Arc Forumnew | comments | leaders | submitlogin
3 points by almkglor 6014 days ago | link | parent

> The easiest solution to your problem might be a callable table whose keys are the types of arguments and whose values are different function implementation.

Yes. But there's a problem: how do you define the table for, say, the variadic function '+ ?

Your solution(s) are approximately what I had in mind, although I think I found some problems with it last night, and I just woke up and can't remember quite yet (brain is still booting or something).



3 points by almkglor 6014 days ago | link

Okay, brain is up and running.

One way to do this is to use settable-fn.arc and have some sort of attachment for a table of argument signatures.

HOWEVER, we will need to formalize on what part of the argument signature to dispatch from.

For example we can consider nex3's 'defm syntax:

  (defm foo ((t arg type) (t arg2 type2))
     ...)
But then, what about optional parameters?

  (defm foo (arg (o arg something))
    ...)
Also: consider the case of 'coerce . It would be trivial to convert from our type to a target type:

  (defm coerce ((t arg our-type) target)
    (case target
      int    (our-type-to-int arg)
      string (our-type-to-string arg)
      ...))
However, how about when we want to convert to our type? How do we express (coerce (string something) 'our-type) ?

-----

3 points by almkglor 6011 days ago | link

Okay, here's the solution I've been thinking of.

  (def coerce (obj typ . rest)
    (apply <base>coerce obj (annotate typ nil) rest))

  (defm <base>coerce ((t obj my-type) (t _ string))
    (convert-my-type-to-string obj))

                                     ; edit: corrected type
  (defm <base>coerce ((t obj string) (t _ my-type))
    (convert-string-to-my-type obj))
Also, for variadic functions:

  (def + rest
    (reduce <base>+ rest))

  (defm <base>+ ((t x string) (t y string))
    (join x y))
Further, optional parameters are simply ignored and considered as part of the rest parameter (i.e. they can't be typed). Basically, the typesystem matches on the first N parameters, where N is how many type parameters you care to define.

Why the first N parameters? So that we can protect against optional parameters defaulting to values that are inappropriate for other types, such as the 'scanner example I gave above.

-----