[Haskell-cafe] Pointless functors
ekirpichov at gmail.com
Fri Mar 13 11:45:43 EDT 2009
Well, yes; then, that means that "an arbitrary element" is also false:
you can't just take an arbitrary element and hope that works well.
Then the original question must be reformulated:
- Are there any functors for which there exists at least one
non-bottom value of type 'f a' for some a, but there does not exist a
function point :: a -> f a such that (a != _|_) => (point a != _|_).
2009/3/13 Brent Yorgey <byorgey at seas.upenn.edu>:
> On Fri, Mar 13, 2009 at 05:35:31PM +0300, Eugene Kirpichov wrote:
>> 'An arbitrary element' means 'undefined will suffice'
>> point x = fmap (const x) undefined
> This is false.
> Prelude> fmap (const 1) [()]
> Prelude> fmap (const 1) undefined
> *** Exception: Prelude.undefined
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