@spec camelize(String.t()) :: String.t()

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"

iex> Macro.camelize("foo/bar")
"Foo.Bar"

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"

@spec classify_atom(atom()) :: :alias | :identifier | :quoted | :unquoted

Classifies an atom based on its possible AST placement.

It returns one of the following atoms:

• :alias - the atom represents an alias

• :identifier - the atom can be used as a variable or local function call (as well as be an unquoted atom)

• :unquoted - the atom can be used in its unquoted form, includes operators and atoms with @ in them

• :quoted - all other atoms which can only be used in their quoted form

Most operators are going to be :unquoted, such as :+, with some exceptions returning :quoted due to ambiguity, such as :"::". Use operator?/2 to check if a given atom is an operator.

Examples

iex> Macro.classify_atom(:foo)
:identifier
iex> Macro.classify_atom(Foo)
:alias
iex> Macro.classify_atom(:foo@bar)
:unquoted
iex> Macro.classify_atom(:+)
:unquoted
iex> Macro.classify_atom(:Foo)
:unquoted
iex> Macro.classify_atom(:"with spaces")
:quoted

Applies a mod, function, and args at compile-time in caller.

This is used when you want to programmatically invoke a macro at compile-time.

@spec dbg(t(), t(), Macro.Env.t()) :: t()

Default backend for Kernel.dbg/2.

This function provides a default backend for Kernel.dbg/2. See the Kernel.dbg/2 documentation for more information.

This function:

• prints information about the given env
• prints information about code and its returned value (using opts to inspect terms)
• returns the value returned by evaluating code

You can call this function directly to build Kernel.dbg/2 backends that fall back to this function.

This function raises if the context of the given env is :match or :guard.

@spec decompose_call(t()) :: {atom(), [t()]} | {t(), atom(), [t()]} | :error

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

iex> Macro.decompose_call(quote(do: {:foo, [], []}))
:error

@spec escape(
  term(),
  keyword()
) :: t()

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 leaves unquote/1 and unquote_splicing/1 expressions 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 most metadata from escaped AST nodes. Note this option changes the semantics of escaped code and it should only be used when escaping ASTs. Defaults to false.

  As an example for :prune_metadata, 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 would get a different MD5 of the module bytecode, 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 data ExUnit needs to keep around. Only the minimal amount of metadata is kept, such as :line, :no_parens and :delimiter.

Comparison to quote/2

The escape/2 function is sometimes confused with 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)
** (ArgumentError) tried to unquote invalid AST: {:a, :b, :c}
Did you forget to escape term using Macro.escape/1?

escape/2 is used to escape values (either directly passed or variable bound), while quote/2 produces syntax trees for expressions.

@spec expand(input(), Macro.Env.t()) :: output()

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.

@spec expand_literals(input(), Macro.Env.t()) :: output()

Expands all literals in ast with the given env.

This function is mostly used to remove compile-time dependencies from AST nodes. In such cases, the given environment is usually manipulated to represent a function:

Macro.expand_literals(ast, %{env | function: {:my_code, 1}})

At the moment, the only expandable literal nodes in an AST are aliases, so this function only expands aliases (and it does so anywhere in a literal).

However, be careful when removing compile-time dependencies between modules. If you remove them but you still invoke the module at compile-time, Elixir will be unable to properly recompile modules when they change.

@spec expand_literals(t(), acc, (t(), acc -> {t(), acc})) :: t() when acc: term()

Expands all literals in ast with the given acc and fun.

fun will be invoked with an expandable AST node and acc and must return a new node with acc. This is a general version of expand_literals/2 which supports a custom expansion function. Please check expand_literals/2 for use cases and pitfalls.

@spec expand_once(input(), Macro.Env.t()) :: output()

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. The expansion happens as if it was expanded by the Elixir compiler and therefore compilation tracers will be invoked and deprecation warnings will be emitted during the expansion.

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

@spec generate_arguments(0, context :: atom()) :: []

@spec 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.

Note the arguments are not unique. If you later on want to access the same variables, you can invoke this function with the same inputs. Use generate_unique_arguments/2 to generate unique arguments that can't be overridden.

Examples

iex> Macro.generate_arguments(2, __MODULE__)
[{:arg1, [], __MODULE__}, {:arg2, [], __MODULE__}]

@spec generate_unique_arguments(0, context :: atom()) :: []

@spec generate_unique_arguments(pos_integer(), context) :: [
  {atom(), [{:counter, integer()}], context},
  ...
]
when context: atom()

Generates AST nodes for a given number of required argument variables using Macro.unique_var/2.

Examples

iex> [var1, var2] = Macro.generate_unique_arguments(2, __MODULE__)
iex> {:arg1, [counter: c1], __MODULE__} = var1
iex> {:arg2, [counter: c2], __MODULE__} = var2
iex> is_integer(c1) and is_integer(c2)
true

@spec inspect_atom(:literal | :key | :remote_call, atom(), keyword()) :: binary()

Inspects atom according to different source formats.

The atom can be inspected according to the three different formats it appears in the AST: as a literal (:literal), as a key (:key), or as the function name of a remote call (:remote_call).

Options

• :escape - a two-arity function used to escape a quoted atom content, if necessary. The function receives the atom content as string and a quote delimiter character, which should always be escaped. By default the content is escaped such that the inspected sequence would be parsed as the given atom.

Examples

As a literal

Literals include regular atoms, quoted atoms, operators, aliases, and the special nil, true, and false atoms.

iex> Macro.inspect_atom(:literal, nil)
"nil"
iex> Macro.inspect_atom(:literal, :foo)
":foo"
iex> Macro.inspect_atom(:literal, :<>)
":<>"
iex> Macro.inspect_atom(:literal, :Foo)
":Foo"
iex> Macro.inspect_atom(:literal, Foo.Bar)
"Foo.Bar"
iex> Macro.inspect_atom(:literal, :"with spaces")
":\"with spaces\""

As a key

Inspect an atom as a key of a keyword list or a map.

iex> Macro.inspect_atom(:key, :foo)
"foo:"
iex> Macro.inspect_atom(:key, :<>)
"<>:"
iex> Macro.inspect_atom(:key, :Foo)
"Foo:"
iex> Macro.inspect_atom(:key, :"with spaces")
"\"with spaces\":"

As a remote call

Inspect an atom the function name of a remote call.

iex> Macro.inspect_atom(:remote_call, :foo)
"foo"
iex> Macro.inspect_atom(:remote_call, :<>)
"<>"
iex> Macro.inspect_atom(:remote_call, :Foo)
"\"Foo\""
iex> Macro.inspect_atom(:remote_call, :"with spaces")
"\"with spaces\""

@spec operator?(name :: atom(), arity()) :: boolean()

Returns true if the given name and arity is an operator.

Examples

iex> Macro.operator?(:not_an_operator, 3)
false
iex> Macro.operator?(:.., 0)
true
iex> Macro.operator?(:+, 1)
true
iex> Macro.operator?(:++, 2)
true
iex> Macro.operator?(:..//, 3)
true

@spec path(t(), (t() -> as_boolean(term()))) :: [t()] | nil

Returns the path to the node in ast for which fun returns a truthy value.

The path is a list, starting with the node in which fun returns a truthy value, followed by all of its parents.

Returns nil if fun returns only falsy values.

Computing the path can be an efficient operation when you want to find a particular node in the AST within its context and then assert something about it.

Examples

iex> Macro.path(quote(do: [1, 2, 3]), & &1 == 3)
[3, [1, 2, 3]]

iex> Macro.path(quote(do: [1, 2]), & &1 == 5)
nil

iex> Macro.path(quote(do: Foo.bar(3)), & &1 == 3)
[3, quote(do: Foo.bar(3))]

iex> Macro.path(quote(do: %{foo: [bar: :baz]}), & &1 == :baz)
[
  :baz,
  {:bar, :baz},
  [bar: :baz],
  {:foo, [bar: :baz]},
  {:%{}, [], [foo: [bar: :baz]]}
]

@spec pipe(t(), t(), integer()) :: t()

Pipes expr into the call_args at the given position.

This function can be used to implement |> like functionality. For example, |> itself is implemented as:

defmacro left |> right do
  Macro.pipe(left, right, 0)
end

expr is the AST of an expression. call_args must be the AST of a call, otherwise this function will raise an error. As an example, consider the pipe operator |>/2, which uses this function to build pipelines.

Even if the expression is piped into the AST, it doesn't necessarily mean that the AST is valid. For example, you could pipe an argument to div/2, effectively turning it into a call to div/3, which is a function that doesn't exist by default. The code will raise unless a div/3 function is locally defined.

@spec postwalk(t(), (t() -> t())) :: t()

This function behaves like prewalk/2, but performs a depth-first, post-order traversal of quoted expressions.

@spec postwalk(t(), any(), (t(), any() -> {t(), any()})) :: {t(), any()}

This functions behaves like prewalk/3, but performs a depth-first, post-order traversal of quoted expressions using an accumulator.

@spec postwalker(t()) :: Enumerable.t()

Returns an enumerable that traverses the ast in depth-first, post-order traversal.

Examples

iex> ast = quote do: foo(1, "abc")
iex> Enum.map(Macro.postwalker(ast), & &1)
[1, "abc", {:foo, [], [1, "abc"]}]

@spec prewalk(t(), (t() -> t())) :: t()

Performs a depth-first, pre-order traversal of quoted expressions.

Returns a new AST where each node is the result of invoking fun on each corresponding node of ast.

Examples

iex> ast = quote do: 5 + 3 * 7
iex> {:+, _, [5, {:*, _, [3, 7]}]} = ast
iex> new_ast = Macro.prewalk(ast, fn
...>   {:+, meta, children} -> {:*, meta, children}
...>   {:*, meta, children} -> {:+, meta, children}
...>   other -> other
...> end)
iex> {:*, _, [5, {:+, _, [3, 7]}]} = new_ast
iex> Code.eval_quoted(ast)
{26, []}
iex> Code.eval_quoted(new_ast)
{50, []}

@spec prewalk(t(), any(), (t(), any() -> {t(), any()})) :: {t(), any()}

Performs a depth-first, pre-order traversal of quoted expressions using an accumulator.

Returns a tuple where the first element is a new AST where each node is the result of invoking fun on each corresponding node and the second one is the final accumulator.

Examples

iex> ast = quote do: 5 + 3 * 7
iex> {:+, _, [5, {:*, _, [3, 7]}]} = ast
iex> {new_ast, acc} = Macro.prewalk(ast, [], fn
...>   {:+, meta, children}, acc -> {{:*, meta, children}, [:+ | acc]}
...>   {:*, meta, children}, acc -> {{:+, meta, children}, [:* | acc]}
...>   other, acc -> {other, acc}
...> end)
iex> {{:*, _, [5, {:+, _, [3, 7]}]}, [:*, :+]} = {new_ast, acc}
iex> Code.eval_quoted(ast)
{26, []}
iex> Code.eval_quoted(new_ast)
{50, []}

@spec prewalker(t()) :: Enumerable.t()

Returns an enumerable that traverses the ast in depth-first, pre-order traversal.

Examples

iex> ast = quote do: foo(1, "abc")
iex> Enum.map(Macro.prewalker(ast), & &1)
[{:foo, [], [1, "abc"]}, 1, "abc"]

@spec quoted_literal?(t()) :: boolean()

Returns true if the given quoted expression represents a quoted literal.

Atoms and numbers are always literals. Binaries, lists, tuples, maps, and structs are only literals if all of their terms are also literals.

Examples

iex> Macro.quoted_literal?(quote(do: "foo"))
true
iex> Macro.quoted_literal?(quote(do: {"foo", 1}))
true
iex> Macro.quoted_literal?(quote(do: {"foo", 1, :baz}))
true
iex> Macro.quoted_literal?(quote(do: %{foo: "bar"}))
true
iex> Macro.quoted_literal?(quote(do: %URI{path: "/"}))
true
iex> Macro.quoted_literal?(quote(do: URI.parse("/")))
false
iex> Macro.quoted_literal?(quote(do: {foo, var}))
false

@spec special_form?(name :: atom(), arity()) :: boolean()

Returns true if the given name and arity is a special form.

@spec struct_info!(module(), Macro.Env.t()) :: [
  %{field: atom(), required: boolean(), default: term()}
]

Extracts the struct information (equivalent to calling module.__info__(:struct)).

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 also capable of expanding structs defined under the module being compiled. Calling this function also adds an export dependency on the given struct.

It will raise ArgumentError if the struct is not available.

@spec to_string(t()) :: String.t()

Converts the given expression AST to a string.

This is a convenience function for converting AST into a string, which discards all formatting of the original code and wraps newlines around 98 characters. See Code.quoted_to_algebra/2 as a lower level function with more control around formatting.

If the AST contains invalid nodes, this function will attempt to inspect them, to aid debugging, although the elements won't be formatted accordingly.

Examples

iex> Macro.to_string(quote(do: foo.bar(1, 2, 3)))
"foo.bar(1, 2, 3)"

@spec to_string(t(), (t(), String.t() -> String.t())) :: String.t()

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

Macro.to_string(quote(do: 1 + 2), fn
  1, _string -> "one"
  2, _string -> "two"
  _ast, string -> string
end)
#=> "one + two"

@spec traverse(t(), any(), (t(), any() -> {t(), any()}), (t(), any() -> {t(), any()})) ::
  {t(), any()}

Performs a depth-first traversal of quoted expressions using an accumulator.

Returns a tuple where the first element is a new AST and the second one is the final accumulator. The new AST is the result of invoking pre on each node of ast during the pre-order phase and post during the post-order phase.

Examples

iex> ast = quote do: 5 + 3 * 7
iex> {:+, _, [5, {:*, _, [3, 7]}]} = ast
iex> {new_ast, acc} =
...>  Macro.traverse(
...>    ast,
...>    [],
...>    fn
...>      {:+, meta, children}, acc -> {{:-, meta, children}, [:- | acc]}
...>      {:*, meta, children}, acc -> {{:/, meta, children}, [:/ | acc]}
...>      other, acc -> {other, acc}
...>    end,
...>    fn
...>      {:-, meta, children}, acc -> {{:min, meta, children}, [:min | acc]}
...>      {:/, meta, children}, acc -> {{:max, meta, children}, [:max | acc]}
...>      other, acc -> {other, acc}
...>    end
...>  )
iex> {:min, _, [5, {:max, _, [3, 7]}]} = new_ast
iex> [:min, :max, :/, :-] = acc
iex> Code.eval_quoted(new_ast)
{5, []}

@spec underscore(atom() | String.t()) :: String.t()

Converts the given argument to a string with the underscore-slash format.

The argument must either be an atom, representing an Elixir module, or a string representing a module without the Elixir. prefix.

This function was designed to format language identifiers/tokens with the underscore-slash format, 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"

iex> Macro.camelize("foo/bar")
"Foo.Bar"

@spec unescape_string(String.t()) :: String.t()

Unescapes characters in a string.

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, and it can be used anywhere that needs to mimic how Elixir parses strings.

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.

@spec unescape_string(String.t(), (non_neg_integer() -> non_neg_integer() | false)) ::
  String.t()

Unescapes characters in a string according to the given mapping.

Check unescape_string/1 if you want to use the same mapping as Elixir single- and double-quoted strings.

Mapping function

The mapping function receives an integer representing the code point of the character it wants to unescape. There are also the special atoms :newline, :unicode, and :hex, which control newline, unicode, and escaping respectively.

Here is the default mapping function implemented by Elixir:

def unescape_map(:newline), do: true
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.

Examples

Using the unescape_map/1 function defined above is easy:

Macro.unescape_string("example\\n", &unescape_map(&1))

@spec unique_var(var, context) :: {var, [{:counter, integer()}], context}
when var: atom(), context: atom()

Generates an AST node representing a unique variable given by the atoms var and context.

Calling this function with the same arguments will generate another variable, with its own unique counter. See var/2 for an alternative.

Examples

iex> {:foo, [counter: c], __MODULE__} = Macro.unique_var(:foo, __MODULE__)
iex> is_integer(c)
true

@spec unpipe(t()) :: [t()]

Breaks a pipeline expression into a list.

The AST for a pipeline (a sequence of applications of |>/2) 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 |>/2 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)).

@spec update_meta(t(), (keyword() -> keyword())) :: t()

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, [], []}

@spec validate(term()) :: :ok | {:error, term()}

Validates the given expressions are valid quoted expressions.

Check the type Macro.t/0 for a complete 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}}

@spec var(var, context) :: {var, [], context} when var: atom(), context: atom()

Generates an AST node representing the variable given by the atoms var and context.

Note this variable is not unique. If you later on want to access this same variable, you can invoke var/2 again with the same arguments. Use unique_var/2 to generate a unique variable that can't be overridden.

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}