[Python-3000] Another overloaded functions usecase

[Guido van Rossum]

Which slide in Norvig's presentation are you referring to?

Why would you have to pass token.type to convert()? I'd think that the
simplest API would be

  convert(token, target).

What am I missing?

--Guido

On 5/18/06, Thomas Dunham <thomas.dunham at gmail.com> wrote:
> Talk on overloaded functions seems a bit sparse recently, and as I rather like
> the idea I wondered if another usecase would be helpful. I reworked
> this from an example published 10 years ago by someone who a couple of
> you guys might see at lunchtime.
>
> Say we want to read a text document in RTF and convert to one of many
> formats. The gang of four describe a builder pattern to do this, and
> their approach requires a class for each type of object to build, and
> another for the director. It's likely that you'd ditch the director if
> you were writing in Python:
>
> class TEXConverter(TextConverter):
>   def convertChar(token):
> ...
>
> builder = TEXConverter()
> ...
>
> act = dict(
>     CHAR=builder.convertChar,
>     FONT=builder.convertFont,
>     PARA=builder.convertParagraph
> )
>
> t = get_token()
> act[t.type](t)
>
>
> Maybe with overloaded functions it could look like this:
>
> target = TEXText()
> Font, Para, Char = inputTypes()
> ...
> token = get_token()
> convert(token, token.type, target)
> ...
> def convert(token, type:Font, target:TEXText):
>     ...
>
> def convert(token, type:Para, target:TEXText):
>     ...
>
> def convert(token, type:Char, target:TEXText):
>     ...
>
> In the original (written in Dylan) Font, Para, Char are instances, and
> it uses == to dispatch on individual objects. I'm suggesting creating
> more classes and dispatching on (possibly singleton) instances of
> them. (The origonal is here: http://norvig.com/design-patterns/)
>
> I don't think this is as elegant as Norvig's version, but I do like
> the way it makes the language do all the dispatching, and looking at
> the function prototypes gives a good impression of the type/operation
> grid that is somewhat hidden in the single-dispatch version.
>
> Tom
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)