Sometimes the apply operator $ can be handy for other purposes too, e.g.
map ($ 1) [cos, sin, tan]
evaluates to
[cos $ 1, sin $ 1, tan $ 1]
[cos 1, sin 1, tan 1]
Note that some consider the following as bad coding style:
f $ g $ h $ x
Instead, the following is preferred
f . g . h $ x
On Mon, Feb 23, 2009 at 7:27 PM, Andrew Wagner
<wagner.andrew@gmail.com> wrote:
The $ function is essentialy a "no-op". That is, it literally does nothing. There is no difference in Haskell between a function and a reference to it. The only purpose of $ is for grouping. The line in question below could have been written identically as "fromQty <- atomically (readTVar fromBal)". The $ groups together everything to the end of the line, and can be used to avoid parentheses that could add noise to the code.
On Mon, Feb 23, 2009 at 1:18 PM, Michael Easter
<codetojoy@gmail.com> wrote:
re: desugaring. Thanks Andrew, that worked great!
I have another, easier question from the same code in RWH:
bogusTransfer qty fromBal toBal = do
fromQty <- atomically $ readTVar fromBal
[snip]
Can someone please explain the ($) function in English? From the type signature, it seems to be an "apply function", but I can't quite explain when we use it. Clearly, it is used throughout RWH but I haven't found a good explanation.
My guess is that it is when we want an "execution" of a function rather than a mere reference to it. Is that accurate?
thanks again
Michael
--
----------------------
Michael Easter
http://codetojoy.blogspot.com: Putting the thrill back in blog
http://youtube.com/ocitv -> Fun people doing serious software engineering
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners