From 29aaba9386be26d0855c42cbc83d9ba1b9ac26c5 Mon Sep 17 00:00:00 2001
From: Gregor Kleen <gkleen@yggdrasil.li>
Date: Fri, 11 Dec 2015 14:50:03 +0100
Subject: somewhat better name

---
 nestedEither.hs | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
 create mode 100644 nestedEither.hs

(limited to 'nestedEither.hs')

diff --git a/nestedEither.hs b/nestedEither.hs
new file mode 100644
index 0000000..64b9412
--- /dev/null
+++ b/nestedEither.hs
@@ -0,0 +1,34 @@
+#! /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)
-- 
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