Heftia

Heftia, a composition of hefty trees and co-Yoneda, is a higher-order effects version of Freer.

The paper

• Casper Bach Poulsen and Cas van der Rest. 2023. Hefty Algebras: Modular Elaboration of Higher-Order Algebraic Effects. Proc. ACM Program. Lang. 7, POPL, Article 62 (January 2023), 31 pages. https://doi.org/10.1145/3571255

inspires this library. Hefty trees, proposed by the above paper, are extensions of free monads, allowing for a straightforward treatment of higher-order effects.

This library offers Heftia monads and Freer monads, encoded into data types in several ways to enable tuning in pursuit of high performance.

Additionally, it's designed to operate as a handler system based on classy-effects, which aims to standardize and unify the definitions of effects in Haskell.

Compared to existing Effect System libraries in Haskell that handle higher-order effects, this library's approach allows for a more effortless and flexible handling of higher-order effects. Here are some examples:

• Two interpretations of the censor effect for Writer

  Let's consider the following Writer effectful program:

  hello :: (Writer String m, Monad m) => m ()
  hello = do
      tell "Hello"
      tell " world!"
  
  censorHello :: (Writer String m, Monad m) => m ()
  censorHello =
      censor
          (\s -> if s == "Hello" then "Goodbye" else s)
          hello
  

  For censorHello, should the final written string be "Goodbye world!"? Or should it be "Hello world!"? With Heftia, you can freely choose either behavior depending on which higher-order effect interpreter (which we call an elaborator) you use.

  main :: IO ()
  main = runFreerEffects do
      (s :: String, _) <-
          interpretTell
              . runElaborate' (elaborateWriterT @String)
              $ censorHello
  
      (sTransactional :: String, _) <-
          interpretTell
              . runElaborate' (elaborateWriterTransactionalT @String)
              $ censorHello
  
      sendIns $ putStrLn $ "Normal: " <> s
      sendIns $ putStrLn $ "Transactional: " <> sTransactional
  

  Using the elaborateWriterT elaborator, you'll get "Goodbye world!", whereas with the elaborateWriterTransactionalT elaborator, you'll get "Hello world!". For more details, please refer to the complete code and the implementation of the elaborator.

• Extracting Multi-shot Delimited Continuations

  In handling higher-order effects, it's easy to work with multi-shot delimited continuations. This enables an almost complete emulation of "Algebraic Effects and Handlers". For more details, please refer to Example 3 - Delimited Continuation .

Furthermore, the structure of Heftia is theoretically straightforward, with ad-hoc elements being eliminated.

Heftia is the second objective of the Sayo Project.

Documentation

Examples with explanations can be found in the docs/examples/ directory.

Future Plans

• Benchmarking

• Enriching the documentation

• Completing missing definitions such as

  • the Heftia monad transformer encoded in tree structure
  • handlers for the Accum, Coroutine, Fresh, Input, Output effect classes

  and others.

License

The license is MPL 2.0. Please refer to the NOTICE. Additionally, this README.md and the documents under the docs/docs-ja directory are licensed under CC BY-SA 4.0.

Your contributions are welcome!

Please see CONTRIBUTING.md.

Credits

Parts of this project have been adapted or inspired by the following resources:

• Hefty Algebras -- The Artifact
  • Copyright (c) 2023 Casper Bach Poulsen and Cas van der Rest
  • License: MIT
  • Modifications: The inspiration for the idea of Heftia. Code was used in the Data.{Free,Hefty}.Sum.