, ptext (sLit "tau_tvs =") <+> ppr tau_tvs
]
- ; (simple_wanted, tc_binds)
- <- simplifyAsMuchAsPossible SimplInfer zonked_wanted
+ -- Make a guess at the quantified type variables
+ -- Then split the constraints on the baisis of those tyvars
+ -- to avoid unnecessarily simplifying a class constraint
+ -- See Note [Avoid unecessary constraint simplification]
+ ; gbl_tvs <- tcGetGlobalTyVars
+ ; zonked_tau_tvs <- zonkTcTyVarsAndFV tau_tvs
+ ; let proto_qtvs = zonked_tau_tvs `minusVarSet` gbl_tvs
+ (perhaps_bound, surely_free)
+ = partitionBag (quantifyMeWC proto_qtvs) zonked_wanted
+ ; emitConstraints surely_free
+
+ -- Now simplify the possibly-bound constraints
+ ; (simplified_perhaps_bound, tc_binds)
+ <- simplifyAsMuchAsPossible SimplInfer perhaps_bound
+ -- Sigh: must re-zonk because because simplifyAsMuchAsPossible
+ -- may have done some unification
; gbl_tvs <- tcGetGlobalTyVars
; zonked_tau_tvs <- zonkTcTyVarsAndFV tau_tvs
- ; zonked_simples <- mapBagM zonkWantedEvVar simple_wanted
+ ; zonked_simples <- mapBagM zonkWantedEvVar simplified_perhaps_bound
; let qtvs = findQuantifiedTyVars apply_mr zonked_simples zonked_tau_tvs gbl_tvs
(bound, free) | apply_mr = (emptyBag, zonked_simples)
| otherwise = partitionBag (quantifyMe qtvs) zonked_simples
| otherwise = tyVarsOfPred pred `intersectsVarSet` qtvs
where
pred = wantedEvVarPred wev
+
+quantifyMeWC :: TyVarSet -> WantedConstraint -> Bool
+quantifyMeWC qtvs (WcImplic implic)
+ = anyBag (quantifyMeWC (qtvs `minusVarSet` ic_skols implic)) (ic_wanted implic)
+quantifyMeWC qtvs (WcEvVar wev)
+ = quantifyMe qtvs wev
\end{code}
+Note [Avoid unecessary constraint simplification]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When inferring the type of a let-binding, with simplifyInfer,
+try to avoid unnecessariliy simplifying class constraints.
+Doing so aids sharing, but it also helps with delicate
+situations like
+ instance C t => C [t] where ..
+ f :: C [t] => ....
+ f x = let g y = ...(constraint C [t])...
+ in ...
+When inferring a type for 'g', we don't want to apply the
+instance decl, because then we can't satisfy (C t). So we
+just notice that g isn't quantified over 't' and partition
+the contraints before simplifying.
+
+This only half-works, but then let-generalisation only half-works.
+
+
Note [Inheriting implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this: