Embedding by Unembedding

This is a proof-of-concept implementation of Embedding by Unembedding.

Some examples can be found in https://github.com/kztk-m/EbU-examples.

Overview

Higher-order abstract syntax is one of the user-friendly ways to represent the EDSL's syntax.

-- Finally-tagless style (Carette+ 09)
class MyLang e where 
  -- ... 
  add  :: e Int -> e Int -> e Int
  let_ :: e a -> (e a -> e b) -> e b

However, since the expression type operator (e) is parameterized only by its guest type, it is unclear how we can implement semantics that refers to environments (technically, semantics that are defined for terms-in-context instead of closed terms). Examples of such semantics include incremental computations, bidirectional transformations, and automatic differentiation.

This library provides operators that address this gap, based on Atkey 09's unembedding---a provably correct isomorphism between de Bruijn-indexed terms and (parametric) higher-order abstract syntax.

data Sem as a = ... -- semantic domain 

addSem :: Sem env Int -> Sem env Int -> Sem env Int 
letSem :: Sem env a -> Sem (a : env) b -> Sem env b 

instance MyLang (EnvI Sem) where 
  -- ...
  add = liftFO2 addSem -- or liftSO (ol0 :. ol0 :. End) addSem 
  let_ = liftSO (ol0 :. ol1 :. End) letSem

References

Kazutaka Matsuda, Samantha Frohlich, Meng Wang, and Nicolas Wu. 2023. Embedding by Unembedding. Proc. ACM Program. Lang. 7, ICFP, Article 189 (August 2023), 47 pages.