From: Rick DeNatale <rick.denatale@...>
Date: 2009-06-18T04:32:04+09:00
Subject: Re: lambda and a Proc

On Wed, Jun 17, 2009 at 2:15 PM, aidy<aidy.lewis@googlemail.com> wrote:
> Hi,
>
> I am a bit unsure on what the difference is between a lambda and a
> Proc?
>
> hello = lambda { puts('aidy, aidy, aidy' ) }
> hello.call
>
> hello = Proc.new{ puts('aidy, aidy, aidy' ) }
> hello.call

Well no differences that affect the above code, but

  a return in a lambda block causes the block to evaluate to the value
of the return, in a proc block it causes a return from the method
where the block was created.
rb(main):011:0> def foo_lambda
irb(main):012:1>   lambda {return 42}
irb(main):013:1> end
=> nil
irb(main):015:0> foo_lambda.call
=> 42

irb(main):028:0> def bar_lambda
irb(main):029:1>   p = lambda {return 42}
irb(main):030:1>   puts "calling p #{p.call}"
irb(main):031:1>   puts "back from call"
irb(main):032:1>   p
irb(main):033:1> end
=> nil
irb(main):034:0> bar_lambda.call
calling p 42
back from call
=> 42


irb(main):005:0> def bar
irb(main):006:1>   p = Proc.new {return 42}
irb(main):007:1>   p.call
irb(main):008:1>   puts "after call"
irb(main):009:1> end
=> nil
irb(main):010:0> bar
=> 42

rb(main):001:0> def foo
irb(main):002:1>   Proc.new {return 42}
irb(main):003:1> end
=> nil
irb(main):004:0> foo.call
LocalJumpError: unexpected return
	from (irb):2:in `foo'
	from (irb):4:in `call'
	from (irb):4

And in Ruby 1.9 lambdas raise an ArgumentError when called with the
wrong number of arguments, a Proc will set missing arguments to nil,
and ignore extra arguments.

The way I think of this is that lambdas act more like nameless methods.

-- 
Rick DeNatale

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