Re: On on (was: [GHC] #971: Add intercalate and split to Data.List)

On 2006-11-02 at 15:28+0100 Nils Anders Danielsson wrote:

On Mon, 30 Oct 2006, Jón Fairbairn wrote:

...

We can write “groupBy (\a b -> p a && p b)” [...]

You mean “groupBy ((&&) `on` p)”, right? ;)

Something very much like that!

Do you consider on to be above or below the Fairbairn threshold, by the way?

Above. It's hard to articulate, but the reasons you give for `on` seem about right, so long as on is defined at a general enough type (and it's hard to beat forall a b c. (a -> a -> b) -> (c -> a) -> c -> t -> b for generality!) To explore this a bit further with a different example, I would have been in favour of introducing (f >< g) (a, b) = (f a, f b) but not mapFst f (a, b) = (f a, b) because the latter is simply (f >< id), which has a nice "parallel" look to it; it seems easier to me to see that (f >< id) . (id >< g) = (f >< g) than that mapFst f . mapSnd g = (f >< g) Now, though, there's &&& in Control.Arrow, and that subsumes

<, so I'm nolonger enthusiastic about (><) (though I might define it locally in circumstances where I think my readers would be unfamiliar with &&&).

One of the considerations is that once you put a name into a library, you'll have the devil's own job getting it out again if you decide it was a mistake. Jón -- Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk