tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
-- Zonk everything in sight
= do { traceTc (text "tcSimplifyRestricted")
- ; wanteds' <- zonkInsts wanteds
+ ; wanteds_z <- zonkInsts wanteds
-- 'ReduceMe': Reduce as far as we can. Don't stop at
-- dicts; the idea is to get rid of as many type
-- HOWEVER, some unification may take place, if we instantiate
-- a method Inst with an equality constraint
; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe)
- ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds'
+ ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds_z
-- Next, figure out the tyvars we will quantify over
; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
ppr _binds,
ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
+ -- Zonk wanteds again! The first call to reduceContext may have
+ -- instantiated some variables.
+ -- FIXME: If red_improve would work, we could propagate that into
+ -- the equality solver, too, to prevent instantating any
+ -- variables.
+ ; wanteds_zz <- zonkInsts wanteds_z
+
-- The first step may have squashed more methods than
-- necessary, so try again, this time more gently, knowing the exact
-- set of type variables to quantify over.
(is_nested_group || isDict inst) = Stop
| otherwise = ReduceMe
env = mkNoImproveRedEnv doc try_me
- ; (_imp, binds, irreds) <- reduceContext env wanteds'
+ ; (_imp, binds, irreds) <- reduceContext env wanteds_zz
-- See "Notes on implicit parameters, Question 4: top level"
; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured