Brent Yorgey
On Mon, Dec 03, 2012 at 03:20:41PM +0000, Miguel Negrao wrote:
A 03/12/2012, às 12:53, Brent Yorgey escreveu:
Is there a syntax in Haskell for partial application that would be something like this the following ?
f a b c d = a +b+ c+ d
g = f _ 1 2 3 Yes, a lambda is the only way to do it.
Given how compact haskell’s syntax can be, is there a reason why implementing such placeholders is not a good idea for haskell ?
Yes: it would introduce horrible ambiguity. Consider
(f (g _ 1 2))
Does it mean
(f (\x -> g x 1 2))
or
(\x -> f (g x 1 2))
? I don't know of any good reason to prefer one over the other. Any rule you come up with is likely to have other weird corner cases you hadn't considered. As I understand it, Scala actually does have this feature, along with lots of confusing, ugly, ad-hoc rules to disambiguate situations like the above.
Another data-point: Mathematica also has this feature, if I correctly understand the above posts [1]. The examples you gave would look like:: (f (g _ 1 2)) ~= f[g[#1, 1, 2]]& (f (\x -> g x 1 2)) ~= f[g[#1, 1, 2]&] (\x -> f (g x 1 2)) ~= f[g[#1, 1, 2]]& So it seems the ambiguity is resolved by having this scope-specifying symbol '&'. [1] http://reference.wolfram.com/mathematica/tutorial/PureFunctions.html -Keshav