
On Mon, Jan 4, 2010 at 4:15 PM, Paul Brauner <[email protected]> wrote:
> I wonder why this isn't some "classic" category theory result (maybe it is ?)
It doesn't hold in category theory in general.
Haskell (or at least a certain subset) is special  many things that
just *look* category theoretical turn out to actually *be* category
theoretical. That's quite a strong statement. So, for example,
functions that have the type signature of a natural transformation are
in fact natural transformations (which tells you nontrivial facts
about the function). And you've found that things that are only
halfdefined functors are actually fullblown functors (subject to
someone providing a rigorous proof of this...). This is useful. If
you're using QuickCheck to convince yourself you've implemented a
functor correctly, you only need to test it on id.

Dan
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