[Python-3000] Argument Decorators (was Re: my take on "typeclasses")

[Bill Birch]

On Thu, 11 May 2006 03:30 pm, Guido van Rossum wrote:

>
> Bill Janssen is proposing that the specific thing a type must do is
> inherit from some abstract base class.
>
> Phillip Eby is countering that that isn't sufficient because he wants
> to be able to make up his own categories and apply these to existing
> types defined by 3rd party libraries.
>
> It would be great if we had a solution that allowed either approach!
Sorry if the following has already been suggested, almost everything has:

Perhaps is we refactor the compiler-generation of annotations slightly so that 
the Type is responsable for handling the annotation, then developers can 
implement their own approaches? Given the blog [1] code:

def foo(x: t1, y: t2) -> t3:
    ...body...

# is more or less equivalent to this:

def foo__(x, y):  # original function
    ...body...

def foo(x, y):    # wrapper function
    x = adapt(x, t1)
    y = adapt(y, t2)
    r = foo__(x, y)
    return adapt(r, t3)

Refactored to push adapt into the type:

def foo(x, y):    # wrapper function
    x = t1.adapt(x)
    y = t2.adapt(y)
    r = foo__(x, y)
    return t3.adapt(r)

Refactor again because even adapt is now in the types. 

def foo(x, y):    # wrapper function
    x = t1.decorateArg(x)
    y = t2.decorateArg(y)
    r = foo__(x, y)
    return t3.decorateReturn(r)

Every Type has decorateArg and decorateReturn methods. They would be different 
for contravariance and covarience reasons. The Python standard types would 
behave as 80% of people expect, whatever that is.  People with particular 
ideas about types would supply their own type classes.  i.e.

   def foo(x: StrictlyT1, y: StrictlyT2) ->StrictlyT3:   # Java fans ;-)
      ...
 
   def foo(x: AdaptT1, y: AdaptT2) ->AdaptT3:  # For fans of PEP 246
     ...

So a library might supply its own type classes consistent with the library 
authors' requirements.

Again, sorry if this is old hat.


----------------References: 
[1] http://www.artima.com/weblogs/viewpost.jsp?thread=87182