Flexible Instances

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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

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• Pro

Cons

• Con
• Con