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# Allow Undecidable Instances
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## Brief Explanation
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There are no restrictions on the form of instance declarations (see [FlexibleInstances](flexible-instances)).
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Implementations ensure termination by making context reduction fail if it exceeds some depth limit.
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## References
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- [ Undecidable instances](http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#undecidable-instances) in the GHC User's guide.
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## Tickets
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<table><tr><th>[\#71](https://gitlab.haskell.org//haskell/prime/issues/71)</th>
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<td>Allow Undecidable Instances</td></tr></table>
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## Pros
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- allows safe instances that would be rejected by conservative termination checks (see [FlexibleInstances](flexible-instances)).
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- offered by GHC and Hugs with appropriate options for several years.
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- experience has shown that increasing the depth limit rarely makes a difference in which programs are accepted.
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## Cons
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- the boundary between legal and illegal programs is unclear, and possibly implementation-dependent. |
