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#ruby2_keywords
#source_location
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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 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, 2]
[[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 con 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.

Public Class Methods

new {|...| block } → a_proc click to toggle source

new → a_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"

 
               static VALUE
rb_proc_s_new(int argc, VALUE *argv, VALUE klass)
{
    VALUE block = proc_new(klass, FALSE, FALSE);

    rb_obj_call_init_kw(block, argc, argv, RB_PASS_CALLED_KEYWORDS);
    return block;
}

Public Instance Methods

prc << g → a_proc click to toggle source

Returns a proc that is the composition of this proc and the given g. The returned proc takes a variable number of arguments, calls g with them then calls this proc with the result.

f = proc {|x| x * x }
g = proc {|x| x + x }
p (f << g).call(2) #=> 16

See #>> for detailed explanations.

 
               static VALUE
proc_compose_to_left(VALUE self, VALUE g)
{
    return rb_proc_compose_to_left(self, to_callable(g));
}

proc === obj → result_of_proc click to toggle source

Invokes the block with obj as the proc's parameter like #call. This allows a proc object to be the target of a when clause in a case statement.

 
               static VALUE
proc_call(int argc, VALUE *argv, VALUE procval)
{
    /* removed */
}

prc >> g → a_proc click to toggle source

Returns a proc that is the composition of this proc and the given g. The returned proc takes a variable number of arguments, calls this proc with them then calls g with the result.

f = proc {|x| x * x }
g = proc {|x| x + x }
p (f >> g).call(2) #=> 8

g could be other Proc, or Method, or any other object responding to call method:

class Parser
  def self.call(text)
     # ...some complicated parsing logic...
  end
end

pipeline = File.method(:read) >> Parser >> proc { |data| puts "data size: #{data.count}" }
pipeline.call('data.json')

See also Object#hash.

 
               static VALUE
proc_hash(VALUE self)
{
    st_index_t hash;
    hash = rb_hash_start(0);
    hash = rb_hash_proc(hash, self);
    hash = rb_hash_end(hash);
    return ST2FIX(hash);
}

inspect() click to toggle source

Alias for: to_s

lambda? → true or false click to toggle source

Returns true if a Proc object is lambda. false if non-lambda.

The lambda-ness affects argument handling and the behavior of return and break.

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 ensures 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.

 
               VALUE
rb_proc_lambda_p(VALUE procval)
{
    rb_proc_t *proc;
    GetProcPtr(procval, proc);

    return proc->is_lambda ? Qtrue : Qfalse;
}

parameters → array click to toggle source

Returns the parameter information of this proc.

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

 
               static VALUE
rb_proc_parameters(VALUE self)
{
    int is_proc;
    const rb_iseq_t *iseq = rb_proc_get_iseq(self, &is_proc);
    if (!iseq) {
        return rb_unnamed_parameters(rb_proc_arity(self));
    }
    return rb_iseq_parameters(iseq, is_proc);
}

ruby2_keywords → proc click to toggle source

Marks the proc as passing keywords through a normal argument splat. This should only be called on procs that accept an argument splat (*args) but not explicit keywords or a keyword splat. It marks the proc such that if the proc is called with keyword arguments, the final hash argument is marked with a special flag such that if it is the final element of a normal argument splat to another method call, and that method call does not include explicit keywords or a keyword splat, the final element is interpreted as keywords. In other words, keywords will be passed through the proc to other methods.

This should only be used for procs that delegate keywords to another method, and only for backwards compatibility with Ruby versions before 2.7.

This method will probably be removed at some point, as it exists only for backwards compatibility. As it does not exist in Ruby versions before 2.7, check that the proc responds to this method before calling it. Also, be aware that if this method is removed, the behavior of the proc will change so that it does not pass through keywords.

module Mod
  foo = ->(meth, *args, &block) do
    send(:"do_#{meth}", *args, &block)
  end
  foo.ruby2_keywords if foo.respond_to?(:ruby2_keywords)
end

 
               static VALUE
proc_ruby2_keywords(VALUE procval)
{
    rb_proc_t *proc;
    GetProcPtr(procval, proc);

    rb_check_frozen(procval);

    if (proc->is_from_method) {
            rb_warn("Skipping set of ruby2_keywords flag for proc (proc created from method)");
            return procval;
    }

    switch (proc->block.type) {
      case block_type_iseq:
        if (proc->block.as.captured.code.iseq->body->param.flags.has_rest &&
                !proc->block.as.captured.code.iseq->body->param.flags.has_kw &&
                !proc->block.as.captured.code.iseq->body->param.flags.has_kwrest) {
            proc->block.as.captured.code.iseq->body->param.flags.ruby2_keywords = 1;
        }
        else {
            rb_warn("Skipping set of ruby2_keywords flag for proc (proc accepts keywords or proc does not accept argument splat)");
        }
        break;
      default:
        rb_warn("Skipping set of ruby2_keywords flag for proc (proc not defined in Ruby)");
        break;
    }

    return procval;
}

source_location → [String, Integer] click to toggle source

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

 
               VALUE
rb_proc_location(VALUE self)
{
    return iseq_location(rb_proc_get_iseq(self, 0));
}

to_proc → proc click to toggle source

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

 
               static VALUE
proc_to_proc(VALUE self)
{
    return self;
}

to_s → string click to toggle source

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

 
               static VALUE
proc_to_s(VALUE self)
{
    const rb_proc_t *proc;
    GetProcPtr(self, proc);
    return rb_block_to_s(self, &proc->block, proc->is_lambda ? " (lambda)" : NULL);
}

Also aliased as: inspect

yield(params,...) → obj click to toggle source

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".

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)