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| author | Ben Gamari <ben@smart-cactus.org> | 2020-01-15 20:10:08 (GMT) |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2020-01-20 20:35:57 (GMT) |
| commit | 7e0bb82b5e267596c18e1086afd6007a00d2c072 (patch) | |
| tree | e187f048b8979f93b7258843f3a277d46cb3fc66 | |
| parent | 14bced999dc8c826a9a10dfd922dadf2d681ffe3 (diff) | |
| download | ghc-7e0bb82b5e267596c18e1086afd6007a00d2c072.zip ghc-7e0bb82b5e267596c18e1086afd6007a00d2c072.tar.gz ghc-7e0bb82b5e267596c18e1086afd6007a00d2c072.tar.bz2 | |
Add missing Note [Improvement from Ground Wanteds]
Closes #17659.
| -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 f8bd367..a53cdac 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 |