From: David Jacobs <developer@...>
Date: 2011-05-13T23:46:38+09:00
Subject: Re: Scope problem (?) in implementing Design Patterns in Ruby

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Hi Richard,

The & has to be adjacent to "all" or "filename" to act as expected. The &
can mean one of two related things when you pass it to a method:

1. It can mean "pass the lambda expression that this variable points to into
a block". That makes the following to snippets equivalent:

# Snippet 1
filter_function = lambda {|x| x == 2 }
[1, 2, 3, 4, 2].select &filter_function

# Snippet 2
[1, 2, 3, 4, 2].select {|x| x == 2 }

2. It can mean "pass the instance method named by a symbol, and treat it as
a block".

That makes these two equivalent:

# Snippet 1
[1, 2, 3, 4, 2].select {|x| x.even? }

# Snippet 2
[1, 2, 3, 4, 2].select(&:even?)

So what my code is doing is calling the all method, for example, and that
method's sole job is to return a lambda that can then be passed in per the
above. You don't have to wrap the lambda in a method like I did. You could
also just say:

all = lambda {|x| x }

I hope that makes things a little clearer!

Cheers,
David
On Fri, May 13, 2011 at 3:31 AM, RichardOnRails <
RichardDummyMailbox58407@uscomputergurus.com> wrote:

> Hi Dave,
>
> I checked out your Version 2.  Added puts stuff and restriction to
> names of files, which should probably be another one of your methods
> instead of my hard-coding.
> Expanded code: http://www.pastie.org/1895917
> Output: http://www.pastie.org/1895932
>
> I have only one question: I put a space between the & and the method
> following it; the code broke.  Is the construct you used documented on-
> line somewhere?  I've peeked at lambda documentation so I've got the
> drift but I haven't used it in any of my code so far.
>
> I copied your memo on the approach and plan to review it and use/
> expand your code after I finish getting Russ Olson's code working or
> giving up on it.  To his credit,  he posted his code on-line so I can
> avoid typos and he included an apparently thorough set of unit tests.
> So I've got a lot of studying to do.
>
> Again,  thanks for your guidance and great ideas,
> Richard
>
>

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