Right, a bit of playing around and I think understand. Maybe.
In your example;
map ($ 3) functionList
I'm assuming that this is Haskel's way of saying "Give the value 3 as an argument to each function in functionList". Playing in Hugs seems to suggest that this is the case;
Hugs> map ($ 3) [(4 +), (5 +), (6 +)]
[7,8,9]
That makes sense. The first arg to map is expecting a function, so we give it a function and a value which just returns the value. I can see why that works.
Rewriting using "flip id" has me stumped though.
Hugs> map (flip id 3) [(4 +), (5 +), (6 +)]
[7,8,9]
So this is saying,
Actually, I don't know. Working it out in my head I would say "id 3" returns 3. But "flip 3" causes an error in Hugs, so this code doesn't work - but clearly it does.
Hugs> :t flip id 3
flip id 3 :: Num a => (a -> b) -> b
Hugs> :t ($ 3)
flip ($) 3 :: Num a => (a -> b) -> b
Is presumably why it works, but I can't work out how to create the type signature of "flip id 3" from the sigs of flip and id.
Help?
Thanks.
Tom
On Tue, Jul 6, 2010 at 12:19 PM, Daniel Fischer
<daniel.is.fischer@web.de> wrote:
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
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