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

[Steven D'Aprano]

On Sat, May 09, 2015 at 10:15:21PM +0300, Koos Zevenhoven wrote:
> 
> On 2015-05-09 21:16, Steven D'Aprano wrote:
[...]

> >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.

It *should* have a different meaning. I want it to have a different 
meaning. Python is not the shell and spam(args) could be a factory 
function which itself returns a callable, e.g. partial, or a decorator. 
We cannot match the shell syntax because Python can do so much more than 
the shell.


> 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.


-- 
Steve