Hask

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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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