Hi everyone,
Here's some code that's giving me an error message I don't understand:
{-# LANGUAGE EmptyDataDecls,
MultiParamTypeClasses,
UndecidableInstances,
FlexibleInstances #-}
data Z
data S n
class Nat n where
toInt :: n -> Int
instance Nat Z where
toInt _ = 0
instance (Nat n) => Nat (S n) where
toInt _ = 1 + toInt (undefined :: n)
instance (Nat n) => Show n where
show _ = show $ toInt (undefined :: n)
-- end of code sample
When I run this through ghci, I get this:
test.hs:19:11:
Overlapping instances for Show Int
arising from a use of `show' at test.hs:19:11-14
Matching instances:
instance Show Int -- Defined in GHC.Show
instance (Nat n) => Show n -- Defined at test.hs:18:9-25
In the first argument of `($)', namely `show'
In the expression: show $ toInt (undefined :: n)
In the definition of `show': show _ = show $ toInt (undefined :: n)
Adding OverlappingInstances to the language pragmas fixes the problem.
My question is: why is this an overlapping instance? It would make
sense if Int was an instance of Nat, but it isn't. Is this just a
limitation in the way overlapping instances are identified?
Mike
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