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Class: Proc

Relationships & Source Files
Inherits: Object
Defined in: proc.c

Overview

Proc objects are blocks of code that have been bound to a set of local variables. Once bound, the code may be called in different contexts and still access those variables.

def gen_times(factor)
  return Proc.new {|n| n*factor }
end

times3 = gen_times(3)
times5 = gen_times(5)

times3.call(12)               #=> 36
times5.call(5)                #=> 25
times3.call(times5.call(4))   #=> 60

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new {|...| ... } ⇒ Proc .new ⇒ Proc

Creates a new Proc object, bound to the current context. new may be called without a block only within a method with an attached block, in which case that block is converted to the Proc object.

def proc_from
  Proc.new
end
proc = proc_from { "hello" }
proc.call   #=> "hello"

Instance Attribute Details

#lambda? ⇒ Boolean (readonly)

Returns true for a Proc object for which argument handling is rigid. Such procs are typically generated by lambda.

A Proc object generated by proc ignores extra arguments.

proc {|a,b| [a,b] }.call(1,2,3)    #=> [1,2]

It provides nil for missing arguments.

proc {|a,b| [a,b] }.call(1)        #=> [1,nil]

It expands a single array argument.

proc {|a,b| [a,b] }.call([1,2])    #=> [1,2]

A Proc object generated by lambda doesn't have such tricks.

lambda {|a,b| [a,b] }.call(1,2,3)  #=> ArgumentError
lambda {|a,b| [a,b] }.call(1)      #=> ArgumentError
lambda {|a,b| [a,b] }.call([1,2])  #=> ArgumentError

lambda? is a predicate for the tricks. It returns true if no tricks apply.

lambda {}.lambda?            #=> true
proc {}.lambda?              #=> false

.new is the same as proc.

Proc.new {}.lambda?          #=> false

lambda, proc and .new preserve the tricks of a Proc object given by & argument.

lambda(&lambda {}).lambda?   #=> true
proc(&lambda {}).lambda?     #=> true
Proc.new(&lambda {}).lambda? #=> true

lambda(&proc {}).lambda?     #=> false
proc(&proc {}).lambda?       #=> false
Proc.new(&proc {}).lambda?   #=> false

A Proc object generated by & argument has the tricks

def n(&b) b.lambda? end
n {}                         #=> false

The & argument preserves the tricks if a Proc object is given by & argument.

n(&lambda {})                #=> true
n(&proc {})                  #=> false
n(&Proc.new {})              #=> false

A Proc object converted from a method has no tricks.

def m() end
method(:m).to_proc.lambda?   #=> true

n(&method(:m))               #=> true
n(&method(:m).to_proc)       #=> true

define_method is treated the same as method definition. The defined method has no tricks.

class C
  define_method(:d) {}
end
C.new.d(1,2)       #=> ArgumentError
C.new.method(:d).to_proc.lambda?   #=> true

define_method always defines a method without the tricks, even if a non-lambda Proc object is given. This is the only exception for which the tricks are not preserved.

class C
  define_method(:e, &proc {})
end
C.new.e(1,2)       #=> ArgumentError
C.new.method(:e).to_proc.lambda?   #=> true

This exception insures that methods never have tricks and makes it easy to have wrappers to define methods that behave as usual.

class C
  def self.def2(name, &body)
    define_method(name, &body)
  end

  def2(:f) {}
end
C.new.f(1,2)       #=> ArgumentError

The wrapper def2 defines a method which has no tricks.

Instance Method Details

#call(params,...) ⇒ Object #[](params,...) ⇒ Object #yield(params,...) ⇒ Object

Alias for #[]. Invokes the block with obj as the proc's parameter like #call. It is to allow a proc object to be a target of when clause in a case statement.

#call(params,...) ⇒ Object #[](params,...) ⇒ Object #yield(params,...) ⇒ Object
Also known as: #call, #===, #yield

Invokes the block, setting the block's parameters to the values in params using something close to method calling semantics. Returns the value of the last expression evaluated in the block.

a_proc = Proc.new {|scalar, *values| values.map {|value| value*scalar } }
a_proc.call(9, 1, 2, 3)    #=> [9, 18, 27]
a_proc[9, 1, 2, 3]         #=> [9, 18, 27]
a_proc.(9, 1, 2, 3)        #=> [9, 18, 27]
a_proc.yield(9, 1, 2, 3)   #=> [9, 18, 27]

Note that prc.() invokes prc.call() with the parameters given. It's syntactic sugar to hide “call”.

For procs created using lambda or ->() an error is generated if the wrong number of parameters are passed to the proc. For procs created using .new or Kernel.proc, extra parameters are silently discarded and missing parameters are set to nil.

a_proc = proc {|a,b| [a,b] }
a_proc.call(1)   #=> [1, nil]

a_proc = lambda {|a,b| [a,b] }
a_proc.call(1)   # ArgumentError: wrong number of arguments (given 1, expected 2)

See also #lambda?.

#arity ⇒ Fixnum

Returns the number of mandatory arguments. If the block is declared to take no arguments, returns 0. If the block is known to take exactly n arguments, returns n. If the block has optional arguments, returns -n-1, where n is the number of mandatory arguments, with the exception for blocks that are not lambdas and have only a finite number of optional arguments; in this latter case, returns n. Keywords arguments will considered as a single additional argument, that argument being mandatory if any keyword argument is mandatory. A proc with no argument declarations is the same as a block declaring || as its arguments.

proc {}.arity                  #=>  0
proc { || }.arity              #=>  0
proc { |a| }.arity             #=>  1
proc { |a, b| }.arity          #=>  2
proc { |a, b, c| }.arity       #=>  3
proc { |*a| }.arity            #=> -1
proc { |a, *b| }.arity         #=> -2
proc { |a, *b, c| }.arity      #=> -3
proc { |x:, y:, z:0| }.arity   #=>  1
proc { |*a, x:, y:0| }.arity   #=> -2

proc   { |x=0| }.arity         #=>  0
lambda { |x=0| }.arity         #=> -1
proc   { |x=0, y| }.arity      #=>  1
lambda { |x=0, y| }.arity      #=> -2
proc   { |x=0, y=0| }.arity    #=>  0
lambda { |x=0, y=0| }.arity    #=> -1
proc   { |x, y=0| }.arity      #=>  1
lambda { |x, y=0| }.arity      #=> -2
proc   { |(x, y), z=0| }.arity #=>  1
lambda { |(x, y), z=0| }.arity #=> -2
proc   { |a, x:0, y:0| }.arity #=>  1
lambda { |a, x:0, y:0| }.arity #=> -2

#binding ⇒ Binding

Returns the binding associated with prc. Note that Kernel#eval accepts either a Proc or a ::Binding object as its second parameter.

def fred(param)
  proc {}
end

b = fred(99)
eval("param", b.binding)   #=> 99

#call(params,...) ⇒ Object #[](params,...) ⇒ Object #yield(params,...) ⇒ Object

Alias for #[].

#curry ⇒ Proc #curry(arity) ⇒ Proc

Returns a curried proc. If the optional arity argument is given, it determines the number of arguments. A curried proc receives some arguments. If a sufficient number of arguments are supplied, it passes the supplied arguments to the original proc and returns the result. Otherwise, returns another curried proc that takes the rest of arguments.

b = proc {|x, y, z| (x||0) + (y||0) + (z||0) }
p b.curry[1][2][3]           #=> 6
p b.curry[1, 2][3, 4]        #=> 6
p b.curry(5)[1][2][3][4][5]  #=> 6
p b.curry(5)[1, 2][3, 4][5]  #=> 6
p b.curry(1)[1]              #=> 1

b = proc {|x, y, z, *w| (x||0) + (y||0) + (z||0) + w.inject(0, &:+) }
p b.curry[1][2][3]           #=> 6
p b.curry[1, 2][3, 4]        #=> 10
p b.curry(5)[1][2][3][4][5]  #=> 15
p b.curry(5)[1, 2][3, 4][5]  #=> 15
p b.curry(1)[1]              #=> 1

b = lambda {|x, y, z| (x||0) + (y||0) + (z||0) }
p b.curry[1][2][3]           #=> 6
p b.curry[1, 2][3, 4]        #=> wrong number of arguments (4 for 3)
p b.curry(5)                 #=> wrong number of arguments (5 for 3)
p b.curry(1)                 #=> wrong number of arguments (1 for 3)

b = lambda {|x, y, z, *w| (x||0) + (y||0) + (z||0) + w.inject(0, &:+) }
p b.curry[1][2][3]           #=> 6
p b.curry[1, 2][3, 4]        #=> 10
p b.curry(5)[1][2][3][4][5]  #=> 15
p b.curry(5)[1, 2][3, 4][5]  #=> 15
p b.curry(1)                 #=> wrong number of arguments (1 for 3)

b = proc { :foo }
p b.curry[]                  #=> :foo

#hash ⇒ Integer

Returns a hash value corresponding to proc body.

See also Object#hash.

#to_s ⇒ String #inspect ⇒ String

Alias for #to_s.

#parameters ⇒ Array

Returns the parameter information of this proc.

prc = lambda{|x, y=42, *other|}
prc.parameters  #=> [[:req, :x], [:opt, :y], [:rest, :other]]

#source_location ⇒ Array, Fixnum

Returns the Ruby source filename and line number containing this proc or nil if this proc was not defined in Ruby (i.e. native).

#to_proc ⇒ Proc

Part of the protocol for converting objects to Proc objects. Instances of class Proc simply return themselves.

#to_s ⇒ String Also known as: #inspect

Returns the unique identifier for this proc, along with an indication of where the proc was defined.

#call(params,...) ⇒ Object #[](params,...) ⇒ Object #yield(params,...) ⇒ Object

Alias for #[].