struct Proc(*T, R)


A Proc represents a function pointer with an optional context (the closure data). It is typically created with a proc literal:

# A proc without arguments
->{ 1 } # Proc(Int32)

# A proc with one argument
->(x : Int32) { x.to_s } # Proc(Int32, String)

# A proc with two arguments:
->(x : Int32, y : Int32) { x + y } # Proc(Int32, Int32, Int32)

The types of the arguments (T) are mandatory, except when directly sending a proc literal to a lib fun in C bindings.

The return type (R) is inferred from the proc's body.

A special new method is provided too:

Proc(Int32, String).new { |x| x.to_s } # Proc(Int32, String)

This form allows you to specify the return type and to check it against the proc's body.

Another way to create a Proc is by capturing a block:

def capture(&block : Int32 -> Int32)
  # block argument is used, so block is turned into a Proc

proc = capture { |x| x + 1 } # Proc(Int32, Int32)                 # => 2

When capturing blocks, the type of the arguments and return type must be specified in the capturing method block signature.

Passing a Proc to a C function

Passing a Proc to a C function, for example as a callback, is possible as long as the Proc isn't a closure. If it is, either a compile-time or runtime error will happen depending on whether the compiler can check this. The reason is that a Proc is internally represented as two void pointers, one having the function pointer and another the closure data. If just the function pointer is passed, the closure data will be missing at invocation time.

Most of the time a C function that allows setting a callback also provide an argument for custom data. This custom data is then sent as an argument to the callback. For example, suppose a C function that invokes a callback at every tick, passing that tick:

lib LibTicker
  fun on_tick(callback : (Int32, Void* ->), data : Void*)

To properly define a wrapper for this function we must send the Proc as the callback data, and then convert that callback data to the Proc and finally invoke it.

module Ticker
  # The callback for the user doesn't have a Void*
  @@box : Pointer(Void)?

  def self.on_tick(&callback : Int32 ->)
    # Since Proc is a {Void*, Void*}, we can't turn that into a Void*, so we
    # "box" it: we allocate memory and store the Proc there
    boxed_data =

    # We must save this in Crystal-land so the GC doesn't collect it (*)
    @@box = boxed_data

    # We pass a callback that doesn't form a closure, and pass the boxed_data as
    # the callback data
    LibTicker.on_tick(->(tick, data) {
      # Now we turn data back into the Proc, using Box.unbox
      data_as_callback = Box(typeof(callback)).unbox(data)
      # And finally invoke the user's callback
    }, boxed_data)

Ticker.on_tick do |tick|
  puts tick

Note that we save the box in @@box. The reason is that if we don't do it, and our code doesn't reference it anymore, the GC will collect it. The C library will of course store the callback, but Crystal's GC has no way of knowing that.

Defined in:


Instance Method Summary

Instance methods inherited from struct Value

==(other : JSON::Any)
==(other : YAML::Any)
, dup dup

Instance methods inherited from class Object

!=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
, =~(other) =~, class class, dup dup, hash(hasher)
, inspect(io : IO)
, itself itself, not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, tap(&block) tap, to_json(io : IO)
, to_pretty_json(indent : String = " ")
to_pretty_json(io : IO, indent : String = " ")
, to_s
to_s(io : IO)
, to_yaml(io : IO)
, try(&block) try, unsafe_as(type : T.class) forall T unsafe_as

Constructor methods inherited from class Object

from_json(string_or_io, root : String) : self
from_json(string_or_io) : self
, from_yaml(string_or_io : String | IO) : self from_yaml

Constructor Detail

def : Pointer(Void), closure_data : Pointer(Void)) #

[View source]

Instance Method Detail

def ==(other : self) #

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def ===(other : self) #

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def ===(other) #

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def arity #

Returns the number of arguments of this Proc.

add = ->(x : Int32, y : Int32) { x + y }
add.arity # => 2

neg = ->(x : Int32) { -x }
neg.arity # => 1

[View source]
def call(*args : *T) : R #

Invokes this Proc and returns the result.

add = ->(x : Int32, y : Int32) { x + y }, 2) # => 3

[View source]
def clone #

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def closure? #

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def closure_data #

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def hash(hasher) #

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def partial(*args : *U) forall U #

Returns a new Proc that has its first arguments fixed to the values given by args.

See Wikipedia, Partial application

add = ->(x : Int32, y : Int32) { x + y }, 2) # => 3

add_one = add.partial(1)  # => 3 # => 11

add_one_and_two = add_one.partial(2) # => 3

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def pointer #

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def to_s(io) #

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