Proc

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

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

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

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

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

The return type 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.

The Proc type

To denote a Proc type you can write:

# A Proc accepting a single Int32 argument and returning a String
Proc(Int32, String)

# A proc accepting no arguments and returning Nil
Proc(Nil)

# A proc accepting two arguments (one Int32 and one String) and returning a Char
Proc(Int32, String, Char)

In type restrictions, generic type arguments and other places where a type is expected, you can use a shorter syntax, as explained in the type:

# An array of Proc(Int32, String, Char)
Array(Int32, String -> Char)

Invoking

To invoke a Proc, you invoke the call method on it. The number of arguments must match the proc's type:

proc = ->(x : Int32, y : Int32) { x + y }
proc.call(1, 2) # => 3

From methods

A Proc can be created from an existing method:

def one
  1
end

proc = ->one
proc.call # => 1

If the method has parameters, you must specify their types:

def plus_one(x)
  x + 1
end

proc = ->plus_one(Int32)
proc.call(41) # => 42

A proc can optionally specify a receiver:

str = "hello"
proc = ->str.count(Char)
proc.call('e') # => 1
proc.call('l') # => 2

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https://crystal-lang.org/reference/syntax_and_semantics/literals/proc.html