[project @ 2003-11-03 15:27:32 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 f06c506..743a34c 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
 
@@ -16,7 +17,7 @@ import Name           ( getSrcLoc )
 import Var             ( Id, TyVar )
 import Class           ( Class, FunDep, classTvsFds )
 import Subst           ( mkSubst, emptyInScopeSet, substTy )
-import TcType          ( Type, ThetaType, SourceType(..), PredType,
+import TcType          ( Type, ThetaType, PredType(..), 
                          predTyUnique, mkClassPred, tyVarsOfTypes, tyVarsOfPred,
                          unifyTyListsX, unifyExtendTysX, tcEqType
                        )
@@ -121,8 +122,8 @@ 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
@@ -148,6 +149,9 @@ grow preds fixed_tvs
 ----------
 type Equation = (TyVarSet, Type, Type) -- These two types should be equal, for some
                                        -- substitution of the tyvars in the tyvar set
+       -- To "execute" the equation, make fresh type variable for each tyvar in the set,
+       -- instantiate the two types with these fresh variables, and then unify.
+       --
        -- For example, ({a,b}, (a,Int,b), (Int,z,Bool))
        -- We unify z with Int, but since a and b are quantified we do nothing to them
        -- We usually act on an equation by instantiating the quantified type varaibles
@@ -157,6 +161,10 @@ 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
@@ -168,7 +176,7 @@ improve :: InstEnv Id               -- Gives instances for given class
 type InstEnv a = Class -> [(TyVarSet, [Type], a)]
 -- This is a bit clumsy, because InstEnv is really
 -- defined in module InstEnv.  However, we don't want
--- to define it (and ClsInstEnv) here because InstEnv
+-- to define it here because InstEnv
 -- is their home.  Nor do we want to make a recursive
 -- module group (InstEnv imports stuff from FunDeps).
 \end{code}
@@ -274,25 +282,31 @@ checkClsFD qtvs fd clas_tvs tys1 tys2
 -- 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), 
+--
+-- 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
+--     (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))]
+                      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
@@ -300,6 +314,12 @@ checkClsFD qtvs fd clas_tvs tys1 tys2
                    qtvs' = filterVarSet (\v -> not (v `elemSubstEnv` unif)) qtvs
                        -- qtvs' are the quantified type variables
                        -- that have not been substituted out
+                       --      
+                       -- Eg.  class C a b | a -> b
+                       --      instance C Int [y]
+                       -- Given constraint C Int z
+                       -- we generate the equation
+                       --      ({y}, [y], z)
   where
     (ls1, rs1) = instFD fd clas_tvs tys1
     (ls2, rs2) = instFD fd clas_tvs tys2