https://github.com/parsonsmatt/exceptiot

A type `ExceptIOT` which uses `IO` for the `MonadError` instance, allowing a `MonadUnliftIO` instance
https://github.com/parsonsmatt/exceptiot

Last synced: 10 days ago
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A type `ExceptIOT` which uses `IO` for the `MonadError` instance, allowing a `MonadUnliftIO` instance

• Host: GitHub
• URL: https://github.com/parsonsmatt/exceptiot
• Owner: parsonsmatt
• License: bsd-3-clause
• Created: 2022-11-22T18:15:32.000Z (almost 2 years ago)
• Default Branch: master
• Last Pushed: 2022-11-22T23:38:14.000Z (almost 2 years ago)
• Last Synced: 2024-03-15T02:52:00.370Z (8 months ago)
• Language: Haskell
• Size: 18.6 KB
• Stars: 5
• Watchers: 3
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: changelog.md
  • License: LICENSE

Awesome Lists containing this project

README

        
# `exceptiot`

Sometimes, it is nice to write code in `MonadError` to indicate that the code

can throw an exception.

```haskell

foo :: MonadError FooError m => Int -> m Char

```

We can call `foo` in `Either FooError` for a pure test, or a `ExceptT FooError

IO` if we are writing in some monad directly. However, sometimes you can't use

`ExceptT`. One possible reason is performance: `ExceptT` can add overhead.

Another reason is `MonadUnliftIO` - `ExceptT` cannot have an instance for this.

You don't want to *stop* using `MonadError`, but you also can't use `ExceptT`

anymore. What can you do?

```haskell

import Control.Monad.Except.IO

main :: IO ()

main = do

    eres <- runExceptIOT (foo 3)

    case eres of

        Left FooError ->

            putStrLn "got FooError"

        Right c ->

            putChar c

```

You can replace `ExceptT` with `ExceptIOT`, and all exception use will switch to

`IO` based exceptions instead of `Either`-based values. This allows you to use

`MonadUnliftIO` without fear.