Write You a Haskell Stephen Diehl

where s’ = foldr Map.delete s as
ftv (Forall as t) = ftv t ‘Set.difference‘ Set.fromList as
instance Substitutable a => Substitutable [a] where
apply = fmap . apply
ftv = foldr (Set.union . ftv) Set.empty
instance Substitutable TypeEnv where
apply s (TypeEnv env) = TypeEnv $ Map.map (apply s) env
ftv (TypeEnv env) = ftv $ Map.elems env
roughout both the typing rules and substitutions we will require a fresh supply of names. In this naive
version we will simply use an infinite list of strings and slice into n’th element of list per a index that we
hold in a State monad. is is a simplest implementation possible, and later we will adapt this name
generation technique to be more robust.
letters :: [String]
letters = [1..] >>= flip replicateM [’a’..’z’]
fresh :: Infer Type
fresh = do
s