Hask refers to a category with types as objects and functions between them as morphisms. However, its use is ambiguous. Sometimes it refers to Haskell (actual Hask), and sometimes it refers to some subset of Haskell where no values are bottom and all functions terminate (platonic Hask). The reason for this is that platonic Hask has lots of nice properties that actual Hask does not, and is thus easier to reason in. There is a faithful functor from platonic Hask to actual Hask allowing programmers to think in the former to write code in the latter.

Consider:

```undef1 = undefined :: a -> b
undef2 = \_ -> undefined
```

Note that these are not the same value:

```seq undef1 () = undefined
seq undef2 () = ()
```

This might be a problem, because `undef1 . id = undef2`. In order to make Hask a category, we define two functions `f` and `g` as the same morphism if `f a = g a` for all `a`. Thus `undef1` and `undef2` are different values, but the same morphism in Hask.

Actual Hask does not have sums, products, or an initial object, and `()` is not a terminal object. The Monad identities fail for almost all instances of the Monad class.

Why Hask isn't as nice as you'd thought.
Initial Object Terminal Object Sum Product
Definition There is a unique function

`u :: Empty -> r`

There is a unique function

`u :: r -> ()`

For any functions

`f :: a -> r`
`g :: b -> r`

there is a unique function `u :: Either a b -> r`

such that: `u . Left = f`
`u . Right = g`

For any functions

`f :: r -> a`
`g :: r -> b`

there is a unique function `u :: r -> (a,b)`

such that: `fst . u = f`
`snd . u = g`

Platonic candidate `u1 r = case r of {}` `u1 _ = ()` `u1 (Left a) = f a`

`u1 (Right b) = g b`

`u1 r = (f r,g r)`
Example failure condition `r ~ ()` `r ~ ()` `r ~ ()`

`f _ = ()`
`g _ = ()`

`r ~ ()`

`f _ = undefined`
`g _ = undefined`

Alternative u `u2 _ = ()` `u2 _ = undefined` `u2 _ = ()` `u2 _ = undefined`
Difference `u1 undefined = undefined`

`u2 undefined = ()`

`u1 _ = ()`

`u2 _ = undefined`

`u1 undefined = undefined`

`u2 undefined = ()`

`u1 _ = (undefined,undefined)`

`u2 _ = undefined`

Result FAIL FAIL FAIL FAIL