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GHC/Type system

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Type system extensions in GHC

GHC comes with a rather large collection of type-system extensions (beyond Haskell 98). They are all documented in the user manual, but this page is a place to record observations, notes, and suggestions on them.

1 Overlapping instances

Here an interesting message about the interaction of existential types and overlapping instances.

2 Indexed data types and indexed newtypes

Indexed data types (including associated data types) are a very recent addition to GHC's type system extensions that is not yet included in the user manual. To use the extension, you need to obtain a version of GHC from its source repository.

3 Stand-alone deriving clauses

Bjorn Bringert has recently implemented "stand-alone deriving" declarations.