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{-
From: Marc van Dongen <dongen@cs.ucc.ie>
Date: Sat, 31 May 1997 19:57:46 +0100 (BST)
panic! (the `impossible' happened):
tcLookupTyVar:a_r6F
Please report it as a compiler bug to glasgow-haskell-bugs@dcs.gla.ac.uk.
If the instance definition for (*) at the end of this toy module
is replaced by the definition that is commented, this all compiles
fine. Strange, because the two implementations are equivalent modulo
the theory {(*) = multiply}.
Remove the `multiply :: a -> a -> a' part, and it compiles without
problems.
SPJ note: the type signature on "multiply" should be
multiply :: Group a => a -> a -> a
-}
module ShouldSucceed( Group, Ring ) where
import qualified Prelude( Ord(..), Eq(..), Num(..) )
import Prelude hiding( Ord(..), Eq(..), Num(..) )
class Group a where
compare :: a -> a -> Prelude.Ordering
fromInteger :: Integer -> a
(+) :: a -> a -> a
(-) :: a -> a -> a
zero :: a
one :: a
zero = fromInteger 0
one = fromInteger 1
-- class (Group a) => Ring a where
-- (*) :: a -> a -> a
-- (*) a b =
-- case (compare a zero) of
-- EQ -> zero
-- LT -> zero - ((*) (zero - a) b)
-- GT -> case compare a one of
-- EQ -> b
-- _ -> b + ((*) (a - one) b)
class (Group a) => Ring a where
(*) :: a -> a -> a
(*) a b = multiply a b
where multiply :: Group b => b -> b -> b
multiply a b
= case (compare a zero) of
EQ -> zero
LT -> zero - (multiply (zero - a) b)
GT -> case compare a one of
EQ -> b
_ -> b + (multiply (a - one) b)
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