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Last active Jan 10, 2016
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walk :: Var -> Subst -> Var
walk var subst = go var subst
where go :: Var -> List Var -> Maybe Var -> Var
go (Unbound k) stack acc =
case (subst k, acc) of
(Nothing, Nothing) -> Unbound k
(Nothing, Just u) -> Cons u v
(Just v, _) -> go v stack acc
go (Cons h t) stack acc = go h (t :: stack) acc
go v [] Nothing = v
go v [] (Just u) = Cons u v
go v (x :: xs) Nothing = go x xs (Just v)
go v (x :: xs) (Some u) = go x xs v (Just (Cons u v))
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