On Tuesday 06 July 2010 13:00:33, Tom Hobbs wrote:
In people's responses to my serialization questions, I've seen them using $.
I didn't know what it was so I've looked it up. Can someone please confirm my understanding of what it does, please?
According to http://en.wikibooks.org/wiki/Haskell/Practical_monads, after the second code sample in the "Return Values" section, it seems to suggest that $ is only used to avoid using so many brackets. Which
"only" is an exaggeration, make it "mostly". Other common uses are map ($ 3) functionList and zipWith ($) functions arguments it's not necessary, you can get the second from zipWith id functions arguments (even using one keystroke less!) and the first from map (flip id 3) functionList or map (\f -> f 3) functionList As for the zipWith, there's a slight advantage in that ($) stands out more than id, without blacking out the rest. As for the map, well, it takes beginners some time usually to figure out what flip id does (and causes surprise that it's even possible, because flip :: (a -> b -> c) -> b -> a -> c id :: t -> t doesn't make it obvious). And the lambda-expression isn't too beautiful either.
seems to make sense, but looking at it's definition in Prelude I really can't see why it's useful.
Yitz gave me the code;
fmap (runGet $ readNames n) $ L.hGetContents h
So can I rewrite this without the $ like this?
fmap (runGet (readNames n)) (L.hGetContents h)
Yes, that's equivalent. But with deeper nesting, judicious use of ($) can make the code much more readable.
Is there any additional benefit to using $ than just not having to write as many brackets?
See above, it can make things more readable in several ways. But it shouldn't be overused. res = f . g . h . i $ j x is better (IMO) than res = f $ g $ h $ i $ j $ x
Thanks,
Tom