patch applied (haskell-prime-status): add ""Make $ left
associative, like application"
Dan Doel
dan.doel at gmail.com
Tue Apr 22 21:02:35 EDT 2008
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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