[Python-ideas] Function composition (was no subject)

[Koos Zevenhoven]

On 8.5.2015 8:19, Rustom Mody wrote:
> On Wed, May 6, 2015 at 6:45 PM, Ivan Levkivskyi <levkivskyi at gmail.com 
> <mailto:levkivskyi at gmail.com>> wrote:
>
>
>     def sunique(lst):
>         return sorted(list(set(lst)))
>
>     vs.
>
>     sunique = sorted @ list @ set
>
>
> I would like to suggest that if composition is in fact added to python 
> its order is 'corrected'
> ie in math there are two alternative definitions of composition
>
> [1] f o g = λ x • g(f(x))
> [2] f o g = λ x • f(g(x))
>
> [2] is more common but [1] is also used
>
> And IMHO [1] is much better for left-to-right reading so your example 
> becomes
> sunique = set @ list @ sorted
> which reads as smoothly as a classic Unix pipeline:
>
> "Unnamed parameter input to set; output inputted to list; output 
> inputted to sort"
>
>

While both versions make sense, [2] is the one that resembles the 
chaining of linear operators or matrices, since column vectors are the 
convention. For the left-to-right pipeline version, some other operator 
might be more appropriate. Also, it would then be more clear to also 
feed x into the pipeline from the left, instead of putting (x) on the 
right like in a normal function call.

As a random example, (root @ mean @ square)(x) would produce the right 
order for rms when using [2].

-- Koos

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150508/8570f274/attachment-0001.html>