fused-effects-exceptions/README.md

# fused-effects-exceptions

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<!--
Setup, hidden from the rendered markdown.

```haskell
{-# LANGUAGE ScopedTypeVariables, TypeApplications #-}
module Main (module Main) where

import Control.Carrier.State.Strict
import Control.Effect.Exception

main :: IO ()
main = pure ()
```
-->


This package provides `Control.Effect.Exception`, a module that wraps the [`Control.Exception`](http://hackage.haskell.org/package/base/docs/Control-Exception.html) API from `base` with the vocabulary provided by the [`fused-effects`](http://hackage.haskell.org/package/fused-effects) library. These functions interact with GHC's support for dynamic exceptions, including functions like `catch` for exception handling and `bracket` for resource management.

Please be aware that injudicious use of these functions may provoke surprising interactions with carriers that thread a monadic state as a parameter, à la the `Control.Carrier.State` types provided by `fused-effects`. For example, a function like `finally`, which does not thread any state from its body to its handler block, may discard state writes in cleanup handlers:

```haskell
discardsState :: IO Char
discardsState = execState 'a' ((throwIO (userError "urk") `finally` put @Char 'z')
    `catch` (\(_ :: IOException) -> pure ()))
```

Though the `put @Char 'z'` statement is evaluated, its effect is ultimately discarded; the result of executing the above is `'a'`. If this behavior is a concern, a `Control.Carrier.State.IORef` carrier is provided, which fixes this issue given access to a `MonadIO` constraint. If it is not a concern (such as if the cleanup block is only run for its effects in `IO`), then the `StateC` carriers from `fused-effects` will suffice. For more information about the issues associated with this approach, consult Alexis King's excellent [Demystifying `MonadBaseControl`](https://lexi-lambda.github.io/blog/2019/09/07/demystifying-monadbasecontrol/).

Prior versions of this package provided a `Catch` effect; this has been excised in favor of the more-general `Control.Effect.Exception`, which provides more functionality without requiring any additional carriers beyond a `Lift IO` effect.