Difference between revisions of "Hask"
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* [http://www.cs.gunma-u.ac.jp/~hamana/Papers/cpo.pdf Makoto Hamana: ''What is the category for Haskell?''] |
* [http://www.cs.gunma-u.ac.jp/~hamana/Papers/cpo.pdf Makoto Hamana: ''What is the category for Haskell?''] |
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+ | A solution approach to the issue of partiality making many of the identities required by categorical constructions not literally true in Haskell: |
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+ | * [http://www.cs.nott.ac.uk/~nad/publications/danielsson-popl2006-tr.pdf Nils A. Danielsson, John Hughes, Patrik Jansson, and Jeremy Gibbons. ''Fast and loose reasoning is morally correct.''] |
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== The seq problem == |
== The seq problem == |
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ghci> <hask>(id . undefined :: Int -> Int) `seq` ()</hask> |
ghci> <hask>(id . undefined :: Int -> Int) `seq` ()</hask> |
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() |
() |
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Revision as of 04:29, 13 November 2009
Hask is the name usually given to the category having Haskell types as objects and Haskell functions between them as morphisms.
A type-constructor that is an instance of the Functor class is an endofunctor on Hask.
A solution approach to the issue of partiality making many of the identities required by categorical constructions not literally true in Haskell:
The seq problem
The right identity law fails in Hask if we distinguish values which can be distinguished by seq
, since:
id . undefined = \x -> id (undefined x) = \x -> undefined x
should be equal to undefined
, but can be distinguished from it using seq
:
ghci>(undefined :: Int -> Int) `seq` ()
* Exception: Prelude.undefined ghci>(id . undefined :: Int -> Int) `seq` ()
()
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