| The attached file (generated by running ghc -fext-core on the Fibheaps
| benchmark from the nofib suite) fails to typecheck:
|
| $ ghc -dcore-lint Fibheaps.hcr
|
| Couldn't match `#' against `*'
| Expected kind: #
| Inferred kind: *
| When checking kinds in `GHC.Prim.(->) (GHC.Prim.State# s)'
I've found what's going on here. Here is the comment from the
definition of the type constructor for (->):
funTyCon = mkFunTyCon funTyConName
(mkArrowKinds [liftedTypeKind, liftedTypeKind]
liftedTypeKind)
-- You might think that (->) should have type (? -> ? -> *), and
you'd be right
-- But if we do that we get kind errors when saying
-- instance Control.Arrow (->)
-- becuase the expected kind is (*->*->*). The trouble is that
the
-- expected/actual stuff in the unifier does not go
contra-variant, whereas
-- the kind sub-typing does. Sigh. It really only matters if
you use (->) in
-- a prefix way, thus: (->) Int# Int#. And this is unusual.
In short, it's really a bug, but not a particularly easy one to fix.
Rumination required. But it probably only happens on a few programs,
right?
S
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