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{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
{- With "hugs -98 +o test.hs" gives me:
ERROR "test.hs":8 - Cannot justify constraints in instance member binding
*** Expression : fromStr
*** Type : FromStr [a] => String -> [a]
*** Given context : FromStr [a]
*** Constraints : FromStr [a]
Adding the constraint "FromStr a" to the declaration of fromStr fixes
the problem, but that seems like it should be redundant. Removing the
second instance (lines 10-11) also fixes the problem, interestingly enough.
/Bjorn Bringert -}
-- August 08: on reflection I think a complaint about overlapping
-- instances for line 8 is absolutely right, so I've changed this to
-- expected-failure
-- Sept 08: on further reflection (!) I'm changing it back
-- See Note [Subtle interaction of recursion and overlap]
-- in TcInstDcls
module ShouldCompile where
class FromStr a where
fromStr :: String -> a
typeError :: FromStr a => a -> a
typeError t = error "type error"
instance FromStr [a] where
fromStr _ = typeError undefined -- line 8
instance FromStr [(String,a)] where -- line 10
fromStr _ = typeError undefined -- line 11
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