[GHC] #12237: Constraint resolution vs. type family resolution vs. TypeErrors
#12237: Constraint resolution vs. type family resolution vs. TypeErrors -------------------------------------+------------------------------------- Reporter: cactus | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple CustomTypeErrors | Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Via http://stackoverflow.com/q/37769351/477476, given the following setup: {{{#!hs {-# LANGUAGE DataKinds, TypeFamilies, TypeOperators #-} {-# LANGUAGE FlexibleContexts, MultiParamTypeClasses, UndecidableInstances #-} import Data.Proxy import GHC.TypeLits type family TEq a b where TEq a a = a TEq a b = TypeError (Text "TEq error") type family F a where F () = () F (a, b) = TEq (F a) (F b) F a = TypeError (Text "Unsupported type: " :<>: ShowType a) }}} and the following usages of `F` and `TEq`: {{{#!hs class (repr ~ F a) => C repr a foo :: (C (F a) a) => proxy a -> () foo _ = () main :: IO () main = print $ foo (Proxy :: Proxy (Int, ())) }}} This results in {{{ * No instance for (C (TEq (TypeError ...) ()) (Int, ())) arising from a use of `foo' * In the second argument of `($)', namely `foo (Proxy :: Proxy (Int, ()))' In the expression: print $ foo (Proxy :: Proxy (Int, ())) In an equation for `main': main = print $ foo (Proxy :: Proxy (Int, ())) }}} but it would be preferable to bail out earlier when `F Int` is resolved to `TypeError ...` instead of propagating that all the way to the `C` constraint. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12237 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12237: Constraint resolution vs. type family resolution vs. TypeErrors -------------------------------------+------------------------------------- Reporter: cactus | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | CustomTypeErrors Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by crockeea): * cc: crockeea (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12237#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12237: Constraint resolution vs. type family resolution vs. TypeErrors -------------------------------------+------------------------------------- Reporter: cactus | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | CustomTypeErrors Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: #11990 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * cc: diatchki (added) * related: => #11990 Comment: This seems fixed in GHC HEAD (c88f31a08943764217b69adb1085ba423c9bcf91, 8.1.20160610), perhaps as a result of #11990. {{{ Test.hs:23:16: error: • Unsupported type: Int • In the second argument of ‘($)’, namely ‘foo (Proxy :: Proxy (Int, ()))’ In the expression: print $ foo (Proxy :: Proxy (Int, ())) In an equation for ‘main’: main = print $ foo (Proxy :: Proxy (Int, ())) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12237#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12237: Constraint resolution vs. type family resolution vs. TypeErrors -------------------------------------+------------------------------------- Reporter: cactus | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: duplicate | Keywords: | CustomTypeErrors Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: #11990 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate Comment: Indeed, this is fixed in 8.0.2. This test case is sufficiently similar to the one in #11990 that I'll close this as a duplicate. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12237#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
participants (1)
-
GHC