#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 augustss): Replying to [comment:13 rwbarton]:
Isn't it analogous to saying that incomplete pattern matches are OK as long as they are inaccessible? (Which they are.) After all, like you said, a closed type family is like a function on types. The type expression `T Bool` has no normal form, for sure, but who says we have to evaluate it?
Yes, it's a lot like a pattern match failure. So let us go with that analogy for a moment. Assume we have these definitions: {{{#!hs foo :: Int -> Int foo 0 = 42 bar = foo 5 - foo 5 }}} Since we know that `x - x = 0` it is tempting to optimize the definition of `bar` to `bar = 0`. This is, of course, wrong since computing `foo 5` will result in bottom. Now compare this to what the type checker does in my original example. It happily assumes that `T Bool` is equal to `T Bool`, but that's wrong. `T Bool` is bottom at type checking time and should be reported as such. I've decided I'm OK with having `T Bool` just floating around in the same way that I'm OK with having `foo 5` floating around in a lazy language. Type families introduce partial functions on the type level, and I think this has to be dealt with. Type expressions can now blow up during type checking. So at the points when a (closed) type family expression has to be evaluated, e.g., for equality comparison, and it cannot be reduced then this should be an error. IMHO. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9636#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler