Arc Forum | I remember that discussion; in fact, I had a response, http://arclanguage.org/it...

Arc Forum

1 point by absz 6002 days ago | link | parent

I remember that discussion; in fact, I had a response, http://arclanguage.org/item?id=3518, in which I proposed this (and I might have been working on this at the time).

The clearest advantage of distinct types is in things like binary trees:

  (tunion binary-tree
    branch (left  binary-tree?)
           (right binary-tree?)
    leaf   (value object))

If you just had

  (vtype branch left right)
  (vtype leaf   value)

you can't quickly tell what's a binary tree and what isn't in the same way, and you lose the invariant that a branch's left and right are both binary trees. At the same time, something more like vtype would remove

  (tunion my-type
    my-type (foo pred)
            (bar int?))

. Actually, if you had something like

  (mac vtype (type . arguments)
    `(tunion ,type ,type ,@arguments))

you could do this:

  (vtype variant1 (x pred))
  (vtype variant2 (a int?) (b object))
  
  (tcase var
    (variant1
      variant1 (do-something var))
    (variant2
      variant2 (do-something-else var))
    else
      (fail))

And you should be able to write a macro that will convert something nicer to that form, e.g.

  (vcase var
    variant1 (do-something var)
    variant2 (do-something-else var)
             (fail))

. I'll work on that and push it to Anarki.

I'm not convinced on the auto-binding; I implemented it that way because the resulting syntax was nice. It might make sense to have an option to work without auto-binding of names, but maybe it won't actually be an issue in practice. car and cdr are a bit atypical because (I hope) you won't be redefining a list type :)