[Haskell-beginners] Pattern matching over functions

[Felipe Almeida Lessa]

On Sun, Dec 11, 2011 at 8:09 PM, Graham Gill <math.simplex at gmail.com> wrote:
> Excellent, thanks Daniel and Felipe.
>
> We don't even need to invoke infinite or undecidable problems, since it's
> easy to construct equal functions for which determining equality would be
> prohibitively expensive. If then you can only check equality for some
> functions, because you want compilation to finish, say, within the
> programmer's lifetime, you lose referential transparency.

Note that the time isn't spent on compilation time, but on run time!
Which is actually worse ;-).

Also note that it is possible to imagine something like

  obviouslyEqual :: a -> a -> Bool

where 'obviouslyEqual x y' is 'True' when it's easy to see that they
are equal or 'False' if you can't decide.  Actually, with GHC you may
say

  {-# LANGUAGE MagicHash #-}

  import GHC.Exts

  obviouslyEqual :: a -> a -> Bool
  obviouslyEqual a b =
    case I# (reallyUnsafePtrEquality# a b) of
      0 -> False
      _ -> True

However, this functions is *not* referentially transparent (exercise:
show an example of how obviouslyEqual breaks referential
transparency).  reallyUnsafePtrEquality# is really unsafe for a reason
=).

Cheers,

-- 
Felipe.