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Hask

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(The seq problem)
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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>
 
()
 
()
 
   
 
{{stub}}
 
{{stub}}

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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