Flexible Instances

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Brief Explanation

In Haskell 98,

• an instance head must have the form C (T u₁ ... u_k), where T is a type constructor defined by a data or newtype declaration (see TypeSynonymInstances) and the u_i are distinct type variables, and
• each assertion in the context must have the form C' v, where v is one of the u_i.

Without restrictions on the form of instances, constraint checking is undecidable (see UndecidableInstances). A conservative rule (used by GHC with -fglasgow-exts) that ensures termination is:

• at least one of the type arguments of the instance head must be a non-variable type, and
• each assertion in the context must have the form C v₁ ... v_n, where the v_i are distinct type variables.

The distinctness requirement prohibits non-terminating instances like

instance C b b => C (Maybe a) b

Note that repeated type variables are permitted in the instance head, e.g.

instance MArray (STArray s) e (ST s)

With this extension, one can write instances like

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

and 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

• Such instances often arise with MultiParamTypeClasses.

Cons

• The above restrictions rule out some useful instances, e.g. derived instances like:

  data Sized s a = N Int (s a)
          deriving (Show)
  

  remain illegal, because the inferred instance has an illegal context:

  instance Show (s a) => Show (Sized s a)
  

  (see  Context reduction errors in the Haskell 98 Report). The following examples from the GHC User's Guide are also forbidden:

  instance C a
  instance (C1 a, C2 a, C3 a) => C a
  

  An alternative criterion using a multiset path ordering would allow the Sized example and the first of these, but the second cannot be shown to terminate without considering other instances in the program.