#15791: typeKind confuses Type and Constraint -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The `typeKind` function takes a type and reports its kind. It gets this wrong, sometimes: if you ask for `typeKind` of the quantified constraint `Eq a => C a` (where `C` is a class), you get `Type`. This is wrong, though I don't have a concrete case where GHC misfires. We ''could'' fix `typeKind` to get this right, but doing so would require making the `FunTy` case recursive (right now, it always returns `Type`). This is wasteful, because most clients of `typeKind` do not care about the distinction between `Type` and `Constraint`. Instead, I propose making a new `tcTypeKind` which behaves correctly in this case. We could also muse about making `tcTypeKind` monadic so that it can look through unification variables. This might also simplify the story around `Note [The tcType invariant]`. Regardless of the "make monadic" idea, I claim that, when this is done, we should have `isPredTy` be functionally equivalent to `(== Constraint) . tcTypeKind`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15791 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler