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# Imprecise Exceptions
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**Ticket:**[\#39](https://gitlab.haskell.org//haskell/prime/issues/39)
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See [ExtensionDescriptionHowto](extension-description-howto) for information on how to write these extension descriptions. Please add any new extensions to the list of [HaskellExtensions](haskell-extensions).
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## Brief Explanation
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In Haskell 98, exceptions may be caught and thrown only in the `IO` monad.
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The `error` function generates failures from pure code, but cannot be caught.
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Exceptions may be thrown by pure code and caught in the `IO` monad.
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Imprecise exceptions may be thrown by pure code and caught in the `IO` monad.
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IO exceptions and `error` are subsumed as special cases.
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## References
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- [ A semantics for imprecise exceptions](http://research.microsoft.com/Users/simonpj/Papers/imprecise-exn.htm), Simon Peyton Jones, Alastair Reid, Tony Hoare, Simon Marlow and Fergus Henderson. PLDI 1999.
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- [ Control.Exception](http://www.haskell.org/ghc/dist/current/docs/libraries/base/Control-Exception.html)
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- [ExtensibleExceptions](extensible-exceptions)
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## Pros
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- more robust programs
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- implemented by GHC and Hugs for several years
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## Cons
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- complicates denotational semantics a little |
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- very tricky to implement properly
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- requires non-trivial run-time support and implementation constraints |
