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
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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