L.S.,
I'm currently writing code where I want some ADTs to be parametric with a
monad. There are some extra conditions that I place on my parameter, but I've
boiled things down to minimal reproducibility. When I define MyADT as follows:
import Data.Typeable
data MyADT m = MyADT (m ())
instance (Typeable1 m, Monad m) => Typeable (MyADT m) where
typeOf t@(MyADT _)
= mkTyCon "MyADT"
`mkTyConApp`
[typeOf1 ((return :: Monad m => MyADT m -> m (MyADT m)) t)]
it compiles fine and GHCi works as expected on a single instance of MyADT:
*Main> typeOf (MyADT (return () :: IO ()))
MyADT IO
However, as soon as I place my data type in a structure of sorts, things break
down:
*Main> typeOf [(MyADT (return () :: IO ()))]
*** Exception: Prelude.undefined
*Main> typeOf (Just (MyADT (return () :: IO ())))
*** Exception: Prelude.undefined
*Main> typeOf ((return :: a -> IO a) (MyADT (return () :: IO ())))
*** Exception: Prelude.undefined
But if I stick a number in a similar structure, typeOf works just fine:
*Main> typeOf ((return :: a -> IO a) 5)
IO Integer
I don't quite understand where the undefined comes from. I'm certainly not
using it anywhere (as shown by the minimal reproduction above). Weirder still,
when I *do* introduce a nice error, that too does not come up, viz.
import Data.Typeable
data MyADT m = MyADT (m ())
instance (Typeable1 m, Monad m) => Typeable (MyADT m) where
typeOf t@(MyADT _)
= error "foobar"
with GHCi-session:
*Main> typeOf (MyADT (return () :: IO ()))
*** Exception: foobar
*Main> typeOf ((return :: a -> IO a) (MyADT (return () :: IO ())))
*** Exception: Prelude.undefined
*Main> typeOf $ Just (MyADT (return () :: IO ()))
*** Exception: Prelude.undefined
*Main> typeOf $ Just 42
Maybe Integer
FYI, I'm using GHC 7.0.3, as installed with the Haskell Platform 2011.2.0.1. Am
I overlooking something? Any help would be appreciated.
Regards,
Philip
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