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#! /usr/bin/env nix-shell
#! nix-shell -i ghci shell.nix
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeSynonymInstances #-}
type (:||:) = Either
infixr 1 :||:
class Contains h n where
extract :: h -> Maybe n
insert :: n -> h
instance Contains (a :||: b) a where
extract (Left a) = Just a
extract _ = Nothing
insert = Left
instance Contains (a :||: b) b where
extract (Right a) = Just a
extract _ = Nothing
insert = Right
instance {-# OVERLAPS #-} Contains h n => Contains (a :||: h) n where
extract (Right a) = extract a
extract _ = Nothing
insert = Right . insert
test :: Maybe String
test = extract (insert "test" :: String :||: Integer :||: Float)
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