Re: [Haskell-cafe] Questions about the Functor class and it's use in "Data types à la carte"

14 Dec 2007


      On Dec 15, 2007 3:15 AM, Ryan Ingram  wrote:
...
On 12/14/07, David Menendez  wrote:
...
And yes, I'm pretty sure that's the only possible implementation for that
type which satisfies the functor laws.
Lets just prove it, then; I'm pretty sure it comes pretty easily from the
free theorem for the type of fmap.
...
...
Now, first note that since we cannot make any choices based on the type of
f, there's no way for f to be relevant to the result of fmap; we have no way
to get something of type "a" besides _|_ to pass to f, and no use for things
of type "b".
Hmmm. Something about that ticks off my "don't play fast and loose
with bottom" detector.

data Val a = Val Int

instance Functor Val where
    fmap f (Val x) = f `seq` Val x

:-)

- Benja

Re: [Haskell-cafe] Questions about the Functor class and it's use in "Data types à la carte"

Benja Fallenstein