patch applied (haskell-prime-status): add ""Make $ left
associative, like application"
Josef Svenningsson
josef.svenningsson at gmail.com
Wed Apr 23 05:09:26 EDT 2008
First I would like to remind everyone of Wadler's law of language design:
http://www.haskell.org/haskellwiki/Wadlers_Law
Having said that I'm now going to argue for yet another color for the bike shed.
I was a little surprised by the list of motivations for changing the
fixity of $ because it didn't contain the one argument I was in fact
expecting to hear.
0) $ was introduced as a combinator for function application.
Therefore we might expect that whenever we have a function application
we can stick a $ in there. But this is not the case. Consider the
following expression:
f x y
There are two applications here and if $ behaved like function
application we would be able to write:
f $ x $ y
But as it is now this expression means something completely different.
I expected this discrepancy between function application and $ to be
the major reason for changing the fixity.
That being said, this change would break a lot of my code as well and
I'm not a big fan of it.
Cheers,
Josef
On Wed, Apr 23, 2008 at 3:02 AM, Dan Doel <dan.doel at gmail.com> wrote:
> On Tuesday 22 April 2008, Simon Marlow wrote:
> > I'm hoping someone will supply some. There seemed to be strong opinion
> > on #haskell that this change should be made, but it might just have been
> > a very vocal minority.
>
> These are the arguments off the top of my head:
>
> 1) Anything of the form:
>
> f $ g $ h $ x
>
> with right associative ($) can instead be written:
>
> f . g . h $ x
>
> where the associativity of ($) doesn't matter. It's not uncommon to want to
> peel off the end of such a pipeline to eliminate a point. For the second
> form, such a translation is:
>
> \x -> f . g . h $ x ==> f . g . h
>
> However:
>
> \x -> f $ g $ h $ x ==> f $ g $ h
>
> Is invalid, so one might argue that writing such pipelines with composition is
> a better habit to get into, as it allows easier cleanup of code in this way
> (if you like somewhat point-free code, that is).
>
> 2) Left associative ($) allows you to eliminate more parentheses. Per #1, any
> parentheses eliminated by right associative ($) can be eliminated by (.) and
> a single ($). However, left associative ($) allows, for instance:
>
> f (g x) (h y) ==> f $ g x $ h y
>
> 3) Left associative ($) is consistent with left associative ($!). The right
> associative version of the latter is inconvenient, because it only allows
> things to be (easily) strictly applied to the last argument of a function.
> Needing to strictly apply to other arguments gives rise to things like:
>
> (f $! x) y z
> ((f $! x) $! y) $! z
>
> Left associative, these are:
>
> f $! x $ y $ z
> f $! x $! y $! z
>
> There may be more arguments, but those are the ones I've heard that I can
> think of at the moment. #3 strikes me as the most likely to bite people (the
> other two are more stylistic issues), but I suppose I don't know the relative
> frequency of strict pipelines (f $! g $! x) versus strict applications at
> non-final arguments.
>
> And I suppose one has to weigh these arguments against breaking lots of code.
>
> Cheers,
> -- Dan
>
>
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