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# Flexible Instances

See ExtensionDescriptionHowto for information on how to write these extension descriptions. Please add any new extensions to the list of HaskellExtensions.

## Brief Explanation

In Haskell 98, instance declarations must have the form `instance` (C1 v1, ..., Cn vn) `=>` C (T u1 ... uk), where T is a type constructor defined by a `data` or `newtype` declaration (see TypeSynonymInstances) and the ui are distinct type variables.

Without restrictions on the form of instances, constraint checking is undecidable (see UndecidableInstances).
A conservative rule that ensures termination (used by GHC with `-fglasgow-exts`) is to require instance heads of the form `instance` (C1 vs1, ..., Cn vsn) `=>` C t1 ... tk, where at least one of the ti is not a type variable (assuming MultiParamTypeClasses). The non-variable restriction can be onerous if OverlappingInstances are permitted.

Note that repeated type variables are permitted.

If the language has FlexibleInstances like

instance C [Bool] where ... instance C [Char] where ...

assertions like `C [a]` can be neither reduced nor rejected, so FlexibleContexts are also needed.

## References

- Instance declarations in the Haskell 98 Report
- Type classes: exploring the design space by Simon Peyton Jones, Mark Jones and Erik Meijer, Haskell Workshop 1997.
- Instance declarations in the GHC's User's Guide.

## Pros

- Pro
- Pro

## Cons

- Con
- Con