From: Kirk Haines <khaines@...>
Date: 2005-10-01T23:47:32+09:00
Subject: Re: Lazy evaluaton (was Re: In your opinion....)

On Saturday 01 October 2005 8:16 am, gabriele renzi wrote:

> > FUN = config_memoized ?
> >  Hash.new do |h,k|
> >    # complex calculate val
> >    h[k] = val
> >  end :
> >  lambda do |k|
> >    # complex calculate val
> >  end
> >
> > # later
> >
> > x = FUN[y]
> >
> > Nothing really fancy I guess but it's nice how seemingly these
> > integrate. (Now, is this another thread split?)
>
> could'nt you just use the default_proc in a Hash?
> irb(main):001:0> fun= Hash.new do |h,k|
> irb(main):002:1*  p 'calculations'
> irb(main):003:1>  h[k]= rand
> irb(main):004:1> end

That is what is in the example above.  However, he has the trinary operator in 
his example.  Depending on the value of config_memoized, one either gets a 
hash with a default block, or a lambda, but in use both of them may appear to 
be the same.

a = Hash.new {|h,k| h[k] = (k * 10000007) % 20}
b = lambda {|k| (k * 10000007) % 20}

a[5]
=> 15
b[5]
=> 15


In the original example, it illustrated how, because lambdas implement #[], 
one could have either a memoizing or a nonmeoizing FUN[] depending on a 
configuration value.

Kind of neat,


Kirk Haines