Can someone explain why the type declaration for `g` is required in the
following?
class RT r t where rt :: r -> t
data D t = Dt t | forall r. RT r t => Dr r
f :: D t -> D t
f = g
where -- g :: D t -> D t
g (Dr r) = Dt (rt r)
As given above, the program evokes these error messages:
with GHC 5.04.2:
Bug4.hs:10:
Could not deduce (RT r t1) from the context (RT r t)
Probable fix:
Add (RT r t1) to the existential context of a data constructor
arising from use of `rt' at Bug4.hs:10
In the first argument of `Dt', namely `(rt r)'
In the definition of `g': Dt (rt r)
with Hugs:
ERROR "Bug4.hs":6 - Cannot justify constraints in explicitly typed
binding
*** Expression : f
*** Type : D a -> D a
*** Given context : ()
*** Constraints : RT _6 a
-- Dean
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