From: Eric Hodel <drbrain@...7.net>
Date: 2005-10-27T16:54:34+09:00
Subject: Re: Enumerable#build

On Oct 26, 2005, at 4:47 AM, Robert Klemme wrote:

> Martin DeMello wrote:
>
>> class Enumerable
>>   def build(seed)
>>     each {|i|
>>       seed.add(yield i)
>>     }
>>   end
>> end
>>
>> and
>>
>> Array#build == Array#<<
>> Hash#build(k,v) == Hash#[]
>> etc
>>
>> So you could say [1,2,3].build({}) {|i| [i, 1]} for the question that
>> prompted the to_hash thread, but also, in general, you can collect
>> into non-array structures (binary trees, e.g.) as long as they
>> implement a suitable #add method.
>>
>> (Of course, you can do ary.inject(seed) {|i| seed.add(f(i)); seed}
>> right now but a #build method makes the common case pretty.)
>>
>
> Looks good.  But,
>
>  - Enumerable is a module... :-)
>
>  - You should add seed as return value.
>
>  - Your first example won't work as Hash#add is nonexistent
>
> An alternative approach would be to reverse the logic and have
> Enumerable#populate:
>
> module Enumerable
>   def populate(*enum)
>     enum = enum.first if enum.size == 1
>     enum.each {|*a| add(yield(*a))}
>     self
>   end
> end
>
> class Hash
>   def populate(*enum)
>     enum = enum.first if enum.size == 1
>     enum.each {|*a| send(:[]=, *yield(*a))}
>     self
>   end
> end
>
>
>>> h={}.populate(1,2,3) {|x| [x, "val #{x}"]}
>>>
> => {1=>"val 1", 2=>"val 2", 3=>"val 3"}
>
> What do you think?

I think you have too much magic.

Enumerable#to_a, Array#fill, Array#flatten and Hash::[] do this  
pretty handily and much more cleanly:

Hash[*ary.fill { |i| [i, "val #{i}"]}.flatten]

$ ruby -e 'p Hash[*(0..3).to_a.fill { |i| [i, "val #{i}"] }.flatten ]'
{0=>"val 0", 1=>"val 1", 2=>"val 2", 3=>"val 3"}

-- 
Eric Hodel - drbrain@segment7.net - http://segment7.net
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