----------
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
-- 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
--
-- 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!
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