[Haskell-cafe] Instance match surprise

[Wojtek Narczyński]

Dear List,

Why does the first instance match? ANY is neither Eq nor Typeable. I 
thought I had some basic understanding of type classes, and now this...

wojtek at biuro:~/src/he$ cat minimatch.hs
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

module MiniMatch where

import Data.Typeable

data A130 = A130 deriving (Eq)
data ANY = ANY

class AtmAcct a
class Against q
class (AtmAcct a, Against q) => Match a q where match :: a -> q -> Bool

instance AtmAcct A130
instance Against ANY

instance (Eq a, Eq q, Typeable a, Typeable q, AtmAcct a, Against q) => 
Match a q where match _ _ = False
instance (AtmAcct a) => Match a ANY where match _ _ = True

m1 = match A130 ANY -- offending line

wojtek at biuro:~/src/he$ ghc minimatch.hs
[1 of 1] Compiling MiniMatch        ( minimatch.hs, minimatch.o )

minimatch.hs:21:6: error:
     • Overlapping instances for Match A130 ANY
         arising from a use of ‘match’
       Matching instances:
         instance (Eq a, Eq q, Typeable a, Typeable q, AtmAcct a,
                   Against q) =>
                  Match a q
           -- Defined at minimatch.hs:18:10
         instance AtmAcct a => Match a ANY -- Defined at minimatch.hs:19:10
     • In the expression: match A130 ANY
       In an equation for ‘m1’: m1 = match A130 ANY

-- 
Thanks,
Wojtek Narczyński