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Macros

Macros are methods that receive AST nodes at compile-time and produce code that is pasted into a program. For example:

macro define_method(name, content)
  def {{name}}
    {{content}}
  end
end

# This generates:
#
#     def foo
#       1
#     end
define_method foo, 1

foo #=> 1

A macro's definition body looks like regular Crystal code with extra syntax to manipulate the AST nodes. The generated code must be valid Crystal code, meaning that you can't for example generate a def without a matching end, or a single when expression of a case, since both of them are not complete valid expressions. Refer to Pitfalls for more information.

Scope

Macros declared at the top-level are visible anywhere. If a top-level macro is marked as private it is only accessible in that file.

They can also be defined in classes and modules, and are visible in those scopes. Macros are also looked-up in the ancestors chain (superclasses and included modules).

For example, a block which is given an object to use as the default receiver by being invoked with with ... yield can access macros defined within that object's ancestors chain:

class Foo
  macro emphasize(value)
    "***#{ {{value}} }***"
  end

  def yield_with_self
    with self yield
  end
end

Foo.new.yield_with_self { emphasize(10) } #=> "***10***"

Macros defined in classes and modules can be invoked from outside of them too:

class Foo
  macro emphasize(value)
    "***#{ {{value}} }***"
  end
end

Foo.emphasize(10) # => "***10***"

Interpolation

You use {{...}} to paste, or interpolate, an AST node, as in the above example.

Note that the node is pasted as-is. If in the previous example we pass a symbol, the generated code becomes invalid:

# This generates:
#
#     def :foo
#       1
#     end
define_method :foo, 1

Note that :foo was the result of the interpolation, because that's what was passed to the macro. You can use the method ASTNode#id in these cases, where you just need an identifier.

Macro calls

You can invoke a fixed subset of methods on AST nodes at compile-time. These methods are documented in a fictitious Crystal::Macros module.

For example, invoking ASTNode#id in the above example solves the problem:

macro define_method(name, content)
  def {{name.id}}
    {{content}}
  end
end

# This correctly generates:
#
#     def foo
#       1
#     end
define_method :foo, 1

Modules and classes

Modules, classes and structs can also be generated:

macro define_class(module_name, class_name, method, content)
  module {{module_name}}
    class {{class_name}}
      def initialize(@name : String)
      end

      def {{method}}
        {{content}} + @name
      end
    end
  end
end

# This generates:
#     module Foo
#       class Bar
#         def initialize(@name : String)
#         end
#
#         def say
#           "hi " + @name
#         end
#       end
#     end
define_class Foo, Bar, say, "hi "

p Foo::Bar.new("John").say # => "hi John"

Conditionals

You use {% if condition %} ... {% end %} to conditionally generate code:

macro define_method(name, content)
  def {{name}}
    {% if content == 1 %}
      "one"
    {% elsif content == 2 %}
      "two"
    {% else %}
      {{content}}
    {% end %}
  end
end

define_method foo, 1
define_method bar, 2
define_method baz, 3

foo #=> one
bar #=> two
baz #=> 3

Similar to regular code, Nop, NilLiteral and a false BoolLiteral are considered falsey, while everything else is considered truthy.

Macro conditionals can be used outside a macro definition:

{% if env("TEST") %}
  puts "We are in test mode"
{% end %}

Iteration

You can iterate a finite amount of times:

macro define_constants(count)
  {% for i in (1..count) %}
    PI_{{i.id}} = Math::PI * {{i}}
  {% end %}
end

define_constants(3)

PI_1 #=> 3.14159...
PI_2 #=> 6.28318...
PI_3 #=> 9.42477...

To iterate an ArrayLiteral:

macro define_dummy_methods(names)
  {% for name, index in names %}
    def {{name.id}}
      {{index}}
    end
  {% end %}
end

define_dummy_methods [foo, bar, baz]

foo #=> 0
bar #=> 1
baz #=> 2

The index variable in the above example is optional.

To iterate a HashLiteral:

macro define_dummy_methods(hash)
  {% for key, value in hash %}
    def {{key.id}}
      {{value}}
    end
  {% end %}
end
define_dummy_methods({foo: 10, bar: 20})
foo #=> 10
bar #=> 20

Macro iterations can be used outside a macro definition:

{% for name, index in ["foo", "bar", "baz"] %}
  def {{name.id}}
    {{index}}
  end
{% end %}

foo #=> 0
bar #=> 1
baz #=> 2

Variadic arguments and splatting

A macro can accept variadic arguments:

macro define_dummy_methods(*names)
  {% for name, index in names %}
    def {{name.id}}
      {{index}}
    end
  {% end %}
end

define_dummy_methods foo, bar, baz

foo #=> 0
bar #=> 1
baz #=> 2

The arguments are packed into an ArrayLiteral and passed to the macro.

Additionally, using * when interpolating an ArrayLiteral interpolates the elements separated by commas:

macro println(*values)
  print {{*values}}, '\n'
end

println 1, 2, 3 # outputs 123\n

Type information

When a macro is invoked you can access the current scope, or type, with a special instance variable: @type. The type of this variable is TypeNode, which gives you access to type information at compile time.

Note that @type is always the instance type, even when the macro is invoked in a class method.

For example:

macro add_describe_methods
  def describe
    "Class is: " + {{ @type.stringify }}
  end
  
  def self.describe
    "Class is: " + {{ @type.stringify }}
  end
end

class Foo
  add_describe_methods
end

Foo.new.describe #=> "Class is Foo"
Foo.describe #=> "Class is Foo"

Constants

Macros can access constants. For example:

VALUES = [1, 2, 3]

{% for value in VALUES %}
  puts {{value}}
{% end %}

If the constant denotes a type, you get back a TypeNode.

Nested macros

It is possible to define a macro which generates one or more macro definitions. You must escape macro expressions of the inner macro by preceding them with a backslash character "\" to prevent them from being evaluated by the outer macro.

macro define_macros(*names)
  {% for name in names %}
    macro greeting_for_{{name.id}}(greeting)
      \{% if greeting == "hola" %}
        "¡hola {{name.id}}!"
      \{% else %}
        "\{{greeting.id}} {{name.id}}"
      \{% end %}
    end
  {% end %}
end

# This generates:
#
#     macro greeting_for_alice
#       {% if greeting == "hola" %}
#         "¡hola alice!"
#       {% else %}
#         "{{greeting.id}} alice"
#       {% end %}
#     end
#     macro greeting_for_bob
#       {% if greeting == "hola" %}
#         "¡hola bob!"
#       {% else %}
#         "{{greeting.id}} bob"
#       {% end %}
#     end
define_macros alice, bob

greeting_for_alice "hello"  #=> "hello alice"
greeting_for_bob "hallo"    #=> "hallo bob"
greeting_for_alice "hej"    #=> "hej alice"
greeting_for_bob "hola"     #=> "¡hola bob!"

Pitfalls

When writing macros (especially outside of a macro definition) it is important to remember that the generated code from the macro must be valid Crystal code by itself even before it is merged into the main program's code. This means, for example, a macro cannot generate a one or more when expressions of a case statement unless case was a part of the generated code.

Here is an example of such an invalid macro:

case 42
{% for klass in [Int32, String] %}
  when {{klass.id}}
    p "is {{klass}}"
{% end %}
end

Notice that case is not within the macro. The code generated by the macro consists solely of two when expressions which, by themselves, is not valid Crystal code. We must include case within the macro in order to make it valid by using begin and end:

{% begin %}
  case 42
  {% for klass in [Int32, String] %}
    when {{klass.id}}
      p "is {{klass}}"
  {% end %}
  end
{% end %}