lambda?

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

A Proc object generated by proc ignore extra arguments.

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

It provides nil for lacked arguments.

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

It expand 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.

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

Proc.new is same as proc.

Proc.new {}.lambda?        => false

lambda, proc and Proc.new preserves 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 same as method definition. The defined method has no tricks.

class C
  define_method(:d) {}
end
C.new.e(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 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 is for a wrapper of define_method. It eases defining a method defining method which defines a usual method which has no tricks.

class << C
  def def2(name, &body)
    define_method(name, &body)
  end
end
class C
  def2(:f) {}
end
C.new.f(1,2)       => ArgumentError

The wrapper, def2, defines a method which has no tricks.

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

    return proc->is_lambda ? Qtrue : Qfalse;
}