struct Proc(*T, R)
Overview
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)
See Proc
literals in the language reference.
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 block end proc = capture { |x| x + 1 } # Proc(Int32, Int32) proc.call(1) # => 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*) end
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 = Box.box(callback) # 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 data_as_callback.call(tick) }, boxed_data) end end Ticker.on_tick do |tick| puts tick end
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:
primitives.crproc.cr
Constructors
- .new(pointer : Pointer(Void), closure_data : Pointer(Void))
- .new(&block : self)
Creates a
Proc
by capturing the given block.
Instance Method Summary
- #==(other : self)
- #===(other : self)
- #===(other)
Case equality.
- #arity
Returns the number of arguments of this
Proc
. - #call(*args : *T) : R
Invokes this
Proc
and returns the result. - #clone
- #closure? : Bool
- #closure_data : Pointer(Void)
- #hash(hasher)
- #partial(*args : *U) forall U
Returns a new
Proc
that has its first arguments fixed to the values given by args. - #pointer : Pointer(Void)
- #to_s(io : IO) : Nil
Instance methods inherited from struct Value
==(other : JSON::Any)==(other : YAML::Any)
==(other) ==, dup dup
Instance methods inherited from class Object
! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as
Class methods inherited from class Object
from_json(string_or_io, root : String)from_json(string_or_io) from_json, from_yaml(string_or_io : String | IO) from_yaml
Constructor Detail
def self.new(&block : self)Source
Creates a Proc
by capturing the given block.
The block argument types are inferred from the Proc
's type arguments. The return type of the block must match the return type specified in the Proc
type.
gt = Proc(Int32, Int32, Bool).new do |x, y| x > y end gt.call(3, 1) # => true gt.call(1, 2) # => false
Instance Method Detail
def ==(other : self)Source
def ===(other : self)Source
def ===(other)Source
Case equality.
The #===
method is used in a case ... when ... end
expression.
For example, this code:
case value when x # something when x when y # something when y end
Is equivalent to this code:
if x === value # something when x elsif y === value # something when y end
Object simply implements #===
by invoking #==
, but subclasses (notably Regex
) can override it to provide meaningful case-equality semantics.
def aritySource
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
def call(*args : *T) : RSource
Invokes this Proc
and returns the result.
add = ->(x : Int32, y : Int32) { x + y } add.call(1, 2) # => 3
def cloneSource
def hash(hasher)Source
def partial(*args : *U) forall USource
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 } add.call(1, 2) # => 3 add_one = add.partial(1) add_one.call(2) # => 3 add_one.call(10) # => 11 add_one_and_two = add_one.partial(2) add_one_and_two.call # => 3
© 2012–2021 Manas Technology Solutions.
Licensed under the Apache License, Version 2.0.
https://crystal-lang.org/api/1.2.1/Proc.html