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{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}
-- from https://ocharles.org.uk/blog/guest-posts/2014-12-19-existential-quantification.html
data HashMap k v = HM -- ... -- actual implementation
class Hashable v where
h :: v -> Int
data HashMapM hm = HashMapM
{ empty :: forall k v . hm k v
, lookup :: Hashable k => k -> hm k v -> Maybe v
, insert :: Hashable k => k -> v -> hm k v -> hm k v
, union :: Hashable k => hm k v -> hm k v -> hm k v
}
data HashMapE = forall hm . HashMapE (HashMapM hm)
-- public
mkHashMapE :: Int -> HashMapE
mkHashMapE = HashMapE . mkHashMapM
-- private
mkHashMapM :: Int -> HashMapM HashMap
mkHashMapM salt = HashMapM { {- implementation -} }
instance Hashable String where
type Name = String
data Gift = G String
giraffe :: Gift
giraffe = G "giraffe"
addGift :: HashMapM hm -> hm Name Gift -> hm Name Gift
addGift mod gifts =
let
HashMapM{..} = mod
in
insert "Ollie" giraffe gifts
-- -------------------------------
santa'sSecretSalt = undefined
sendGiftToOllie = undefined
traverse_ = undefined
sendGifts =
case mkHashMapE santa'sSecretSalt of
HashMapE (mod@HashMapM{..}) ->
let
gifts = addGift mod empty
in
traverse_ sendGiftToOllie $ lookup "Ollie" gifts
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