# Hask

### From HaskellWiki

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## Revision as of 23:59, 17 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

id . undefined = \x -> id (undefined x) = \x -> undefined x

undefined

seq

(undefined :: Int -> Int) `seq` ()

* Exception: Prelude.undefinedghci>

(id . undefined :: Int -> Int) `seq` ()

()

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