[project @ 2003-08-20 15:07:56 by simonmar]

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

diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs
index 1c71ed5..6fd587a 100644 (file)
--- a/ghc/compiler/types/FunDeps.lhs
+++ b/ghc/compiler/types/FunDeps.lhs
@@ -123,8 +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
@@ -150,6 +150,9 @@ 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
@@ -280,14 +283,15 @@ 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!
@@ -311,6 +315,12 @@ 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