#9637: Type level programming needs an error function -------------------------------------+------------------------------------- Reporter: augustss | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #9180, #9636 | Differential Rev(s): Phab:D1236 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * milestone: => 8.0.1 Old description:
When doing type level programming you often need a way to report errors. I suggest a closed type family called Error with special semantics. It is kinded like this Error :: Symbol -> k1 -> k2
If this function has to be reduced during constraint solving it will simply generate a type error with the given string and printing the type as extra information (this kind can be generalized a lot if we want to).
I've found that having this function gives more accurate and earlier error messages.
Wiki design page: [wiki:CustomTypeErros]
New description: When doing type level programming you often need a way to report errors. I suggest a closed type family called Error with special semantics. It is kinded like this Error :: Symbol -> k1 -> k2 If this function has to be reduced during constraint solving it will simply generate a type error with the given string and printing the type as extra information (this kind can be generalized a lot if we want to). I've found that having this function gives more accurate and earlier error messages. Wiki design page: [wiki:Proposal/CustomTypeErrors] -- Comment: It sounds like this will make it in for 8.0.1 with an implementation by Iavor Diatchki. Thanks Iavor! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9637#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler