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 :). Code follows: --- {-# OPTIONS -fglasgow-exts #-} {- Need this for "instance Foo [String]" declarations -} module PLRE where -- Perl-Like Regular Expressions import Text.Regex -- Perl-Like =~ operator, which changes behaviour depending on its calling -- context class RegExContext a where (=~) :: String -> String -> a instance RegExContext Bool where s =~ re = case matchRegex (mkRegex re) s of Nothing -> False Just x -> True instance RegExContext [String] where s =~ re = case matchRegex (mkRegex re) s of Nothing -> [] Just x -> x boolContextTest string regEx = case string =~ regEx of True -> print True False -> print False stringListContextTest string regEx = case string =~ regEx of (a:x) -> print ("First match: " ++ a) _ -> error "No subexpression matches" --- Some test output for you: *PLRE> boolContextTest "foo" "^f" True *PLRE> boolContextTest "foo" "^g" False *PLRE> stringListContextTest "foo" "^(.)" "First match: f" *PLRE> stringListContextTest "goo" "^(.)" "First match: g" *PLRE> stringListContextTest "" "^(.)" *** Exception: No subexpression matches Note that you have a fairly severe restriction if you want to use =~ in your code: the Haskell compiler must be able to determine a concrete type for the context that =~ is used in. i.e. if stringListContextTest was defined as: stringListContextTest string regEx = case string =~ regEx of (a:x) -> print a _ -> error "No subexpression matches" The compiler can't concretise a type for 'a', and it'll complain about not having an instance for RegExContext [a] (which is fair enough). Even with this restriction, I'm sure it'll still be useful. It shouldn't be a bit leap to define other Perl-ish operators in this fashion, such as !~, or even s/.../. Have the appropriate amount of fun! 1. Actually, I wanted to turn Haskell into a language more suitable for text processing, but that doesn't sound as evil. -- % Andre Pang : trust.in.love.to.save