What does "subtyping" mean in a prototype language with structural
types? There is no such thing as a "parent" or "child" type, so the
type hierarchy context — the only context where "subtyping" means
something to me — doesn't exist.
Consider these two records:
let x = [
add: { a, b -> a + b },
subtract: { a, b -> a - b }
]
let y = [
add: { a, b -> a + b },
multiply: { a, b -> a * b }
]
Which is the parent? Which is the child?
If you do something like this:
let xs = x :: [x]
let ys = y :: [y]
let xys = x :: y :: [x]
Then the types are all well-defined: list of things like x, list of
things like y, list of things like x-or-y. You can't turn "xys" into
a list of xs or a list of ys, because it's neither.
The only place where I've found subtyping issues is with code like this:
let foundXs = xys.findAll { it -> it.like(x) }
(Where "like" is "structurally identical to".)
The list "foundXs" is now a list of "x"-like structures, even though
the type is a list of "x-or-y"-like structures. But that—like a
couple of the similar cases I've encountered—are what *should be*
match statements being dropped in strange places, usually because of
an OOP accent in the programmer. And it's easy to define a
properly-typed findAllLike, if you really wanted to stick to it:
let findAllLike = {list,type -> list match {
case () => ()
case (a:type) :: rest => a::findAllLike(rest, type)
case a::rest => findAllLike(rest)
}}
So it doesn't seem to be a problem to reject importing the idea of
"casting up", which is good since it doesn't even really mean anything
anyway in this context.
~~ Robert.
Love Your Enemy: A Campaign to Regain Human Dignity Through Nonviolence
http://www.mettacenter.org/mc/projects/love-your-enemy
On Wed, Aug 10, 2011 at 11:47 AM, John Cowan <johnwco...@gmail.com> wrote:
> On Wed, Aug 10, 2011 at 10:29 AM, Robert Fischer
> <smokejumpe...@gmail.com> wrote:
>
>> Can you define "contra-" and "co-" variance for me in the context of
>> structural (not nominal) typing?
>
> Whether a type system is structurally or nominally typed is orthogonal
> to whether it has subtyping. Algol 68 records, which are structural
> types, have no subtypes; but Go interface types, which are also
> structural types, do have subtypes. The co- and contravariance
> problem arises wherever there are static types with subtyping.
>
> --
> GMail doesn't have rotating .sigs, but you can see mine at
> http://www.ccil.org/~cowan/signatures
>
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