#2661: Associated type synonyms not fully simplified in GHCi
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Reporter: kupopo | Owner:
Type: bug | Status: new
Priority: normal | Component: GHCi
Version: 6.8.2 | Severity: minor
Keywords: | Testcase:
Architecture: x86 | Os: Linux
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The following code works as expected when compiled in GHC, or even when
loaded into GHCi and run with "main":
{{{
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleContexts #-}
data Z = Z
data S n = S n
class Peano n where
mkPeano :: n
toInt :: n -> Int
instance Peano Z where
mkPeano = Z
toInt Z = 0
instance Peano n => Peano (S n) where
mkPeano = S mkPeano
toInt (S n) = 1 + toInt n
class (Peano a, Peano b, Peano (Sum a b)) => BinOp a b where
type Sum a b
instance BinOp Z Z where type Sum Z Z = Z
instance Peano a => BinOp Z (S a) where type Sum Z (S a) = S a
instance Peano a => BinOp (S a) Z where type Sum (S a) Z = S a
instance BinOp a b => BinOp (S a) (S b) where
type Sum (S a) (S b) = S (S (Sum a b))
add :: BinOp a b => a -> b -> Sum a b
add _ _ = mkPeano
main = do
print $ toInt $ add Z $ add Z Z
}}}
However, if the contents of main is typed at the GHCi prompt (toInt $ add
Z $ add Z Z), it complains that {{{No instance for (BinOp Z (Sum Z Z))
arising from a use of `add' at <interactive>:1:8-12}}}. Evidently it
stops short before evaluating that Sum Z Z = Z and therefore BinOp Z Z is
in fact an instance. This is the case both in 6.8.3 as well as
6.10.0.20080921.
If "seq 1" is inserted, as in "toInt $ seq 1 $ add Z $ add Z Z", then GHCi
again behaves consistently with GHC. This is therefore a workaround,
albeit inconvenient.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2661>
GHC <http://www.haskell.org/ghc/>
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