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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 {|n| n*factor }

times3 = gen_times(3)
times5 = gen_times(5)               #=> 36                #=> 25   #=> 60

Public Class Methods

new {|...| block } → a_proc click to toggle source
new → a_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 = proc_from { "hello" }   #=> "hello"
               static VALUE
rb_proc_s_new(int argc, VALUE *argv, VALUE klass)
    VALUE block = proc_new(klass, FALSE);

    rb_obj_call_init(block, argc, argv);
    return block;

Public Instance Methods

arity → fixnum click to toggle source

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
               static VALUE
proc_arity(VALUE self)
    int arity = rb_proc_arity(self);
    return INT2FIX(arity);
binding → binding click to toggle source

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 {}

b = fred(99)
eval("param", b.binding)   #=> 99
               static VALUE
proc_binding(VALUE self)
    VALUE bindval, envval;
    const rb_proc_t *proc;
    const rb_iseq_t *iseq;
    rb_binding_t *bind;

    GetProcPtr(self, proc);
    envval = rb_vm_proc_envval(proc);
    iseq = proc->block.iseq;
    if (SYMBOL_P(iseq)) goto error;
    if (RUBY_VM_IFUNC_P(iseq)) {
        struct vm_ifunc *ifunc = (struct vm_ifunc *)iseq;
        if (IS_METHOD_PROC_IFUNC(ifunc)) {
            VALUE method = (VALUE)ifunc->data;
            envval = env_clone(envval, method_receiver(method), method_cref(method));
            iseq = rb_method_iseq(method);
        else {
            rb_raise(rb_eArgError, "Can't create Binding from C level Proc");

    bindval = rb_binding_alloc(rb_cBinding);
    GetBindingPtr(bindval, bind);
    bind->env = envval;

    if (iseq) {
        bind->path = iseq->body->location.path;
        bind->first_lineno = FIX2INT(rb_iseq_first_lineno(iseq));
    else {
        bind->path = Qnil;
        bind->first_lineno = 0;

    return bindval;
curry → a_proc click to toggle source
curry(arity) → a_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
               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;

    rb_scan_args(argc, argv, "01", &arity);
    if (NIL_P(arity)) {
        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);
hash → integer click to toggle source

Returns a hash value corresponding to proc body.

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 LONG2FIX(hash);
inspect() click to toggle source
Alias for: to_s
lambda? → true or false click to toggle source

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. {}.lambda?          #=> false

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

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

lambda(&proc {}).lambda?     #=> false
proc(&proc {}).lambda?       #=> false {}).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(& {})              #=> 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,2)       #=> ArgumentError   #=> 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,2)       #=> ArgumentError   #=> 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)

  def2(:f) {}
end,2)       #=> ArgumentError

The wrapper def2 defines a method which has no tricks.

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 unnamed_parameters(rb_proc_arity(self));
    return rb_iseq_parameters(iseq, is_proc);
source_location → [String, Fixnum] 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)

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)
    VALUE str = 0;
    rb_proc_t *proc;
    const char *cname = rb_obj_classname(self);
    const rb_iseq_t *iseq;
    const char *is_lambda;

    GetProcPtr(self, proc);
    iseq = proc->block.iseq;
    is_lambda = proc->is_lambda ? " (lambda)" : "";

    if (RUBY_VM_NORMAL_ISEQ_P(iseq) && rb_iseq_check(iseq)) {
        int first_lineno = 0;

        if (iseq->body->line_info_table) {
            first_lineno = FIX2INT(rb_iseq_first_lineno(iseq));
        str = rb_sprintf("#<%s:%p@%"PRIsVALUE":%d%s>", cname, (void *)self,
                         iseq->body->location.path, first_lineno, is_lambda);
    else if (SYMBOL_P(iseq)) {
        str = rb_sprintf("#<%s:%p(&%+"PRIsVALUE")%s>", cname, (void *)self,
                         (VALUE)iseq, is_lambda);
    else {
        str = rb_sprintf("#<%s:%p%s>", cname, (void *)proc->block.iseq,

    if (OBJ_TAINTED(self)) {
    return str;
Also aliased as: inspect

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