27 lines
2.4 KiB
Markdown
27 lines
2.4 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://travis-ci.org/patrickt/fused-effects-exceptions)
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This package provides two useful effects for handling exceptions thrown from pure code or IO with GHC's `Control.Exception.throw` function.
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## Control.Effect.Resource
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This effect provides `bracket`, `finally`, and `onException` functions capable of allocating and freeing scarce resources in the presence of GHC's exceptions. It is similar in functionality to the [`resourcet`](http://hackage.haskell.org/package/resourcet) package.
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This effect was included in prior versions of `fused-effects`, but has been moved to this package due to the surprising interactions it can have with the `Control.Carrier.State` carriers provided by `fused-effects`. If you use the `ResourceC` and `StateC` effects in conjunction, writes inside a `finally` block may be discarded, since `finally` discards the result of its cleanup handler:
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```haskell
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λ runM (runResource (runState 'a' (modify (succ @Char) `finally` modify (succ . succ @Char))))
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('b', ())
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```
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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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## Control.Effect.State
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This effect is similar to the `MonadCatch` and `MonadThrow` classes provided by the `exceptions` package. It delegates to `catch` from `Control.Exception`. An additional `catchSync` primitive is provided to handle the common case of catching only synchronous exceptions. Its implementation was extracted from one originally written by Josh Vera.
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This effect displays the same behavior associated with `Resource` in that carriers like `Control.Carrier.State.Strict` which rely on return types to propagate state may drop state information.
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