From: "David A. Black" <dblack@...>
Date: 2009-07-08T20:29:37+09:00
Subject: Re: Symbol#to_proc with arguments

Hi --

On Wed, 8 Jul 2009, James Coglan wrote:

> 2009/7/8 Max Williams <toastkid.williams@gmail.com>
>
>> hey folks
>>
>> I've often wondered if i can use Symbol#to_proc with arguments - reading
>> Dave Thomas's explanation on this page
>> http://pragdave.pragprog.com/pragdave/2005/11/symbolto_proc.html it
>> looks like i would be able to do this:
>>
>>>> array = %w(apple banana peach plum pear)
>> => ["apple", "banana", "peach", "plum", "pear"]
>>>> array.collect(&:split, 'l')
>> SyntaxError: compile error
>> (irb):12: syntax error, unexpected ',', expecting ')'
>> array.collect(&:split, 'l')
>>                      ^
>> (irb):12: syntax error, unexpected ')', expecting $end
>>  from (irb):12
>>
>> But it fails as you can see.  Trying to fully grasp what's going on with
>> Symbol#to_proc makes my head hurt a little bit (it is only 9.16am here)
>> and i'm clearly not getting it.  Can anyone explain?
>
>
>
> The block argument (prefixed with '&') must be the last argument in the list
> when calling a method, so passing 'l' in your example is not valid syntax.
> You could rewrite Symbol#to_proc to take arguments, and call #to_proc
> explicitly when calling #collect.
>
> class Symbol
>  def to_proc(*args)
>    lambda { |o| o.__send__(self, *args) }
>  end
> end
>
> %w[apple banana peach plum pear].collect &:split.to_proc('l')
> #=> [["app", "e"], ["banana"], ["peach"], ["p", "um"], ["pear"]]

That's going to be incomprehensible, except by guesswork, to anyone
who doesn't know that you've modified Symbol. It also undoes whatever
benefit you might get from the &obj idiom automatically calling
#to_proc. I'd just do:

   array.map {|e| e.split('l') }

No real reason to second-guess Ruby on this. If you really want to
modify Symbol, creating an entirely new method would probably be
better:

   array.map(&:split.to_argument_accepting_proc('l'))

though either way I admit it doesn't feel necessary or adviseable to
me.


David

-- 
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