[Haskell-cafe] Why is (+++) not a functor?

[David Barbour]

On Tue, Jul 12, 2011 at 6:55 PM, Felipe Almeida Lessa <
felipe.lessa at gmail.com> wrote:

> ArrowChoice's (+++) documentation [1] says that "this is in general
> not a functor" [1].  What does it mean for (+++) to be or not to be a
> functor?
>
>
The same note is made for (***). If you review Ross Patterson's paper [1],
you'll see a note: "Note that *** does not in general preserve composition;
for example, the order in which effects occur is significant." I expect the
note in Control.Arrow is an oblique reference to this 'fact'.

[1] http://www.soi.city.ac.uk/~ross/papers/notation.pdf
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