2008/4/24 Twan van Laarhoven
Cale Gibbard wrote:
Hello,
In keeping with my small but seemingly extremely controversial suggestions for changes to the Prelude, here's a suggestion which I think is elegant and worth considering for the Haskell' Prelude:
Rename fmap to map (like it was in Haskell 1.4), and define (.) as a synonym for it.
One thing I fear (though that fear may be irrational) is that you get code that looks like "(.) . ((.) . (.) .)". To me, and I expect to many people, map and composition are different things, and used in different ways. If both are written as a dot it will take extra mental effort to decipher the meaning of a program. The potential for writing code that resembles the worst outputs of the @pl lambdabot plugin also becomes larger.
This is why I recommend having (.) only be a synonym for map (which would be the method of Functor).
Cale: do you have some real world examples of code you wrote using (.) = fmap?
I haven't used the convention in anything too large, but I've found it rather convenient and natural in the case of, for instance, IO, to be able to write things like map read . lines . getContents. I've played around with it in a lot of small cases and not found ambiguity to be much of a problem. It turns out that the functor is basically always determined by the last thing in the chain of (.) applications, so it remains sensible.
Secondly, I am really fond of the Applicative notation <$>, which goes great together with <*>. A lighter notation would be nice, but I see no good way to do that. (Perhaps we need to add syntactic sugar for idiom brackets?)
Yeah, that's something to think about. I agree that the appearance of <$> mixes well with the other Applicative operators, and should likely remain a part of that library. Adding special syntactic support for Applicative could be very nice though. - Cale