[Haskell-cafe] lawless instances of Functor

Dan Piponi dpiponi at gmail.com
Mon Jan 4 18:42:13 EST 2010

On Mon, Jan 4, 2010 at 3:26 PM, Derek Elkins <derek.a.elkins at gmail.com> wrote:

> Yes, I have the same problem...Basically, I'm
> pretty sure the construction of that free theorem doesn't rely on any
> of the actual details...

For a long time I've thought such a higher order free theorem must
exist, and I've mentioned it to a few people, and searched hard for a
paper on it, but I haven't seen an actual statement and proof.

> At this point, though, I haven't put
> much effort into proving that the free theorem holds uniformly

Well I encourage you to as I've a hunch the correctly generalised
theorem will be quite pretty. I'd have a go but the style of proof for
these sorts of things is outside of my domain of

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