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

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] which just evaluates to [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 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 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

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