Hask

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.

• Makoto Hamana: What is the category for Haskell?

A solution approach to the issue of partiality making many of the identities required by categorical constructions not literally true in Haskell:

• Nils A. Danielsson, John Hughes, Patrik Jansson, and Jeremy Gibbons. Fast and loose reasoning is morally correct.

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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