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
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.
