Use implication constraints to improve type inference

[ghc-hetmet.git] / compiler / types / FunDeps.lhs

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 06248b7..cad292b 100644 (file)
--- a/compiler/types/FunDeps.lhs
+++ b/compiler/types/FunDeps.lhs
@@ -10,7 +10,7 @@ It's better to read it as: "if we know these, then we're going to know these"
 \begin{code}
 module FunDeps (
        Equation, pprEquation,
-       oclose, grow, improve, 
+       oclose, grow, improve, improveOne,
        checkInstCoverage, checkFunDeps,
        pprFundeps
     ) where
@@ -229,6 +229,63 @@ improve inst_env preds
        -- In any case, improvement *generates*, rather than
        -- *consumes*, equality constraints
 
+improveOne :: (Class -> [Instance])
+          -> Pred_Loc
+          -> [Pred_Loc]
+          -> [(Equation,Pred_Loc,Pred_Loc)]
+
+-- Just do improvement triggered by a single, distinguised predicate
+
+improveOne inst_env pred@(IParam ip ty, _) preds
+  = [ ((emptyVarSet, [(ty,ty2)]), pred, p2) 
+    | p2@(IParam ip2 ty2, _) <- preds
+    , ip==ip2
+    , not (ty `tcEqType` ty2)]
+
+improveOne inst_env pred@(ClassP cls tys, _) preds
+  | tys `lengthAtLeast` 2
+  = instance_eqns ++ pairwise_eqns
+       -- NB: we put the instance equations first.   This biases the 
+       -- order so that we first improve individual constraints against the
+       -- instances (which are perhaps in a library and less likely to be
+       -- wrong; and THEN perform the pairwise checks.
+       -- The other way round, it's possible for the pairwise check to succeed
+       -- and cause a subsequent, misleading failure of one of the pair with an
+       -- instance declaration.  See tcfail143.hs for an example
+  where
+    (cls_tvs, cls_fds) = classTvsFds cls
+    instances         = inst_env cls
+    rough_tcs         = roughMatchTcs tys
+
+       -- NOTE that we iterate over the fds first; they are typically
+       -- empty, which aborts the rest of the loop.
+    pairwise_eqns :: [(Equation,Pred_Loc,Pred_Loc)]
+    pairwise_eqns      -- This group comes from pairwise comparison
+      = [ (eqn, pred, p2)
+       | fd <- cls_fds
+       , p2@(ClassP cls2 tys2, _) <- preds
+       , cls == cls2
+       , eqn <- checkClsFD emptyVarSet fd cls_tvs tys tys2
+       ]
+
+    instance_eqns :: [(Equation,Pred_Loc,Pred_Loc)]
+    instance_eqns      -- This group comes from comparing with instance decls
+      = [ (eqn, p_inst, pred)
+       | fd <- cls_fds         -- Iterate through the fundeps first, 
+                               -- because there often are none!
+       , let rough_fd_tcs = trimRoughMatchTcs cls_tvs fd rough_tcs
+       , ispec@(Instance { is_tvs = qtvs, is_tys = tys_inst, 
+                           is_tcs = mb_tcs_inst }) <- instances
+       , not (instanceCantMatch mb_tcs_inst rough_tcs)
+       , eqn <- checkClsFD qtvs fd cls_tvs tys_inst tys
+       , let p_inst = (mkClassPred cls tys_inst, 
+                       ptext SLIT("arising from the instance declaration at")
+                       <+> ppr (getSrcLoc ispec))
+       ]
+
+improveOne inst_env eq_pred preds
+  = []
+
 ----------
 checkGroup :: (Class -> [Instance])
           -> [Pred_Loc]