[tlug] Re: Today's TSAC Meeting

[John Fremlin <john@example.com>]

Edward Middleton wrote:
> Y-Combinators were discussed in this meeting and John Fremlin questioned
> whether they they were useful for anything.  Here[1] is an example using
> the Y-Combinator[2] in ruby to implement autovivication in Ruby hashes

I am sure that there are real uses for the Y Combinator. It is important 
in some more primitive languages. We were talking about Haskell which is 
quite advanced.

> 1. http://www.eecs.harvard.edu/~cduan/technical/ruby/ycombinator.shtml

If you looked at this code and actually understood what it was doing, I 
think it is basically this: (well at least it has the same results when 
I ran it)

def make_hash_func()
  Hash.new { |h, k| h[k] = make_hash_func }
end

hash = make_hash_func

# Now this will work:
hash['a']['b']['c']['d']['e']['f']['g'] = 1

       # And hash looks like:
hash # => {"a"=>{"b"=>{"c"=>{"d"=>{"e"=>{"f"=>{"g"=>1}}}}}}}


I don't know much Ruby so please correct me if I am wrong.

> 2. http://en.wikipedia.org/wiki/Y_combinator

The Y Combinator is for making an anonymous recursive function.

I am sure that there are real uses for it. Can you provide an example in 
Haskell? Seeing as it is part of the standard library one would hope 
that it was at least occasionally used.