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| -rw-r--r-- | compiler/typecheck/TcCanonical.hs | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/compiler/typecheck/TcCanonical.hs b/compiler/typecheck/TcCanonical.hs index f8bd367252..a53cdac72e 100644 --- a/compiler/typecheck/TcCanonical.hs +++ b/compiler/typecheck/TcCanonical.hs @@ -543,6 +543,19 @@ mk_strict_superclasses rec_clss ev tvs theta cls tys = do { sc_ev <- newDerivedNC loc sc_pred ; mk_superclasses rec_clss sc_ev [] [] sc_pred } +{- Note [Improvement from Ground Wanteds] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose class C b a => D a b +and consider + [W] D Int Bool +Is there any point in emitting [D] C Bool Int? No! The only point of +emitting superclass constraints for W/D constraints is to get +improvement, extra unifications that result from functional +dependencies. See Note [Why adding superclasses can help] above. + +But no variables means no improvement; case closed. +-} + mk_superclasses :: NameSet -> CtEvidence -> [TyVar] -> ThetaType -> PredType -> TcS [Ct] -- Return this constraint, plus its superclasses, if any |