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? Tom