[project @ 2002-09-13 15:02:25 by simonpj]

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

diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs
index 4854e0c..79f62fb 100644 (file)
--- a/ghc/compiler/types/FunDeps.lhs
+++ b/ghc/compiler/types/FunDeps.lhs
@@ -7,6 +7,7 @@ It's better to read it as: "if we know these, then we're going to know these"
 
 \begin{code}
 module FunDeps (
+       Equation, pprEquation, pprEquationDoc,
        oclose, grow, improve, checkInstFDs, checkClsFD, pprFundeps
     ) where
 
@@ -20,6 +21,7 @@ import TcType         ( Type, ThetaType, SourceType(..), PredType,
                          predTyUnique, mkClassPred, tyVarsOfTypes, tyVarsOfPred,
                          unifyTyListsX, unifyExtendTysX, tcEqType
                        )
+import PprType         (  )
 import VarSet
 import VarEnv
 import Outputable
@@ -121,8 +123,8 @@ oclose preds fixed_tvs
 \begin{code}
 grow :: [PredType] -> TyVarSet -> TyVarSet
 grow preds fixed_tvs 
-  | null pred_sets = fixed_tvs
-  | otherwise     = loop fixed_tvs
+  | null preds = fixed_tvs
+  | otherwise  = loop fixed_tvs
   where
     loop fixed_tvs
        | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs
@@ -157,6 +159,10 @@ type Equation = (TyVarSet, Type, Type)     -- These two types should be equal, for s
        --
 
 
+pprEquationDoc (eqn, doc) = vcat [pprEquation eqn, nest 2 doc]
+
+pprEquation (qtvs, t1, t2) = ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs))
+                                                 <+> ppr t1 <+> ptext SLIT(":=:") <+> ppr t2
 
 ----------
 improve :: InstEnv Id          -- Gives instances for given class
@@ -264,20 +270,40 @@ mkEqnMsg (pred1,from1) (pred2,from2)
          nest 2 (sep [ppr pred2 <> comma, nest 2 from2])]
  
 ----------
-checkClsFD :: TyVarSet                         -- The quantified type variables, which
-                                       -- can be instantiated to make the types match
+checkClsFD :: TyVarSet                         -- Quantified type variables; see note below
           -> FunDep TyVar -> [TyVar]   -- One functional dependency from the class
           -> [Type] -> [Type]
           -> [Equation]
 
 checkClsFD qtvs fd clas_tvs tys1 tys2
+-- 'qtvs' are the quantified type variables, the ones which an be instantiated 
+-- to make the types match.  For example, given
+--     class C a b | a->b where ...
+--     instance C (Maybe x) (Tree x) where ..
+-- and an Inst of form (C (Maybe t1 t2), 
+-- then we will call checkClsFD with
+--
+--     qtvs = {x}, tys1 = [Maybe x,  Tree x]
+--                 tys2 = [Maybe t1, t2]
+--
+-- We can instantiate x to t1, and then we want to force
+--     Tree x [t1/x]  :=:   t2
+
 -- We use 'unify' even though we are often only matching
 -- unifyTyListsX will only bind variables in qtvs, so it's OK!
   = case unifyTyListsX qtvs ls1 ls2 of
        Nothing   -> []
-       Just unif -> [ (qtvs', substTy full_unif r1, substTy full_unif r2)
+       Just unif -> -- pprTrace "checkFD" (vcat [ppr_fd fd,
+                    --                        ppr (varSetElems qtvs) <+> (ppr ls1 $$ ppr ls2),
+                    --                        ppr unif]) $ 
+                    [ (qtvs', substTy full_unif r1, substTy full_unif r2)
                     | (r1,r2) <- rs1 `zip` rs2,
                       not (maybeToBool (unifyExtendTysX qtvs unif r1 r2))]
+                       -- Don't include any equations that already hold
+                       -- taking account of the fact that any qtvs that aren't 
+                       -- already instantiated can be instantiated to anything at all
+                       -- NB: qtvs, not qtvs' because unifyExtendTysX only tries to
+                       --     look template tyvars up in the substitution
                  where
                    full_unif = mkSubst emptyInScopeSet unif
                        -- No for-alls in sight; hmm