error proper. For example, it can be used to rescue
the earlier :badarg
error too, prior to transformation:
try do
:erlang.error(:badarg)
rescue
ErlangError -> :ok
end
Catching throws and exits
The catch clause can be used to catch throws values and exits.
try do
exit(:shutdown)
catch
:exit, :shutdown -> IO.puts "Exited with shutdown reason"
end
try do
throw(:sample)
catch
:throw, :sample ->
IO.puts "sample thrown"
end
catch values also support :error
, as in Erlang, although it is
commonly avoided in favor of raise/rescue control mechanisms.
After clauses
An after
clause allows you to define cleanup logic that will be invoked both
when the tried block of code succeeds and also when an error is raised. Note
that the process will exit as usually when receiving an exit signal that causes
it to exit abruptly and so the after
clause is not guaranteed to be executed.
Luckily, most resources in Elixir (such as open files, ETS tables, ports, sockets,
etc.) are linked to or monitor the owning process and will automatically clean
themselves up if that process exits.
File.write!("tmp/story.txt", "Hello, World")
try do
do_something_with("tmp/story.txt")
after
File.rm("tmp/story.txt")
end
Else clauses
Else clauses allow the result of the expression to be pattern matched on:
x = 2
try do
1 / x
rescue
ArithmeticError ->
:infinity
else
y when y < 1 and y > -1 ->
:small
_ ->
:large
end
If an else clause is not present and no exceptions are raised, the result of the expression will be returned:
x = 1
^x =
try do
1 / x
rescue
ArithmeticError ->
:infinity
end
However, when an else clause is present but the result of the expression does not match any of the patterns an exception will be raised. This exception will not be caught by a catch or rescue in the same try:
x = 1
try do
try do
1 / x
rescue
# The TryClauseError can not be rescued here:
TryClauseError ->
:error_a
else
0 ->
:small
end
rescue
# The TryClauseError is rescued here:
TryClauseError ->
:error_b
end
Similarly, an exception inside an else clause is not caught or rescued inside the same try:
try do
try do
nil
catch
# The exit(1) call below can not be caught here:
:exit, _ ->
:exit_a
else
_ ->
exit(1)
end
catch
# The exit is caught here:
:exit, _ ->
:exit_b
end
This means the VM no longer needs to keep the stacktrace once inside
an else clause and so tail recursion is possible when using a try
with a tail call as the final call inside an else clause. The same
is true for rescue
and catch
clauses.
Variable handling
Since an expression inside try
may not have been evaluated
due to an exception, any variable created inside try
cannot
be accessed externally. For instance:
try do
x = 1
do_something_that_may_fail(same_arg)
:ok
catch
_, _ -> :failed
end
x #=> unbound variable "x"
In the example above, x
cannot be accessed since it was defined
inside the try
clause. A common practice to address this issue
is to return the variables defined inside try
:
x =
try do
x = 1
do_something_that_may_fail(same_arg)
x
catch
_, _ -> :failed
end
Unquotes the given expression from inside a macro.
Examples
Imagine the situation you have a variable value
and
you want to inject it inside some quote. The first attempt
would be:
value = 13
quote do
sum(1, value, 3)
end
Which would then return:
{:sum, [], [1, {:value, [], quoted}, 3]}
Which is not the expected result. For this, we use unquote:
iex> value = 13
iex> quote do
...> sum(1, unquote(value), 3)
...> end
{:sum, [], [1, 13, 3]}
Unquotes the given list expanding its arguments. Similar
to unquote/1
.
Examples
iex> values = [2, 3, 4]
iex> quote do
...> sum(1, unquote_splicing(values), 5)
...> end
{:sum, [], [1, 2, 3, 4, 5]}
Used to combine matching clauses.
Let’s start with an example:
iex> opts = %{width: 10, height: 15}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height),
...> do: {:ok, width * height}
{:ok, 150}
If all clauses match, the do
block is executed, returning its result.
Otherwise the chain is aborted and the non-matched value is returned:
iex> opts = %{width: 10}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height),
...> do: {:ok, width * height}
:error
Guards can be used in patterns as well:
iex> users = %{"melany" => "guest", "bob" => :admin}
iex> with {:ok, role} when not is_binary(role) <- Map.fetch(users, "bob"),
...> do: {:ok, to_string(role)}
{:ok, "admin"}
As in for/1
, variables bound inside with/1
won’t leak;
“bare expressions” may also be inserted between the clauses:
iex> width = nil
iex> opts = %{width: 10, height: 15}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> double_width = width * 2,
...> {:ok, height} <- Map.fetch(opts, :height),
...> do: {:ok, double_width * height}
{:ok, 300}
iex> width
nil
An else
option can be given to modify what is being returned from
with
in the case of a failed match:
iex> opts = %{width: 10}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height) do
...> {:ok, width * height}
...> else
...> :error ->
...> {:error, :wrong_data}
...> end
{:error, :wrong_data}
If there is no matching else
condition, then a WithClauseError
exception is raised.
Creates a tuple.
More information about the tuple data type and about functions to manipulate
tuples can be found in the Tuple
module; some functions for working with
tuples are also available in Kernel
(such as Kernel.elem/2
or
Kernel.tuple_size/1
).
AST representation
Only two-item tuples are considered literals in Elixir and return themselves
when quoted. Therefore, all other tuples are represented in the AST as calls to
the :{}
special form.
iex> quote do
...> {1, 2}
...> end
{1, 2}
iex> quote do
...> {1, 2, 3}
...> end
{:{}, [], [1, 2, 3]}