From: Brian Candler <B.Candler@...>
Date: 2003-07-22T01:43:46+09:00
Subject: Re: Proposal: Array#to_h, to simplify hash generation

On Tue, Jul 22, 2003 at 01:23:07AM +0900, Simon Strandgaard wrote:
> May I suggest a Enum#build routine, working like this pseudo code
> 
> [1, 2, 3].build(Hash) { |i| i, f(i) }
> #=> {1=>"f(1)", 2=>"f(3)", 3=>"f(3)"}
> 
> [1, 2, 3].build(String) { |i| i.class }
> #=> "FixnumFixnumFixnum"
> 
> [1, 2, 3].build(Array) { |i| Regex.new(i.to_s) }
> #=> [/1/, /2/, /3/]
> 
> Is this madness, appending elements this way ?
> Especialy will this work for hash ?

Hmm, interesting idea: a way to delegate collection to different classes
without having to clutter up Enumerable with collect_hash, collect_string
etc.

If a class implements a standard "add element" method, perhaps <<, then how
about this:

class Array    # repeat for module Enumerable as well
  def collect(dest=Array,*args)
    res = dest.new(*args)
    each do |item|
      res << yield(item)
    end
    res
  end
end

class Hash
  def <<(kvpair)
    self[kvpair[0]] = kvpair[1]
  end
end

a = [["one","two"],["three","four"],["five","six"]]

p a.collect {|e| e[0]}            #>> ["one", "three", "five"]
p a.collect(String,"") {|e| e[1]} #>> "twofoursix"
p a.collect(Hash) {|e| e}         #>> {"five"=>"six", "three"=>"four", "one"=>"two"}

The bit I don't like is Hash#<< taking an array of two elements, rather than
having two arguments, but I guess that's what you get from having a method
rather than a proc.

A minor quibble comes out of this: why doesn't String.new with no arguments
just return an empty string? In 1.6.8 it gives an ArgumentError exception.

Your examples work, with minor modification:

def f(i) i*i; end
p [1, 2, 3].collect(Hash) { |i| [i, f(i)] }
# {1=>1, 2=>4, 3=>9}

p [1, 2, 3].collect(String,"") { |i| i.class.to_s }
# "FixnumFixnumFixnum"

p [1, 2, 3].collect(Array) { |i| Regexp.new(i.to_s) }
# [/1/, /2/, /3/]

Cheers,

Brian.