From: "trans.  (T. Onoma)" <transami@...>
Date: 2004-09-19T23:20:08+09:00
Subject: Re: Array#index block and rdetect

On Sunday 19 September 2004 07:59 am, Robert Klemme wrote:
> "trans. (T. Onoma)" <transami@runbox.com> schrieb im Newsbeitrag
> news:200409190606.22327.transami@runbox.com...
>
> > Tiny RCR suggestion:
> >
> > Array#index optionally take a block like Enumerable#detect does.
> >
> >    ['a','b','c'].index{|e| e == 'b'}  # => 1
> >    ['a','b','c'].index{|e| e == 'c'}  # => 2
> >    ['a','a','a'].index{|e| e == 'a'}  # => 0
> >    ['a','a','a'].index{|e| e == 'b'}  # => nil

But this Array#index {block) is acceptable, yes?

> > Question: why isn't #index a method of Enumerable, considering
> > #each_with_index is? (FYI: 'alias find detect'.)
>
> Because not all collections keep a definite order as Array does.  For
> example, hashes do not keep any certain order of elements hence indexing
> does not make any sense.

Couldn't the index of a hash be considered the key? So the block of #index 
when applied is the value. i.e.

    {:a=>'a',:b=>'b',:c=>'b'}.index{|e| e == 'b'}  # => :b
  
But then, of course, there would be no telling which key you might get. 
Suppose that could be considered a problem.

> > Lastly, what about an rdetect?
>
> Not in Enumerable because each only iterates in one direction.  (Ok, you
> could go through it an output the last but that would not work for
> Enumerables that return infinite elements.)

I see.

Hmm... sometimes it seems like these are quite categorized as finely as they 
might be.

T.

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