#13443: Typeclass resolution errors quite puzzling -------------------------------------+------------------------------------- Reporter: tomjaguarpaw | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * milestone: 8.6.1 => Old description:

This ticket is based on a post I made to haskell-cafe: https://mail.haskell.org/pipermail/haskell-cafe/2016-August/124622.html

Here's a program with an odd error message (GHC 8.0.1):

{{{ data A a = A a deriving Eq data B = B

main :: IO () main = print (A B == A B)

test/main.hs:5:15: error: • No instance for (Eq B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B) }}}

I get an error about `Eq B` even though it's `Eq A` that is manifestly required at the call site. This error is odder when `A` and `B` are defined far away from the use of `==`.

This is even odder:

{{{ data A a = A a data B = B

instance Ord a => Eq (A a) where

main :: IO () main = print (A B == A B)

test/main.hs:7:15: error: • No instance for (Ord B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B) }}}

Now not only is the type puzzling (`B` instead of `A`) but the *class* is puzzling (`Ord` instead of `Eq`). This occurred to me in practice because `Data.Graph.Inductive.PatriciaTree.Gr` has `(Eq a, Ord b) => Eq (Gr a b)`.

It would have been a lot more helpful to see

{{{ * No instance for (Ord B) * arising from (Eq A) * arising from the use of '==' }}}

Does anyone agree with me that GHC should produce the full trace when it fails to resolve instances rather than just the proximal failure?

New description: This ticket is based on a post I made to haskell-cafe: https://mail.haskell.org/pipermail/haskell-cafe/2016-August/124622.html Here's a program with an odd error message (GHC 8.0.1): {{{#!hs data A a = A a deriving Eq data B = B main :: IO () main = print (A B == A B) test/main.hs:5:15: error: • No instance for (Eq B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B) }}} I get an error about `Eq B` even though it's `Eq A` that is manifestly required at the call site. This error is odder when `A` and `B` are defined far away from the use of `==`. This is even odder: {{{#!hs data A a = A a data B = B instance Ord a => Eq (A a) where main :: IO () main = print (A B == A B) test/main.hs:7:15: error: • No instance for (Ord B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B) }}} Now not only is the type puzzling (`B` instead of `A`) but the *class* is puzzling (`Ord` instead of `Eq`). This occurred to me in practice because `Data.Graph.Inductive.PatriciaTree.Gr` has `(Eq a, Ord b) => Eq (Gr a b)`. It would have been a lot more helpful to see {{{ * No instance for (Ord B) * arising from (Eq A) * arising from the use of '==' }}} Does anyone agree with me that GHC should produce the full trace when it fails to resolve instances rather than just the proximal failure? -- Comment: Demilestoning as no one has stepped up to carry this out yet. Do ping if this sounds like the sort of project you would like to try. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13443#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler