Python from Wise Guy's Viewpoint

[Andreas Rossberg]

Pascal Costanza wrote:
> 
>> As an example of the kind of "overloading" (or type dispatch, if you 
>> want)
>> you cannot express in dynamically typed lagnuages: in Haskell, you can
>> naturally define functions which are overloaded on their return type, 
>> i.e.
>> you don't need any value at all at runtime to do the "dispatch". For
>> example, consider an overloaded function fromString, that produces 
>> values of
>> potentially arbitrary types from strings.
> 
> Wrong.

Not sure how the "wrong" relates to the quoted paragraph, but I guess 
you mean my conjecture that you cannot dispatch on return types with 
dynamic typing.

> (defmethod from-string-expansion (to-type string)
>   (if (or (subtypep to-type 'sequence)
>           (subtypep to-type 'character)
>           (eq to-type t))
>     `(coerce ,string ',to-type)
>     `(coerce (read-from-string ,string) ',to-type)))
> 
> (defmacro set-from-string (x string &environment env)
>   (multiple-value-bind
>     (bound localp declarations)
>     (variable-information x env)
>     (declare (ignore bound localp))
>     (let ((type (or (cdr (assoc 'type declarations)) t)))
>       `(setf ,x ,(from-string-expansion type string)))))

Interesting example, thanks for showing it. I'm not fluent enough in 
Lisp to understand how this actually works but it does not seem to be 
extensible in a compositional way (you have to insert all type cases by 
hand). And does the resolution work transitively? I.e. if I write some 
complex function f using fromString somewhere, performing arbitrary 
calculations on its return value of type t, and returning something of a 
type containing t, is all this code parametric in t such that I can call 
f expecting arbitrary result types? All this would be automatic in Haskell.

Also note that your transcript shows that your approach indeed requires 
*explicit* type annotations, while you would rarely need them when doing 
the same thing in a language like Haskell.

Anyway, your code is complicated enough to make my point that the static 
type system gives you similar expressiveness with less fuss.

	- Andreas

-- 
Andreas Rossberg, rossberg at ps.uni-sb.de

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