[Haskell-beginners] Re: the ($) function (was desugaring an example from RWH)

[Andrew Wagner]

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 at 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
>
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