[Haskell-beginners] ($) operator

Brent Yorgey byorgey at seas.upenn.edu
Sat Jan 24 17:52:37 EST 2009

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].


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