[GHC] #16050: Instance resolution error message unclear, because of missing kind information
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- consider the following modules: {{{#!hs module A where (.) :: forall (a :: TYPE 'UnliftedRep) (b :: TYPE 'UnliftedRep) (c :: TYPE 'UnliftedRep). (b -> c) -> (a -> b) -> (a -> c) (.) f g = \x -> f (g x) data UList (a :: TYPE 'UnliftedRep) where UNil :: UList a UCons :: a -> UList a -> UList a mapFB :: forall (a :: TYPE 'UnliftedRep) (elt :: TYPE 'UnliftedRep) (lst :: Type). (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst mapFB c f = \x ys -> c (f x) ys {-# RULES "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f . g) #-} }}} {{{#!hs module B where import Control.Category ((.)) data UList (a :: TYPE 'UnliftedRep) where UNil :: UList a UCons :: a -> UList a -> UList a mapFB :: forall (a :: TYPE 'UnliftedRep) (elt :: TYPE 'UnliftedRep) (lst :: Type). (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst mapFB c f = \x ys -> c (f x) ys {-# RULES "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f . g) #-} }}} Module 'A' works fine. Module 'B', fails with the following error: {{{#!hs • No instance for (Category (->)) arising from a use of ‘.’ • In the second argument of ‘mapFB’, namely ‘(f . g)’ In the expression: mapFB c (f . g) When checking the transformation rule "mapFB" | line| "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f . g) | ^^^ }}} I expected this failure because of the kind mismatch; the category instance for `(->)` obviously requires that it be kinded `Type -> Type -> Type`. However, it confused someone I am teaching, who said to me that they didn't understand the error, since they expected it to work as `(->)` does indeed have a Category instance. (They are very unfamiliar with Levity-Polymorphism). My question is this: Would it be preferable to include such kind information in the error message? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by chessai): Now that I think of this, this confused me a lot more the first time I came across such an error. The error message just says nothing about the kinds, so saying that an instance doesn't exist is pretty confusing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I agree. For equality mis-matches, GHC already tries not to report `T Int` does not match `T Int`, by seeing if the only differences are in the kinds. (The logic is in `TcErrors.pprWithExplicitKindsWhenMismatch`.) It would be tiresome, but not hard, to do the same for instance matching, printing the kinds if the only reason for the lack of a match is in the kinds. Any volunteers? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13992, #14146 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #13992, #14146 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: chessai Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by chessai): * owner: (none) => chessai * related: #13992, #14146 => Comment: I can work on this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: chessai Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13992, #14146 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #13992, #14146 Comment: (The related tickets were removed for some mysterious reason.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: chessai Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13992, #14146 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by monoidal): This looks to be the same problem, but for equality: {{{ #!haskell {-# LANGUAGE GADTs, TypeOperators, PolyKinds #-} import GHC.Types data a :~: b where Refl :: a :~: a foo :: TYPE a :~: TYPE b foo = Refl }}} gives an error message {{{ • Couldn't match type ‘'LiftedRep’ with ‘'LiftedRep’ ‘a’ is a rigid type variable bound by the type signature for: foo :: * :~: * at Repr.hs:7:1-24 ‘b’ is a rigid type variable bound by the type signature for: foo :: * :~: * at Repr.hs:7:1-24 }}} To see the problem, you need to use `-fprint-explicit-runtime-reps`. (I'm not sure if this should be a separate ticket.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16050: Instance resolution error message unclear, because of missing kind information -------------------------------------+------------------------------------- Reporter: chessai | Owner: chessai Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13992, #14146 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Indeed comment:6 is a separate bug: #16074 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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