On Jan 16, 2007, at 7:22 PM, David House wrote:
Hey all,
I've written a chapter for the Wikibook that attempts to teach some
basic Category Theory in a Haskell hacker-friendly fashion.
http://en.wikibooks.org/wiki/Haskell/Category_theory
In the section on the category laws you say that the identity
morphism should satisfy
f . idA = idB . f
This is not strong enough. You need
f . idA = f = idB . f
Unfortunately, fixing this means that the category Hask is no longer
a category since
_|_ . id = \x -> _|_ ≠ _|_
Also it's a bit strange to state that morphisms are closed under
composition after the associativity law. Wouldn't it be nicer to
introduce composition as a total operation off the bat?
/ Ulf
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