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Simon Peyton Jones authored
------------------------------------ Desugar existential matches correctly ------------------------------------ Consider data T = forall a. Ord a => T a (a->Int) f (T x f) True = ...expr1... f (T y g) False = ...expr2.. When we put in the tyvars etc we get f (T a (d::Ord a) (x::a) (f::a->Int)) True = ...expr1... f (T b (e::Ord a) (y::a) (g::a->Int)) True = ...expr2... After desugaring etc we'll get a single case: f = \t::T b::Bool -> case t of T a (d::Ord a) (x::a) (f::a->Int)) -> case b of True -> ...expr1... False -> ...expr2... *** We have to substitute [a/b, d/e] in expr2! ** Originally I tried to use (\b -> let e = d in expr2) a to do this substitution. While this is "correct" in a way, it fails Lint, because e::Ord b but d::Ord a. So now I simply do the substitution properly using substExpr.
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