[Haskell-cafe] Re: unary pattern matching

Simon Marlow simonmarhaskell at gmail.com
Wed Feb 1 06:19:13 EST 2006

John Meacham wrote:
> On Fri, Jan 27, 2006 at 04:57:14AM -0500, Cale Gibbard wrote:
>> Or if we're going to allow @ as an infix operator, we could use (@
>> pat), reminiscent of section notation. (exp @) of course would make no
>> sense, seeing as there's no representation for patterns as values.
> oooh. I like that. what fixity do you think @ should have? perhaps even
> lower than $... 

I like it too, but I discovered you can get pretty close with a type class:

class Match a b where
   (@@) :: a -> b -> Bool

instance Match a b => Match (Maybe a) (Maybe b) where
   Just a @@ Just b = a @@ b
   Nothing @@ Nothing = True
   _ @@ _ = False

instance Match a () where
   x @@ () = True

The trick is that () behaves like a wildcard.  eg:

*Main> Just undefined @@ Just ()
*Main> Just Nothing @@ Just ()
*Main> Just Nothing @@ Just (Just ())
*Main> Just Nothing @@ Just Nothing

     Ambiguous type variables `a', `a1' in the constraint:
       `Match a a1' arising from use of `@@' at <interactive>:1:13-14
     Probable fix: add a type signature that fixes these type variable(s)
*Main> Just (Just undefined) @@ Just ()
*Main> Just (Just undefined) @@ Just (Just ())


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