Add bidirectional project

rauhala.info/projects/2021-01-21-bidirectional.md

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+---
+title: bidirectional
+github: https://github.com/MasseR/bidirectional
+issues: https://github.com/MasseR/bidirectional/issues
+badge: https://github.com/MasseR/bidirectional/workflows/Test/badge.svg
+---
+
+Bidirectional serialization based on Lysxia's post on [Monadic profunctors for bidirectional programming](https://blog.poisson.chat/posts/2017-01-01-monadic-profunctors.html).
+
+
+Let's assume we have a parser like the following
+
+``` haskell
+int :: Parser (ReaderT String Maybe) (Writer [Int]) Int Int
+int = parser (ReaderT readMaybe) (\x -> x <$ tell [show x])
+```
+
+Then you can use the parser for parsing:
+
+```
+> runReaderT (decode int) "3"
+Just 3
+```
+
+Or for encoding:
+
+```
+> execWriter (encode int 3)
+["3"]
+```
+
+Or combine both of them
+
+```
+> runReaderT (decode int) $ head $ execWriter $ encode int 3
+Just 3
+```