[Haskell-beginners] Why is this not a "category"?

[Kim-Ee Yeoh]

On Sat, Mar 29, 2014 at 12:18 PM, John M. Dlugosz
<ngnr63q02 at sneakemail.com>wrote:

> "(Harder.) If we add another morphism to the above example, it fails to be
> a category. Why? Hint: think about associativity of the composition
> operation."


One has to be careful with crowd-sourced wikipedia-like learning material.
They aren't always helpful, although 80% of the time they are.

In this case, the 2 exercises probably belong right after the section on
"Category Laws" but _before_ "Hask, the Haskell category."

That way, there's less confusion about what the exercises refer to and what
they do NOT refer to. They certainly don't refer to Hask.

That said, the exercises presume acquaintance with partial orders / posets
so there's some web-trawling to be done if the requirement is unmet.

And I'll also argue that the exercises aren't helpful. They may have their
place in some musty math textbook. But for a general audience, it's like
having some obscure concept (category) explained in terms of another
obscure concept (partial order), with no-one getting any wiser.

p.s. if you are still stuck email me off-list for the answer.

-- Kim-Ee
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