36 lines
2.5 KiB
Markdown
36 lines
2.5 KiB
Markdown
# fused-effects-exceptions
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[](https://hackage.haskell.org/package/fused-effects-exceptions)
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[](LICENSE)
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[](https://github.com/fused-effects/fused-effects-exceptions/actions)
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<!--
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Setup, hidden from the rendered markdown.
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```haskell
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{-# LANGUAGE ScopedTypeVariables, TypeApplications #-}
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module Main (module Main) where
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import Control.Carrier.State.Strict
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import Control.Effect.Exception
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main :: IO ()
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main = pure ()
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```
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-->
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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.
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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:
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```haskell
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discardsState :: IO Char
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discardsState = execState 'a' ((throwIO (userError "urk") `finally` put @Char 'z')
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`catch` (\(_ :: IOException) -> pure ()))
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```
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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/).
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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.
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