From: Josh Cheek <josh.cheek@...>
Date: 2010-06-17T04:03:23+09:00
Subject: Re: strange behaviour if exploding parameters

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On Wed, Jun 16, 2010 at 9:01 AM, Jarmo Pertman <jarmo.p@gmail.com> wrote:

> Hello!
>
> I've just happened to find strange behaviour when trying to explode
> parameters.
>
> Why doesn't it work?
> irb(main):007:0> lambda {|*parameters| *parameters.class}
> SyntaxError: compile error
> (irb):7: syntax error, unexpected '}', expecting tCOLON2 or '[' or '.'
>        from (irb):7
>        from :0
>
>
> class is Array if 0 or more parameters given
> irb(main):009:0> l = lambda {|*parameters| p *parameters.class}
> => #<Proc:0x04b7e510@(irb):9>
> irb(main):010:0> l[]
> Array
> => nil
>
>
> class is Array if some parameters given
> irb(main):012:0> l = lambda {|*parameters| x = *parameters; p x.class}
> => #<Proc:0x04b2c880@(irb):12>
> irb(main):013:0> l[1,2]
> Array
> => nil
>
>
> class is NilClass instead of Array?
> irb(main):014:0> l[]
> NilClass
> => nil
>
> Any ideas why class is NilClass instead of Array if *parameters is
> assigned to local variable? I'd expect it to be always an Array and if
> no parameters given then an empty Array.
>
> Best Regards,
> Jarmo
> --
> Posted via http://www.ruby-forum.com/.
>
>
Hi, I think you might be a bit confused about how * works in your param
list. It will aggregate all the params into a single array:
lambda { |*params| params }.call 1 , 2 , 3 # => [1, 2, 3]


So you can call class on it
lambda { |*params| params.class }.call 1 , 2 , 3 # => Array


return is a method invocation (at least in my mind) so you can "explode" the
brackets here too, and it becomes like "return 1 , 2 , 3" which is
syntactically the same as "call 1 , 2 , 3"
lambda { |*params| return *params }.call 1 , 2 , 3 # => [1, 2, 3]


But there are two things that don't really make sense. "1 , 2 , 3" all by
itself is not valid Ruby. Leaving it on its own line like this
lambda { |*params| *params }.call 1 , 2 , 3 # => syntax error
Is like saying this
lambda { |*params| 1 , 2 , 3 }.call 1 , 2 , 3 # => syntax error


Since that is not valid, then what does it mean to say *params.class ? Well,
first of all, the .class is evaluated first (not sure how to say that,
"higher binding" maybe?) The class is an Array. so this becomes "*Array"
which is probably not what you want, and only works, again, when passing it
to return.
lambda { |*params| return *params.class }.call 1 , 2 , 3 # => Array
lambda { |*params| *params.class }.call 1 , 2 , 3 # => syntax error




So, in summary, I think this is what you were looking for:
lambda { |*params| params.class }.call 1 , 2 , 3 # => Array

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