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# Extension Name
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See [ExtensionDescriptionHowto](extension-description-howto) for information on how to write these extension descriptions. Please add any new extensions to the list of [HaskellExtensions](haskell-extensions).
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## Brief Explanation
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Associated types are data type and type synonym declarations in type classes. They are useful to denote type-indexed data types (i.e., generic programming) and functions over types. Applications include the typical applications of functional dependencies, but also self-optimising libraries and APIs that resemble ML module signatures.
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## References
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- [ Associated Types with Class](http://www.cse.unsw.edu.au/~chak/papers/CKPM05.html) by Manuel M. T. Chakravarty, Gabriele Keller, Simon Peyton Jones, and Simon Marlow, in POPL'05.
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- [ Associated Type Synonyms](http://www.cse.unsw.edu.au/~chak/papers/CKP05.html) by Manuel M. T. Chakravarty, Gabriele Keller, and Simon Peyton Jones, in ICFP'05.
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- [ phrac](http://www.cse.unsw.edu.au/~pls/repos/phrac/) (Darcs repo) is a prototype implementation of HM type inference with ATs.
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## Pros
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- Associated types are a functional alternative to [FunctionalDependencies](functional-dependencies), which are actually relational. (Why use relational programming on the value level if we don't do it on the value level?)
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- They avoid some problems with FDs (described in the above papers).
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- They closes the gap reported in [ A comparative study of language support for generic programming](http://www.osl.iu.edu/publications/prints/2003/comparing_generic_programming03.pdf)
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## Cons
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- Only a prototype implementation so far |
