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

[Koos Zevenhoven]

Just a small correction to my below email (the definition of "feed"):

On 10.5.2015 18:15, Koos Zevenhoven wrote:
> On 10.5.2015 5:56, Steven D'Aprano wrote:
>
> [...]
>>> 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.
>> No. I think it is absolutely vital to distinguish by syntax the
>> difference between composition and function application, and not try to
>> "do what I mean". DWIM software has a bad history of doing the wrong
>> thing.
>>
>> Every other kind of callable uses obj(arg) to call it: types, functions,
>> methods, partial objects, etc. We shouldn't make function composition
>> try to be different. If I write sqrt at 100 I should get a runtime error,
>> not 10.
>>
>> I don't mind if the error is delayed until I actually try to call the
>> composed object, but at some point I should get a TypeError that 100 is
>> not callable.
>>
>
> That is in fact a part of why I added a function call () to the sketch 
> in my recent post (extended partial operator, there using ->). This 
> way, the composition operator would never do the actual call by 
> itself, but instead make a partial. But I admit that (sqrt at 100)() 
> still would give 10, not the runtime error you want (which may indeed 
> cause problems with callable arguments). It only solves half the problem.
>
> Another way to feed the left-to-right | composition from the left 
> would of course be
>
> (feed(x) | spam | eggs | cheese)()   # feed would be just  def 
> feed(x): return x
>

Sorry, I messed that up. "feed" would of course be:

  def feed(x):
      def feeder():
          return x
      return feeder


> But I'm not sure I like it. Luckily, (cheese @ eggs @ spam)(x) does 
> not have this problem. However, if cheese, eggs and spam were matrix 
> transformations, one would write
>
> cheese @ eggs @ spam @ x
>
> But perhaps numpy would want to bridge this gap with extended behavior 
> (allow calling numpy functions with @ or "calling a matrix 
> transformation" with () ). Or perhaps not :).
>
> -- Koos
>
>
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