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1 point by cchooper 5738 days ago | link | parent

Ah...but...what if you want to define a generic function that operates differently on lists and bools (i.e. not a scanner, but a general generic function). I haven't had a close look at Arc-3F yet, so maybe I need to play around a bit more to uderstand what you're saying :)

1 point by almkglor 5738 days ago | link

Well, a "list" is a "scanner". So your "not a scanner" doesn't make sense, at least from the point of view of Arc-F.

However if you mean "list" as in sequence of cons cells:

  (def works-on-cons-cells-and-bools (x)
    (err "this works only on cons cells and bools!"))
  (defm works-on-cons-cells-and-bools ((t x cons))
    (work-on-cons-cells x))
  (defm works-on-cons-cells-and-bools ((t x bool))
    (work-on-bool x))
Note that you can even define a unifying "type class" function which ensures that the given data is a cons cell or a bool, or is convertible to one (i.e. an analog to 'scanner). For example, you might want a "hooper" type class:

  (def hooper (x)
    (err "Not convertible to a bool or cons cell" x))
  (defm hooper ((t x cons))
  (defm hooper ((t x bool))
Then you can convert works-on-cons-cells-and-bools with the type class:

  (def work-on-hooper (x)
    (works-on-cons-cells-and-bools (hooper x)))
Then, someone can make a type which supports the "hooper" type class by either overloading hooper (and returning a true hooper), or overloading hooper and works-on-cons-cells-and-bools:

choice one:

  (defm hooper ((t x my-type))
    (convert-my-type-to-cons x))
choice two:

  (defm hooper ((t x my-type))
  (defm works-on-cons-cells-and-bools ((t x my-type))
    (work-on-my-type x))