From: Rick DeNatale <rick.denatale@...>
Date: 2009-11-11T04:45:55+09:00
Subject: Re: RCR enumerable extra into core

On Tue, Nov 10, 2009 at 10:47 AM, Brian Candler <b.candler@pobox.com> wrote:
> Roger Pack wrote:
>> we currently have
>> list.map => Enumerator
>>
>> so should
>> list.map(:method)
>>
>> best return an array or an enumerator?
>
> The ultimate conclusion of this: all Enumerable methods should return
> other Enumerators. When you want a real array, then add .to_a to the
> end.

And you base this ultimate conclusion on what?

Prior to Ruby 1.9, most Enumerable methods required that a block be
passed and would raise an error if not. There are  few, such as to_a
which did not.

If you wanted an enumerator you needed to do something like

(1..100).enum_for(:inject, 0)

In Ruby 1.9 the enumerable methods got a little more relaxed, they
work as before if a block IS given, and if not they return an
enumerator instead.

So, my ultimate conclusion is that enumerable methods should almost
always use a block passed as an argument to do whatever they do, and
for Ruby 1.9 at least return an enumerator if a block is not passed
which will do whatever the enumerable itself would do with a block.

And a Ruby method can't distinguish wheter it was called with syntax like

   some_method { # some block
                        }


  or

   some_method(&something_which_understands_to_proc)

In both cases block_given? inside the method invocation will return true.

Now as I understand it, the proposal would be to make

  list.map(:method)

do the same thing as

 list.map(&:method)

One problem with this is what to do with something like

 list.inject(:initial_value)

Where instead of a proc, :initial_value represents the initial value
for an enumerator.  I would argue that only the caller knows which one
he/she wants.

That "&" is pretty important in distinguishing these two cases.



-- 
Rick DeNatale

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