Type Synonym Instances
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).
- allow TypeSynonymInstances
- 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 = ...with
data GenT param a = ... type T = GenT Default
- Since constraints on the instance head refer to the expanded type, errors can be more obscure.
This change would make type synonyms a bit more like newtypes. NewtypeDeriving makes newtypes a bit more like type synonyms since they can be made to inherit properties of the underlying type, albeit (usefully) selectively. Having both type synonyms and newtype is a bit confusing, and the more alike they are, the more confusing it is. I guess looking for a single replacement is not an option for Haskell', but at least we should consider if automatic derivation for newtypes does not supersede this one as it would make it significantly less tiresome to introduce a newtype for purposes of abbreviation.