function operators

Hans Nowak wurmy at earthlink.net
Tue Nov 27 03:05:39 CET 2001


"James A. H. Skillen" wrote:
> 
> Has anyone ever wished that Python had operators defined on functions?
> For example, suppose you want the function f(x) = cos(x) + sin(x).
> You could do:
> 
>     def f(x):
>         return cos(x) + sin(x)
> 
> or
> 
>     f = lambda x: cos(x) + sin(x)
> 
> but wouldn't:
> 
>     f = cos + sin

Assuming this would ever be implemented, then it would work on "normal"
functions, but problems would occur with other callable objects. Most
notably, classes that override the + operator. In other words, if a and
b are instances of a class that implements both __add__ and __call__,
what would

  f = a + b

mean? Would it mean

  f = a.__add__(b)

or

  f = lambda x: a(...) + b(...)

like with regular functions? Ambiguity occurs.

Then again, it may be possible to write a wrapper class that holds a
function, is callable through __call__, *and* implements __add__ and
other operators, that do what you want... I was going to give it a try,
but then I got high... ;-)

> There is also another tremendously useful operator on functions:
> composition.
> 
> For want of a better symbol, how about a new keyword "on" to be
> composition.
> So to get the function f(x) = cos(sin(x)) I could do
> 
>     f = cos on sin
> 
> Why? Well this is useful if you use map, filter etc.

But a compose function is easily defined; for example, in
1.5.2-compatible
code:

    def compose(f, g):
        return lambda x, f=f, g=g: f(g(x))

or in 2.1:

    from __future__ import nested_scopes

    def compose(f, g):
        return lambda x: f(g(x))

...so whether it's worthwile to add an extra operator for this, is
doubtful.
    
> but with the new syntax I could write
> 
>     map(cos on sin, range(10))

It does look nice, though. :-)

> Does this make sense, or have I been doing too much mathematics? ;-)

Python isn't a functional language, but many features of such languages
can be implemented with a little trickery.

Regards,

--Hans



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