From 989039af2ce80a2b1c42b6d9a9e2dda815b6449d Mon Sep 17 00:00:00 2001 From: simonpj Date: Thu, 17 Apr 2003 14:12:15 +0000 Subject: [PATCH] [project @ 2003-04-17 14:12:15 by simonpj] Comments --- ghc/compiler/types/FunDeps.lhs | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs index f8a9fba..6fd587a 100644 --- a/ghc/compiler/types/FunDeps.lhs +++ b/ghc/compiler/types/FunDeps.lhs @@ -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 @@ -280,6 +283,7 @@ 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), -- then we will call checkClsFD with -- @@ -287,7 +291,7 @@ checkClsFD qtvs fd clas_tvs tys1 tys2 -- 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 -- 1.7.10.4