#9636: Function with type error accepted -------------------------------------+------------------------------------- Reporter: augustss | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:12 augustss]:
To me saying that `T Bool` is OK because it's inaccessible is akin to saying that type incorrect expressions are OK as long as they are inaccessible. After all, if you don't use them, they can't cause any harm. But for expressions we have decided that this isn't acceptable.
I actually would say that an inaccessible expression is ''always'' type- correct, because inaccessibility means that there is a proof of ''false'' in the context, and thus anything is possible. But that's perhaps a story for another day.
I guess making `T a` behave would require something like kind classes.
It's a little worse than that, I think. Say `x :: Z`. In `show x`, we know `Z` must be in the `Show` class, and we also know that ''anything'' of type `Z` is a valid parameter to `show`. Thus, we have substitution, because substitution preserves types. But, in closed type families, we have a stranger situation: `T a` where `a :: *` is acceptable, but `T Bool` is not. I think kind classes (which can be kludged today) don't solve this problem.
But I'm not asking for the moon. :) I'd just like the compiler to tell me when it finds something that is clearly not going to work, like `T Bool`. Exactly under what conditions and how it tells me, I don't care.
This is certainly possible. It's just that the behavior around this feature won't be complete and might not be predictable in corner cases. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9636#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler