Hask

Revision as of 23:59, 17 November 2009 by F.s. (Talk | contribs)

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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