#15674: GADT's displayed type is misleading -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1-beta1 Keywords: | Operating System: Windows Architecture: x86_64 | Type of failure: Poor/confusing (amd64) | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHCi's `:t` alleges these three data constructors have the same type `:: Int -> T Int` (modulo name of the type constructor): {{{#!hs data DG a where MkDG :: Int -> DG Int MkDG2 :: (a ~ Int) => a -> DG a data family DF a data instance DF Int where MkDF :: Int -> DF Int }}} I tried switching on verbosity flags, but to no avail: `-fprint-explicit- foralls, -fprint-equality-relations`, and a few others. The `DG` constructors are GADTs, the `DF` constructor is not. So it's not hard to see different type-level behaviour: {{{#!hs f (MkDF x) = x -- accepted without a signature -- f :: DF Int -> Int -- inferred signature, or you can spec it -- f :: DF a -> a -- rejected: signature is too general/ -- a is a rigid type variable g (MkDG x) = x -- } without a signature, rejected g (MkDG2 x) = x -- } "untouchable" type (Note) -- g :: DG Int -> Int -- } g accepted with either sig -- g :: DG a -> a -- } ?but MkDG doesn't return DG a, allegedly }}} '''Note:''' at least the error message re `MkDG2` does show its type as `:: (forall a). (a ~ Int) => DG a -> a`. But doggedly `MkDG :: Int -> DG Int`. "Untouchable" error messages are a fertile source of questions on StackOverflow. The message does say `Possible fix: add a type signature for 'g'`, but gives no clue what the signature might be. If you've imported these data constructors from a library, such that you can't (easily) see they're GADTs, couldn't `:t` give you better help to show what's going on? Or should one of the verbosity flags already do this? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15674 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler