Elixir v1.9.2 Macro View Source
Conveniences for working with macros.
Custom Sigils
To create a custom sigil, define a function with the name
sigil_{identifier}
that takes two arguments. The first argument will be
the string, the second will be a charlist containing any modifiers. If the
sigil is lower case (such as sigil_x
) then the string argument will allow
interpolation. If the sigil is upper case (such as sigil_X
) then the string
will not be interpolated.
Valid modifiers include only lower and upper case letters. Other characters will cause a syntax error.
The module containing the custom sigil must be imported before the sigil syntax can be used.
Examples
defmodule MySigils do
defmacro sigil_x(term, [?r]) do
quote do
unquote(term) |> String.reverse()
end
end
defmacro sigil_x(term, _modifiers) do
term
end
defmacro sigil_X(term, [?r]) do
quote do
unquote(term) |> String.reverse()
end
end
defmacro sigil_X(term, _modifiers) do
term
end
end
import MySigils
~x(with #{"inter" <> "polation"})
#=>"with interpolation"
~x(with #{"inter" <> "polation"})r
#=>"noitalopretni htiw"
~X(without #{"interpolation"})
#=>"without \#{"interpolation"}"
~X(without #{"interpolation"})r
#=>"}\"noitalopretni\"{# tuohtiw"
Link to this section Summary
Functions
Converts the given string to CamelCase format.
Decomposes a local or remote call into its remote part (when provided), function name and argument list.
Recursively escapes a value so it can be inserted into a syntax tree.
Receives an AST node and expands it until it can no longer be expanded.
Receives an AST node and expands it once.
Generates AST nodes for a given number of required argument variables using
Macro.var/2
.
Returns true
if the given name and arity is an operator.
Pipes expr
into the call_args
at the given position
.
Performs a depth-first, post-order traversal of quoted expressions.
Performs a depth-first, post-order traversal of quoted expressions using an accumulator.
Performs a depth-first, pre-order traversal of quoted expressions.
Performs a depth-first, pre-order traversal of quoted expressions using an accumulator.
Returns true
if the given quoted expression is an AST literal.
Returns true
if the given name and arity is a special form.
Expands the struct given by module
in the given env
.
Converts the given expression AST to a string.
Performs a depth-first traversal of quoted expressions using an accumulator.
Converts the given atom or binary to underscore format.
Unescapes the given chars.
Unescapes the given chars according to the map given.
Breaks a pipeline expression into a list.
Applies the given function to the node metadata if it contains one.
Validates the given expressions are valid quoted expressions.
Generates an AST node representing the variable given
by the atoms var
and context
.
Link to this section Types
Specs
Represents expressions in the AST
Specs
Represents literals in the AST
Specs
Abstract Syntax Tree (AST)
Link to this section Functions
Specs
Converts the given string to CamelCase format.
This function was designed to camelize language identifiers/tokens,
that's why it belongs to the Macro
module. Do not use it as a general
mechanism for camelizing strings as it does not support Unicode or
characters that are not valid in Elixir identifiers.
Examples
iex> Macro.camelize("foo_bar")
"FooBar"
If uppercase characters are present, they are not modified in any way as a mechanism to preserve acronyms:
iex> Macro.camelize("API.V1")
"API.V1"
iex> Macro.camelize("API_SPEC")
"API_SPEC"
Specs
Decomposes a local or remote call into its remote part (when provided), function name and argument list.
Returns :error
when an invalid call syntax is provided.
Examples
iex> Macro.decompose_call(quote(do: foo))
{:foo, []}
iex> Macro.decompose_call(quote(do: foo()))
{:foo, []}
iex> Macro.decompose_call(quote(do: foo(1, 2, 3)))
{:foo, [1, 2, 3]}
iex> Macro.decompose_call(quote(do: Elixir.M.foo(1, 2, 3)))
{{:__aliases__, [], [:Elixir, :M]}, :foo, [1, 2, 3]}
iex> Macro.decompose_call(quote(do: 42))
:error
Specs
Recursively escapes a value so it can be inserted into a syntax tree.
Examples
iex> Macro.escape(:foo)
:foo
iex> Macro.escape({:a, :b, :c})
{:{}, [], [:a, :b, :c]}
iex> Macro.escape({:unquote, [], [1]}, unquote: true)
1
Options
:unquote
- when true, this function leavesunquote/1
andunquote_splicing/1
statements unescaped, effectively unquoting the contents on escape. This option is useful only when escaping ASTs which may have quoted fragments in them. Defaults to false.:prune_metadata
- when true, removes metadata from escaped AST nodes. Note this option changes the semantics of escaped code and it should only be used when escaping ASTs, never values. Defaults to false.As an example,
ExUnit
stores the AST of every assertion, so when an assertion fails we can show code snippets to users. Without this option, each time the test module is compiled, we get a different MD5 of the module byte code, because the AST contains metadata, such as counters, specific to the compilation environment. By pruning the metadata, we ensure that the module is deterministic and reduce the amount of dataExUnit
needs to keep around.
Comparison to Kernel.SpecialForms.quote/2
The escape/2
function is sometimes confused with Kernel.SpecialForms.quote/2
,
because the above examples behave the same with both. The key difference is
best illustrated when the value to escape is stored in a variable.
iex> Macro.escape({:a, :b, :c})
{:{}, [], [:a, :b, :c]}
iex> quote do: {:a, :b, :c}
{:{}, [], [:a, :b, :c]}
iex> value = {:a, :b, :c}
iex> Macro.escape(value)
{:{}, [], [:a, :b, :c]}
iex> quote do: value
{:value, [], __MODULE__}
iex> value = {:a, :b, :c}
iex> quote do: unquote(value)
{:a, :b, :c}
escape/2
is used to escape values (either directly passed or variable
bound), while Kernel.SpecialForms.quote/2
produces syntax trees for
expressions.
Receives an AST node and expands it until it can no longer be expanded.
Note this function does not traverse the AST, only the root node is expanded.
This function uses expand_once/2
under the hood. Check
it out for more information and examples.
Receives an AST node and expands it once.
The following contents are expanded:
- Macros (local or remote)
- Aliases are expanded (if possible) and return atoms
- Compilation environment macros (
__CALLER__/0
,__DIR__/0
,__ENV__/0
and__MODULE__/0
) - Module attributes reader (
@foo
)
If the expression cannot be expanded, it returns the expression itself. This function does not traverse the AST, only the root node is expanded.
expand_once/2
performs the expansion just once. Check expand/2
to perform expansion until the node can no longer be expanded.
Examples
In the example below, we have a macro that generates a module
with a function named name_length
that returns the length
of the module name. The value of this function will be calculated
at compilation time and not at runtime.
Consider the implementation below:
defmacro defmodule_with_length(name, do: block) do
length = length(Atom.to_charlist(name))
quote do
defmodule unquote(name) do
def name_length, do: unquote(length)
unquote(block)
end
end
end
When invoked like this:
defmodule_with_length My.Module do
def other_function, do: ...
end
The compilation will fail because My.Module
when quoted
is not an atom, but a syntax tree as follows:
{:__aliases__, [], [:My, :Module]}
That said, we need to expand the aliases node above to an atom, so we can retrieve its length. Expanding the node is not straightforward because we also need to expand the caller aliases. For example:
alias MyHelpers, as: My
defmodule_with_length My.Module do
def other_function, do: ...
end
The final module name will be MyHelpers.Module
and not
My.Module
. With Macro.expand/2
, such aliases are taken
into consideration. Local and remote macros are also
expanded. We could rewrite our macro above to use this
function as:
defmacro defmodule_with_length(name, do: block) do
expanded = Macro.expand(name, __CALLER__)
length = length(Atom.to_charlist(expanded))
quote do
defmodule unquote(name) do
def name_length, do: unquote(length)
unquote(block)
end
end
end
Specs
generate_arguments(0, context :: atom()) :: []
generate_arguments(pos_integer(), context) :: [{atom(), [], context}, ...] when context: atom()
Generates AST nodes for a given number of required argument variables using
Macro.var/2
.
Examples
iex> Macro.generate_arguments(2, __MODULE__)
[{:arg1, [], __MODULE__}, {:arg2, [], __MODULE__}]
Specs
Returns true
if the given name and arity is an operator.
Specs
Pipes expr
into the call_args
at the given position
.
Specs
Performs a depth-first, post-order traversal of quoted expressions.
Specs
Performs a depth-first, post-order traversal of quoted expressions using an accumulator.
Specs
Performs a depth-first, pre-order traversal of quoted expressions.
Specs
Performs a depth-first, pre-order traversal of quoted expressions using an accumulator.
Specs
Returns true
if the given quoted expression is an AST literal.
Specs
Returns true
if the given name and arity is a special form.
Specs
struct!(module, Macro.Env.t()) :: %module{} when module: module()
Expands the struct given by module
in the given env
.
This is useful when a struct needs to be expanded at compilation time and the struct being expanded may or may not have been compiled. This function is even capable of expanding structs defined under the module being compiled.
It will raise CompileError
if the struct is not available.
Specs
Converts the given expression AST to a string.
The given fun
is called for every node in the AST with two arguments: the
AST of the node being printed and the string representation of that same
node. The return value of this function is used as the final string
representation for that AST node.
This function discards all formatting of the original code.
Examples
iex> Macro.to_string(quote(do: foo.bar(1, 2, 3)))
"foo.bar(1, 2, 3)"
iex> Macro.to_string(quote(do: 1 + 2), fn
...> 1, _string -> "one"
...> 2, _string -> "two"
...> _ast, string -> string
...> end)
"one + two"
Specs
Performs a depth-first traversal of quoted expressions using an accumulator.
Specs
Converts the given atom or binary to underscore format.
If an atom is given, it is assumed to be an Elixir module, so it is converted to a binary and then processed.
This function was designed to underscore language identifiers/tokens,
that's why it belongs to the Macro
module. Do not use it as a general
mechanism for underscoring strings as it does not support Unicode or
characters that are not valid in Elixir identifiers.
Examples
iex> Macro.underscore("FooBar")
"foo_bar"
iex> Macro.underscore("Foo.Bar")
"foo/bar"
iex> Macro.underscore(Foo.Bar)
"foo/bar"
In general, underscore
can be thought of as the reverse of
camelize
, however, in some cases formatting may be lost:
iex> Macro.underscore("SAPExample")
"sap_example"
iex> Macro.camelize("sap_example")
"SapExample"
iex> Macro.camelize("hello_10")
"Hello10"
Specs
Unescapes the given chars.
This is the unescaping behaviour used by default in Elixir
single- and double-quoted strings. Check unescape_string/2
for information on how to customize the escaping map.
In this setup, Elixir will escape the following: \0
, \a
, \b
,
\d
, \e
, \f
, \n
, \r
, \s
, \t
and \v
. Bytes can be
given as hexadecimals via \xNN
and Unicode code points as
\uNNNN
escapes.
This function is commonly used on sigil implementations
(like ~r
, ~s
and others) which receive a raw, unescaped
string.
Examples
iex> Macro.unescape_string("example\\n")
"example\n"
In the example above, we pass a string with \n
escaped
and return a version with it unescaped.
Specs
unescape_string(String.t(), (non_neg_integer() -> non_neg_integer() | false)) :: String.t()
Unescapes the given chars according to the map given.
Check unescape_string/1
if you want to use the same map
as Elixir single- and double-quoted strings.
Map
The map must be a function. The function receives an integer representing the code point of the character it wants to unescape. Here is the default mapping function implemented by Elixir:
def unescape_map(unicode), do: true
def unescape_map(hex), do: true
def unescape_map(?0), do: ?0
def unescape_map(?a), do: ?\a
def unescape_map(?b), do: ?\b
def unescape_map(?d), do: ?\d
def unescape_map(?e), do: ?\e
def unescape_map(?f), do: ?\f
def unescape_map(?n), do: ?\n
def unescape_map(?r), do: ?\r
def unescape_map(?s), do: ?\s
def unescape_map(?t), do: ?\t
def unescape_map(?v), do: ?\v
def unescape_map(e), do: e
If the unescape_map/1
function returns false
, the char is
not escaped and the backslash is kept in the string.
Hexadecimals and Unicode code points will be escaped if the map
function returns true
for ?x
. Unicode code points if the map
function returns true
for ?u
.
Examples
Using the unescape_map/1
function defined above is easy:
Macro.unescape_string("example\\n", &unescape_map(&1))
Specs
Breaks a pipeline expression into a list.
The AST for a pipeline (a sequence of applications of |>
) is similar to the
AST of a sequence of binary operators or function applications: the top-level
expression is the right-most :|>
(which is the last one to be executed), and
its left-hand and right-hand sides are its arguments:
quote do: 100 |> div(5) |> div(2)
#=> {:|>, _, [arg1, arg2]}
In the example above, the |>
pipe is the right-most pipe; arg1
is the AST
for 100 |> div(5)
, and arg2
is the AST for div(2)
.
It's often useful to have the AST for such a pipeline as a list of function applications. This function does exactly that:
Macro.unpipe(quote do: 100 |> div(5) |> div(2))
#=> [{100, 0}, {{:div, [], [5]}, 0}, {{:div, [], [2]}, 0}]
We get a list that follows the pipeline directly: first the 100
, then the
div(5)
(more precisely, its AST), then div(2)
. The 0
as the second
element of the tuples is the position of the previous element in the pipeline
inside the current function application: {{:div, [], [5]}, 0}
means that the
previous element (100
) will be inserted as the 0th (first) argument to the
div/2
function, so that the AST for that function will become {:div, [], [100, 5]}
(div(100, 5)
).
Specs
Applies the given function to the node metadata if it contains one.
This is often useful when used with Macro.prewalk/2
to remove
information like lines and hygienic counters from the expression
for either storage or comparison.
Examples
iex> quoted = quote line: 10, do: sample()
{:sample, [line: 10], []}
iex> Macro.update_meta(quoted, &Keyword.delete(&1, :line))
{:sample, [], []}
Specs
Validates the given expressions are valid quoted expressions.
Checks the Macro.t/0
for the specification of a valid
quoted expression.
It returns :ok
if the expression is valid. Otherwise it returns a tuple in the form of
{:error, remainder}
where remainder
is the invalid part of the quoted expression.
Examples
iex> Macro.validate({:two_element, :tuple})
:ok
iex> Macro.validate({:three, :element, :tuple})
{:error, {:three, :element, :tuple}}
iex> Macro.validate([1, 2, 3])
:ok
iex> Macro.validate([1, 2, 3, {4}])
{:error, {4}}
Specs
Generates an AST node representing the variable given
by the atoms var
and context
.
Examples
In order to build a variable, a context is expected.
Most of the times, in order to preserve hygiene, the
context must be __MODULE__/0
:
iex> Macro.var(:foo, __MODULE__)
{:foo, [], __MODULE__}
However, if there is a need to access the user variable, nil can be given:
iex> Macro.var(:foo, nil)
{:foo, [], nil}