In my effort to turn Haskell into a language more like Perl (muahaha)[1], I got a bit fed up and implemented something like Perl 5's =~ binding operator (a.k.a. "regex" operator); I thought maybe somebody else here might find it useful. Perl has the concept of 'contexts': a function does something different depending on what type its caller expects back from the function. Sounds like a perfect abuse of type classes, to me :).
And, just for kicks, here's Perl 6's "smart match" (~~) operator, which works much like ==, but in a more "Do What I Mean!" fashion, matching any type to any other type:
---
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-}
module SmartMatch where
import Text.Regex
class TruthValue a where truth :: a -> Bool
instance TruthValue Bool where truth = id
instance TruthValue Int where truth 0 = False truth _ = True
instance TruthValue String where truth "" = False truth _ = True
instance TruthValue a => TruthValue [a] where truth = all truth
class SmartMatch a b where (~~) :: a -> b -> Bool
instance Eq a => SmartMatch a a where a ~~ b = a == b
instance SmartMatch c d => SmartMatch d c where a ~~ b = b ~~ a
instance TruthValue a => SmartMatch Bool a where a ~~ b = a == truth b
instance SmartMatch String Regex where
s ~~ re = case matchRegex re s of
Nothing -> False
Just x -> True
---
-- % Andre Pang : trust.in.love.to.save _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
