From: Brian Candler <b.candler@...>
Date: 2009-06-23T04:36:39+09:00
Subject: Re: map shall not return an Enumerator ( was re guru help )

Robert Dober wrote:
> I believe that the confusion arises from the fact that map returns an
> Enumerator and that just seems quite flawed at second thought (or is
> this third thought ;).
> 
> Why the heck does map return an Enumerator? If I wanted that I surely
> would have called to_enum !

I agree that map and select returning an Enumerator, in the way they do 
in 1.8.7/1.9, is pretty pointless. But if map without a block (and 
select without a block etc) are not useful, but I don't think it helps 
to overload them in the way you want either. I'd rather get an error 
raised, as per 1.8.6.

Aside: what's more interesting to me is "horizontal" execution of 
enumerators - that is, passing each value along instead of building 
intermediate arrays - and thus being able to run map/select on infinite 
lists. See:

http://redmine.ruby-lang.org/issues/show/708
http://redmine.ruby-lang.org/issues/show/707

There's also an implementation of this in the facets library.

In this case, if you write

  infinite.map { |x| x*2 }.select { |x| x % 3 == 0 } ...

then an Enumerator is returned at each stage of the chain. However you 
still need to provide a block to map and a block to select, of course, 
so it's not the same as #map without block returning an Enumerator.

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