1 files changed, 34 insertions, 0 deletions

diff --git a/nestedEither.hs b/nestedEither.hs
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+#! /usr/bin/env nix-shell
+#! nix-shell -i ghci shell.nix
+
+-- Type indexed nested Eithers
+
+{-# 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)