When trying to compile the following module, ghc-3.02 dies with the
following message:
panic! (the `impossible' happened):
tcInstType:1
-------------------------------------------
module Impossible where
type T a = Int
class C a where foo :: Int -> IO a
instance C (T a) where foo = undefined
bar :: C a => IO a
bar = foo 12 >>= foo
-------------------------------------------
Section 4.3.2 of the Haskell report (Instance Declarations) states:
The general form of the corresponding instance declaration is:
instance c' => C (T u1 ... uk) where { d }
where k>=0 and T is not a type synonym.
Therefore, the above program is illegal (Hugs tells me this, too). But
things are even stranger when bar is removed:
-------------------------------------------
module Impossible where
type T a = Int
class C a where foo :: Int -> IO a
instance C (T a) where foo = undefined
-------------------------------------------
ghc-3.02 happily compiles this. Bug or feature?
--
Sven Panne Tel.: +49/89/2178-2235
LMU, Institut fuer Informatik FAX : +49/89/2178-2211
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