#11621: GHC doesn't see () as a Constraint in type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.1 checker) | Keywords: Resolution: | ConstraintKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11715, #13742 | Differential Rev(s): Phab:D5413 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I wish it was really fixed, but it isn't. Try this {{{ type family F2 b where F2 'True = () F2 'False = NotFound }}} That elicits {{{ T11621.hs:12:14: error: * Expected a type, but `NotFound' has kind `Constraint' * In the type `NotFound' In the type family declaration for `F2' | 12 | F 'False = NotFound | ^^^^^^^^ }}} GHC is generally very good about being robustly order-independent; nothing depends on the order in which the type inference engine encounters code. But this is a rare counter example. The hack is described in `Note [Inferring tuple kinds]` in `TcHsType`. In `F`, GHC sees `NotFound` first, and that tells it that the answer is `Constraint`. IN `F2` it sees `()` first and guesses (wrongly) `*`. So I would say not-fixed. I can think of ad-hoc ways to fix this -- such as having a built-in constraint `TK k` which means `k` must be either `*` or `Constraint`. But I have thus far lacked the time and energy to think it through enough or implement it. Especially since #11715 is still open. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11621#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler