Class: Proc

A Proc object is an encapsulation of a block of code, which can be stored in a local variable, passed to a method or another Proc, and can be called. Proc is an essential concept in Ruby and a core of its functional programming features.

square = Proc.new {|x| x**2 }

square.call(3)  #=> 9
# shorthands:
square.(3)      #=> 9
square[3]       #=> 9

Proc objects are closures, meaning they remember and can use the entire context in which they were created.

def gen_times(factor)
  Proc.new {|n| n*factor } # remembers the value of factor at the moment of creation
end

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

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

Creation

There are several methods to create a Proc

• Use the Proc class constructor:

  proc1 = Proc.new {|x| x**2 }

• Use the Kernel.proc method as a shorthand of .new:

  proc2 = proc {|x| x**2 }

• Receiving a block of code into proc argument (note the &):

  def make_proc(&block)
    block
  end
  
  proc3 = make_proc {|x| x**2 }

• Construct a proc with lambda semantics using the Kernel.lambda method (see below for explanations about lambdas):

  lambda1 = lambda {|x| x**2 }

• Use the Lambda proc literal syntax (also constructs a proc with lambda semantics):

  lambda2 = ->(x) { x**2 }

Lambda and non-lambda semantics

Procs are coming in two flavors: lambda and non-lambda (regular procs). Differences are:

• In lambdas, return and break means exit from this lambda;

• In non-lambda procs, return means exit from embracing method (and will throw LocalJumpError if invoked outside the method);

• In non-lambda procs, break means exit from the method which the block given for. (and will throw LocalJumpError if invoked after the method returns);

• In lambdas, arguments are treated in the same way as in methods: strict, with ArgumentError for mismatching argument number, and no additional argument processing;

• Regular procs accept arguments more generously: missing arguments are filled with nil, single Array arguments are deconstructed if the proc has multiple arguments, and there is no error raised on extra arguments.

Examples:

# return in non-lambda proc, b, exits m2.
# (The block +{ return }+ is given for m1 and embraced by m2.)
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1 { return }; $a << :m2 end; m2; p $a
#=> []

# break in non-lambda proc, b, exits m1.
# (The block +{ break }+ is given for m1 and embraced by m2.)
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1 { break }; $a << :m2 end; m2; p $a
#=> [:m2]

# next in non-lambda proc, b, exits the block.
# (The block +{ next }+ is given for m1 and embraced by m2.)
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1 { next }; $a << :m2 end; m2; p $a
#=> [:m1, :m2]

# Using proc method changes the behavior as follows because
# The block is given for proc method and embraced by m2.
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&proc { return }); $a << :m2 end; m2; p $a
#=> []
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&proc { break }); $a << :m2 end; m2; p $a
# break from proc-closure (LocalJumpError)
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&proc { next }); $a << :m2 end; m2; p $a
#=> [:m1, :m2]

# return, break and next in the stubby lambda exits the block.
# (lambda method behaves same.)
# (The block is given for stubby lambda syntax and embraced by m2.)
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&-> { return }); $a << :m2 end; m2; p $a
#=> [:m1, :m2]
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&-> { break }); $a << :m2 end; m2; p $a
#=> [:m1, :m2]
$a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&-> { next }); $a << :m2 end; m2; p $a
#=> [:m1, :m2]

p = proc {|x, y| "x=#{x}, y=#{y}" }
p.call(1, 2)      #=> "x=1, y=2"
p.call([1, 2])    #=> "x=1, y=2", array deconstructed
p.call(1, 2, 8)   #=> "x=1, y=2", extra argument discarded
p.call(1)         #=> "x=1, y=", nil substituted instead of error

l = lambda {|x, y| "x=#{x}, y=#{y}" }
l.call(1, 2)      #=> "x=1, y=2"
l.call([1, 2])    # ArgumentError: wrong number of arguments (given 1, expected 2)
l.call(1, 2, 8)   # ArgumentError: wrong number of arguments (given 3, expected 2)
l.call(1)         # ArgumentError: wrong number of arguments (given 1, expected 2)

def test_return
  #=> { return 3 }.call      # just returns from lambda into method body
  proc { return 4 }.call    # returns from method
  return 5
end

test_return # => 4, return from proc

Lambdas are useful as self-sufficient functions, in particular useful as arguments to higher-order functions, behaving exactly like Ruby methods.

Procs are useful for implementing iterators:

def test
  [[1, 2], [3, 4], [5, 6]].map {|a, b| return a if a + b > 10 }
                            #  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
end

Inside map, the block of code is treated as a regular (non-lambda) proc, which means that the internal arrays will be deconstructed to pairs of arguments, and return will exit from the method test. That would not be possible with a stricter lambda.

You can tell a lambda from a regular proc by using the #lambda? instance method.

Lambda semantics is typically preserved during the proc lifetime, including &-deconstruction to a block of code:

p = proc {|x, y| x }
l = lambda {|x, y| x }
[[1, 2], [3, 4]].map(&p) #=> [1, 3]
[[1, 2], [3, 4]].map(&l) # ArgumentError: wrong number of arguments (given 1, expected 2)

The only exception is dynamic method definition: even if defined by passing a non-lambda proc, methods still have normal semantics of argument checking.

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

This exception ensures that methods never have unusual argument passing conventions, and makes it easy to have wrappers defining 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 receives body as a non-lambda proc, yet defines a method which has normal semantics.

Conversion of other objects to procs

Any object that implements the #to_proc method can be converted into a proc by the & operator, and therefore can be consumed by iterators.

class Greeter
  def initialize(greeting)
    @greeting = greeting
  end

  def to_proc
    proc {|name| "#{@greeting}, #{name}!" }
  end
end

hi = Greeter.new("Hi")
hey = Greeter.new("Hey")
["Bob", "Jane"].map(&hi)    #=> ["Hi, Bob!", "Hi, Jane!"]
["Bob", "Jane"].map(&hey)   #=> ["Hey, Bob!", "Hey, Jane!"]

Of the Ruby core classes, this method is implemented by ::Symbol, ::Method, and ::Hash.

:to_s.to_proc.call(1)           #=> "1"
[1, 2].map(&:to_s)              #=> ["1", "2"]

method(:puts).to_proc.call(1)   # prints 1
[1, 2].each(&method(:puts))     # prints 1, 2

{test: 1}.to_proc.call(:test)       #=> 1
%i[test many keys].map(&{test: 1})  #=> [1, nil, nil]

Orphaned Proc

return and break in a block exit a method. If a Proc object is generated from the block and the Proc object survives until the method is returned, return and break cannot work. In such case, return and break raises ::LocalJumpError. A Proc object in such situation is called as orphaned Proc object.

Note that the method to exit is different for return and break. There is a situation that orphaned for break but not orphaned for return.

def m1(&b) b.call end; def m2(); m1 { return } end; m2 # ok
def m1(&b) b.call end; def m2(); m1 { break } end; m2 # ok

def m1(&b) b end; def m2(); m1 { return }.call end; m2 # ok
def m1(&b) b end; def m2(); m1 { break }.call end; m2 # LocalJumpError

def m1(&b) b end; def m2(); m1 { return } end; m2.call # LocalJumpError
def m1(&b) b end; def m2(); m1 { break } end; m2.call # LocalJumpError

Since return and break exits the block itself in lambdas, lambdas cannot be orphaned.

Numbered parameters

Numbered parameters are implicitly defined block parameters intended to simplify writing short blocks:

# Explicit parameter:
%w[test me please].each { |str| puts str.upcase } # prints TEST, ME, PLEASE
(1..5).map { |i| i**2 } # => [1, 4, 9, 16, 25]

# Implicit parameter:
%w[test me please].each { puts _1.upcase } # prints TEST, ME, PLEASE
(1..5).map { _1**2 } # => [1, 4, 9, 16, 25]

Parameter names from _1 to _9 are supported:

[10, 20, 30].zip([40, 50, 60], [70, 80, 90]).map { _1 + _2 + _3 }
# => [120, 150, 180]

Though, it is advised to resort to them wisely, probably limiting yourself to _1 and _2, and to one-line blocks.

Numbered parameters can’t be used together with explicitly named ones:

[10, 20, 30].map { |x| _1**2 }
# SyntaxError (ordinary parameter is defined)

To avoid conflicts, naming local variables or method arguments _1, _2 and so on, causes a warning.

_1 = 'test'
# warning: `_1' is reserved as numbered parameter

Using implicit numbered parameters affects block’s arity:

p = proc { _1 + _2 }
l = lambda { _1 + _2 }
p.parameters     # => [[:opt, :_1], [:opt, :_2]]
p.arity          # => 2
l.parameters     # => [[:req, :_1], [:req, :_2]]
l.arity          # => 2

Blocks with numbered parameters can’t be nested:

%w[test me].each { _1.each_char { p _1 } }
# SyntaxError (numbered parameter is already used in outer block here)
# %w[test me].each { _1.each_char { p _1 } }
#                    ^~

Numbered parameters were introduced in Ruby 2.7.