[Python-3000] signature annotation in the function signature or a separate line

[Samuele Pedroni]

Guido van Rossum wrote:
>>But maybe I'm misremembering the discussion, maybe decorators make it
>>very difficult to visually scan for function definitions, and maybe
>>people want all that garbage in their function signature.
> 
> 
> They don't want it, but if they're forced to have it occasionally
> they'll cope. I still think you're way overestimating the importance
> of this use case.
> 

Given that the meaning of annotations is meant not be predefined,
given that people are comining with arbitrarely verbose examples
thereof, given the precedent of type inferenced languages
that use a separate line for optional type information I think
devising a way to have the annotation on a different line
with a decorator like introduction instead of mixed with
the function head would be saner:

One possibility would be to have a syntax for signature expressions
and then allow them as decorators with the obvious effect of attaching
themself:

@sig int,int -> int
def f(a,b):
     return a+b

or with argument optional argument names:

@sig a: int,b: int -> int
def f(a,b):
     return a+b

sig expressions (possibly with parens) would be first class
and be able to appear anywhere an expression is allowed,
they would produce an object embedding the signature information.

So both of these would be possible:

@typecheck
@sig int,int -> int
def f(a,b):
     return a+b

@typecheck(sig int,int -> int)
def f(a,b):
     return a+b

For example having first-class signatures would help express nicely 
reflective queries on overloaded/generic functions, etc...

regards.