Lambda Library

This is a sample project to compare how side-effects can be managed and tested in Haskell using different techniques.

Each alternative has it's own app<N> and test<N> project, and they all share common library code from src.

There are some docs describing each alternative

• Part 1, Introduction
• Part 2, Console Handler
• Part 3, Task
• Part 4, Type Classes
• Part 5, ReaderT design pattern

Alternatives

• app1 uses IO functions direcly. The tests replace & capture the STDIN/STDOUT.
• app2 uses the handler pattern to be able to replace the functions to interact with the console in tests.
• app3 similar to app2 but restricts which other side-effects can happen by introducing a Task to replace IO.
• app4 uses custom type classes to restrict other side-effects from happening.
• app5 similar to app4 but using ReaderT to keep the application state

Additional alternatives based on packages

• app-bluefin similar to app2 and app3 but uses bluefin package as a type level alternative of the handler pattern.
• 🚧 app-effectful uses effectful
  • TBD tests
• 🚧 app-polysemy
• ❌ app-capabilities similar to app5 but uses Capabilities package.
  • ❌ Capabilities packages only works for base >= 4.5 && 4.6. We are using 4.17. I wanted to try it out based on its blog post

Running the code

The code was developed with ghc 9.4.8 and cabal 3.10.3.0 as they were the recommended version at time.

% ghcup run --ghc 9.4.8 --cabal 3.10.3.0 -- zsh
# or % ghcup run --ghc recommended --cabal recommended -- zsh

% ghc --version
The Glorious Glasgow Haskell Compilation System, version 9.4.8

% cabal --version
cabal-install version 3.10.3.0
compiled using version 3.10.3.0 of the Cabal library

Each app and test can be run with cabal directly.

% cabal run app1
% cabal test app1-test