Version 3 (modified by ross@…, 10 years ago) (diff)

Type Synonym Instances

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Brief Explanation

Haskell 98 permits only type constructors defined using data or newtype in instance heads.

The proposal is to allow type synonyms (fully applied, as ever) in instance heads. These would be fully expanded before any other restrictions on instance heads were checked (see FlexibleInstances and OverlappingInstances).

Not very useful without either FlexibleInstances or UndecidableInstances.



  • Offered by GHC and Hugs for several years.
  • Synonyms can cut large instance heads to a manageable size, e.g. when building a monad using monad transformers.
  • Allows a library author to generalize types while retaining source compatibility with old client instances. e.g. replacing
    data T a = ...
    data GenT param a = ...
    type T = GenT Default


  • Since constraints on the instance head refer to the expanded type, errors can be more obscure.