From: Robert Klemme <shortcutter@...>
Date: 2007-11-16T19:23:21+09:00
Subject: Re: Shadowing bug??

2007/11/15, Vasco Andrade e Silva <vascoas@gmail.com>:
> Robert Klemme wrote:
> > 2007/11/14, Vasco Andrade e Silva <vascoas@gmail.com>:
> >> >
> >> 1
> >>
> >> > f_ko({:xpto => "ohoh"})
> >> :in_lambda
> >> "ohoh"
> >> {:xpto=>"ohoh"}
> >> "ohoh"
> >> {:xpto=>"ohoh"} # :-)
> >
> > Just let me add that even though this is fixed IMHO it is a bad idea
> > to use the same identifier in both cases if you want to keep them
> > separate.  This can likely cause confusion.
>
> I don't agreed with you. Shadowing, IMHO, is a very useful (and
> powerful) way to express semantic and execute "best practices".
>
> Example:
> Imagine you want to process a tree (as i wanted). Suppose that in the
> process of that computation you call "defn_to_sexp". This method
> receives a node (tagged with :defn).
> In this case i wanted to do a "special" process to the args branch of
> the tree. So, and because i didn't want to declare another function
> visible from class scope, i created a proc (process_args). This proc
> should receive the tree branch (i.e. "node") and other variable witch
> represents a computation over some other branch of defn's node.
>
> Here goes a simple sample:
> def defn_to_sexp(node)
>   process_args = lambda do |n, has_block|
>     ...
>   end
>   ...
>   has_block = process(...)
>   process_args.call(args_node(node), hash_block)
>   ...
> end
>
> i have more semantic if i can call the word "node" instead of the word
> "n" in lambda's arg: I want to shadow the node variable.
> This is a case, IMHO, where it's a good idea "to use the same identifier
> in both cases".
>
> In this case i would wanted to write:
> def defn_to_sexp(node)
>   process_args = lambda do |node, has_block|
>     ... # i don't want to access to the outer node
>         # and "node" says more than "n"
>   end
>   ...
>   has_block = process(...)
>   process_args.call(args_node(node), hash_block)
>   ...
> end

IMHO you are abusing a lambda to get a nested method.  This likely
also has performance impacts since the block has to be recreated on
every invocation of the method.  In this case I'd rather use a private
helper method.

But I can see how you would want to use the same identifier in some
cases.  I can only speak for myself: I did not feel the urge to do
this (yet).

Cheers

robert

-- 
use.inject do |as, often| as.you_can - without end