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

Gregory Salvan apieum at gmail.com
Sat May 9 22:41:24 CEST 2015 
pipeline operator may be confusing with bitwise operator.
In this case :
eggs = arg | spam | cheese

Is eggs a composed function or string of bits ?


2015-05-09 21:15 GMT+02:00 Koos Zevenhoven <koos.zevenhoven at aalto.fi>:

>
> On 2015-05-09 21:16, Steven D'Aprano wrote:
>
>> On Sat, May 09, 2015 at 11:38:38AM -0400, Ron Adam wrote:
>>
>>  How about an operator for partial?
>>>
>>>            root @ mean @ map $ square(xs)
>>>
>> Apart from the little matter that Guido has said that $ will never be
>> used as an operator in Python, what is the association between $ and
>> partial?
>>
>> Most other operators have either been used for centuries e.g. + and - or
>> at least decades e.g. * for multiplication because ASCII doesn't have
>> the × symbol. The barrier to using a completely arbitrary symbol with no
>> association to the function it plays should be considered very high.
>>
>> I would only support an operator for function composition if it was at
>> least close to the standard operators used for function composition in
>> other areas. @ at least suggests the ∘ used in mathematics, e.g.
>> sin∘cos, but | is used in pipelining languages and shells and could be
>> considered, e.g. ls | wc.
>>
>> My own preference would be to look at @ as the closest available ASCII
>> symbol to ∘ and use it for left-to-right composition, and | for
>> left-to-right function application. E.g.
>>
>> (spam @ eggs @ cheese)(arg) is equivalent to spam(eggs(cheese(arg)))
>>
>> (spam | eggs | cheese)(arg) is equivalent to cheese(eggs(spam(arg)))
>>
>> also known as compose() and rcompose().
>> We can read "@" as "of", "spam of eggs of cheese of arg", and | as
>> a pipe, "spam(arg) piped to eggs piped to cheese".
>>
>
> For me these are by far the most logical ones too, for exactly the same
> reasons (and because of the connection of @ with matrix multiplication and
> operators that operate from the left).
>
>  It's a pity we can't match the shell syntax and write:
>>
>> spam(args)|eggs|cheese
>>
>> but that would have a completely different meaning.
>>
>>
>
> But it does not need to have a different meaning. You could in addition
> have:
>
> spam @ eggs @ cheese @ arg   #  equivalent to spam(eggs(cheese(arg)))
>
> arg | spam | eggs | cheese    # equivalent to cheese(eggs(spam(arg)))
>
> Here, arg would thus be recognized as not a function.
>
> In this version, your example of spam(args)|eggs|cheese would do exactly
> the same operation as (spam | eggs | cheese)(args) :-).
>
>
>  David Beazley has a tutorial on using coroutines in pipelines:
>>
>> http://www.dabeaz.com/coroutines/
>>
>> where he ends up writing this:
>>
>>      f = open("access-log")
>>      follow(f,
>>             grep('python',
>>             printer()))
>>
>>
>> Coroutines grep() and printer() make up the pipeline. I cannot help but
>> feel that the | syntax would be especially powerful for this sort of
>> data processing purpose:
>>
>>      # could this work using some form of function composition?
>>      follow(f, grep('python')|printer)
>>
>>
>>
>>
> This seems promising!
>
>
> -- Koos
>
>
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