Class: Proc

Inherits:
Object show all
Defined in:
proc.c,
proc.c

Overview

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 Proc.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.

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.new {|...| ... } ⇒ Proc .new ⇒ Proc

Creates a new Proc object, bound to the current context. Proc::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"

Overloads:

  • .new {|...| ... } ⇒ Proc

    Yields:

    • (...)

    Returns:

  • .new ⇒ Proc

    Returns:



827
828
829
830
831
832
833
834
 
# File 'proc.c', line 827

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;
}

Instance Method Details

#<<(g) ⇒ Proc

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 Proc#>> for detailed explanations.

Returns:



3366
3367
3368
3369
3370
 
# File 'proc.c', line 3366

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

#===(*args) ⇒ Object

call-seq:

proc === obj   -> result_of_proc

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



926
927
928
929
930
 
# File 'proc.c', line 926

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

#>>(g) ⇒ Proc

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 Method#>> and Method#<<.

Returns:



3419
3420
3421
3422
3423
 
# File 'proc.c', line 3419

static VALUE
proc_compose_to_right(VALUE self, VALUE g)
{
    return rb_proc_compose_to_right(self, to_callable(g));
}

#[](*args) ⇒ Object

call-seq:

prc.call(params,...)   -> obj
prc[params,...]        -> obj
prc.(params,...)       -> obj
prc.yield(params,...)  -> obj

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 Proc.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 Proc#lambda?.



926
927
928
929
930
 
# File 'proc.c', line 926

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

#arity ⇒ Integer

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. Keyword arguments will be 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   { |a=0| }.arity         #=>  0
lambda { |a=0| }.arity         #=> -1
proc   { |a=0, b| }.arity      #=>  1
lambda { |a=0, b| }.arity      #=> -2
proc   { |a=0, b=0| }.arity    #=>  0
lambda { |a=0, b=0| }.arity    #=> -1
proc   { |a, b=0| }.arity      #=>  1
lambda { |a, b=0| }.arity      #=> -2
proc   { |(a, b), c=0| }.arity #=>  1
lambda { |(a, b), c=0| }.arity #=> -2
proc   { |a, x:0, y:0| }.arity #=>  1
lambda { |a, x:0, y:0| }.arity #=> -2

Returns:



1053
1054
1055
1056
1057
1058
 
# File 'proc.c', line 1053

static VALUE
proc_arity(VALUE self)
{
    int arity = rb_proc_arity(self);
    return INT2FIX(arity);
}

#binding ⇒ Binding

Returns the binding associated with prc.

def fred(param)
  proc {}
end

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

Returns:



3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
 
# File 'proc.c', line 3106

static VALUE
proc_binding(VALUE self)
{
    VALUE bindval, binding_self = Qundef;
    rb_binding_t *bind;
    const rb_proc_t *proc;
    const rb_iseq_t *iseq = NULL;
    const struct rb_block *block;
    const rb_env_t *env = NULL;

    GetProcPtr(self, proc);
    block = &proc->block;

  again:
    switch (vm_block_type(block)) {
      case block_type_iseq:
	iseq = block->as.captured.code.iseq;
	binding_self = block->as.captured.self;
	env = VM_ENV_ENVVAL_PTR(block->as.captured.ep);
	break;
      case block_type_proc:
	GetProcPtr(block->as.proc, proc);
	block = &proc->block;
	goto again;
      case block_type_symbol:
	goto error;
      case block_type_ifunc:
	{
	    const struct vm_ifunc *ifunc = block->as.captured.code.ifunc;
	    if (IS_METHOD_PROC_IFUNC(ifunc)) {
		VALUE method = (VALUE)ifunc->data;
		VALUE name = rb_fstring_lit("<empty_iseq>");
		rb_iseq_t *empty;
		binding_self = method_receiver(method);
		iseq = rb_method_iseq(method);
		env = VM_ENV_ENVVAL_PTR(block->as.captured.ep);
		env = env_clone(env, method_cref(method));
		/* set empty iseq */
		empty = rb_iseq_new(NULL, name, name, Qnil, 0, ISEQ_TYPE_TOP);
		RB_OBJ_WRITE(env, &env->iseq, empty);
		break;
	    }
	    else {
	      error:
		rb_raise(rb_eArgError, "Can't create Binding from C level Proc");
		return Qnil;
	    }
	}
    }

    bindval = rb_binding_alloc(rb_cBinding);
    GetBindingPtr(bindval, bind);
    RB_OBJ_WRITE(bindval, &bind->block.as.captured.self, binding_self);
    RB_OBJ_WRITE(bindval, &bind->block.as.captured.code.iseq, env->iseq);
    rb_vm_block_ep_update(bindval, &bind->block, env->ep);
    RB_OBJ_WRITTEN(bindval, Qundef, VM_ENV_ENVVAL(env->ep));

    if (iseq) {
	rb_iseq_check(iseq);
	RB_OBJ_WRITE(bindval, &bind->pathobj, iseq->body->location.pathobj);
	bind->first_lineno = FIX2INT(rb_iseq_first_lineno(iseq));
    }
    else {
	RB_OBJ_WRITE(bindval, &bind->pathobj,
		     rb_iseq_pathobj_new(rb_fstring_lit("(binding)"), Qnil));
	bind->first_lineno = 1;
    }

    return bindval;
}

#call(*args) ⇒ Object

call-seq:

prc.call(params,...)   -> obj
prc[params,...]        -> obj
prc.(params,...)       -> obj
prc.yield(params,...)  -> obj

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 Proc.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 Proc#lambda?.



926
927
928
929
930
 
# File 'proc.c', line 926

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

#clone ⇒ Object

:nodoc:



163
164
165
166
167
168
169
 
# File 'proc.c', line 163

static VALUE
proc_clone(VALUE self)
{
    VALUE procval = rb_proc_dup(self);
    CLONESETUP(procval, self);
    return procval;
}

#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 (given 4, expected 3)
p b.curry(5)                 #=> wrong number of arguments (given 5, expected 3)
p b.curry(1)                 #=> wrong number of arguments (given 1, expected 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 (given 1, expected 3)

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

Overloads:



3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
 
# File 'proc.c', line 3261

static VALUE
proc_curry(int argc, const VALUE *argv, VALUE self)
{
    int sarity, max_arity, min_arity = rb_proc_min_max_arity(self, &max_arity);
    VALUE arity;

    if (rb_check_arity(argc, 0, 1) == 0 || NIL_P(arity = argv[0])) {
	arity = INT2FIX(min_arity);
    }
    else {
	sarity = FIX2INT(arity);
	if (rb_proc_lambda_p(self)) {
	    rb_check_arity(sarity, min_arity, max_arity);
	}
    }

    return make_curry_proc(self, rb_ary_new(), arity);
}

#dup ⇒ Object 



919
920
921
922
923
924
925
926
927
928
929
 
# File 'vm.c', line 919

VALUE
rb_proc_dup(VALUE self)
{
    VALUE procval;
    rb_proc_t *src;

    GetProcPtr(self, src);
    procval = proc_create(rb_cProc, &src->block, src->is_from_method, src->is_lambda);
    RB_GC_GUARD(self); /* for: body = rb_proc_dup(body) */
    return procval;
}

#hash ⇒ Integer

Returns a hash value corresponding to proc body.

See also Object#hash.

Returns:



1349
1350
1351
1352
1353
1354
1355
1356
1357
 
# File 'proc.c', line 1349

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);
}

#lambda? ⇒ Boolean

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

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

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

Proc.new is the same as proc.

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

lambda, proc and Proc.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.

Returns:

  • (Boolean) 


274
275
276
277
278
279
280
281
 
# File 'proc.c', line 274

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

    return proc->is_lambda ? Qtrue : Qfalse;
}

#parameters ⇒ Array

Returns the parameter information of this proc.

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

Returns:



1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
 
# File 'proc.c', line 1290

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

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

Returns:



3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
 
# File 'proc.c', line 3526

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 ⇒ Array, Integer

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

Returns ].

Returns:



1255
1256
1257
1258
1259
 
# File 'proc.c', line 1255

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

#to_proc ⇒ Proc

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

Returns:



1415
1416
1417
1418
1419
 
# File 'proc.c', line 1415

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

#to_s ⇒ String Also known as: inspect

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

Returns:



1399
1400
1401
1402
1403
1404
1405
 
# File 'proc.c', line 1399

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);
}

#yield(*args) ⇒ Object

call-seq:

prc.call(params,...)   -> obj
prc[params,...]        -> obj
prc.(params,...)       -> obj
prc.yield(params,...)  -> obj

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 Proc.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 Proc#lambda?.



926
927
928
929
930
 
# File 'proc.c', line 926

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