On Sat, Jan 24, 2009 at 07:37:29PM +0000, John Hartnup wrote:
Hi.
I'm working through Real World Haskell, and although it's going well (I just finished the exercise to write a glob matcher without using a regex library, and I'm pleased as punch), I keep seeing the ($) operator, and I'm not sure I understand its use. If the book explains it, I've been unable to find it.
Empirically, it seems like: a $ b c d e f .. is equivalent to .. a (b c d e f)
But is that it's only purpose? To placate the LISP haters by removing parentheses?
(1 +) 2 does the same thing as (1 +) $ 2, and has the same type.
Am I missing something?
More generally, in a language with first-class and higher-order functions like Haskell, it's simply useful to have a function which performs function application. For example, one place that ($) comes in handy is when applying each function in a list of functions to a single input value. Using ($) you can just write something like map ($5) [(+1), (^2), abs] which evaluates to [6,25,5]. -Brent