[Haskell-beginners] Good style on "." and "$" pipeline?
byorgey at seas.upenn.edu
Tue Aug 21 22:43:53 CEST 2012
On Tue, Aug 21, 2012 at 09:34:49PM +0100, Carlos J. G. Duarte wrote:
> Hi. Often I see the following pattern to build function "pipelines":
> x = f1 . f2 $ fn arg
> They use the "$" just before the last function call. I never got much into
> it: when to use the "." or the "$"? I think today I figured it out:
> => "." is a function composition operator: used to "merge" functions;
> => "$" is a function application operator: used to apply a function to its
> If this reasoning is correct, I think the following should be a more
> adequate pattern:
> x = f1 . f2 . fn $ arg
> To merge/compose all functions first and apply the composed function to
> its parameter(s).
> Am I correct on this? Thx
Yes, you are absolutely correct. But sometimes people do something
f1 . f2 $ fn arg
if for some reason they are thinking of "fn arg" as a "primitive"
starting point, and then applying a pipeline of functions f2, f1 to
that. But it is really just a different point of view. In any case,
both (f1 . f2 . fn $ arg) and (f1 . f2 $ fn arg) are perfectly good
style. Having more than one $, like (f1 $ f2 $ fn $ arg), is frowned
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