Felix has been able to do OO for some time, it just isn't sugared.
Here's an example:
//////////////////
// An OO experiment
fun X (var x:int, var y:int) = {
proc setx(a:int) { x = a; }
proc sety(a:int) { x = a; }
fun getx() => x;
fun gety () => y;
fun sum () => x + y;
return (setx=setx, sety=sety, getx=getx, sum=sum);
}
var a = X(2,3);
println$ a.getx();
a.setx(22);
println$ a.getx(), a.sum();
var b = a :>> (sum:1->int);
proc f(v:(sum:1->int))
{
match v with
| (sum= ?pp)=> println$ pp();
endmatch;
}
f b;
/////////////
Here, X can be viewed as a constructor for an object.
The type of the object is:
(
setx:int->void,
sety:int->void,
getx: unit -> int,
gety: unit -> int,
sum: unit -> int
)
that is, it's a record of functions. Notice that the "data" is 100% encapsulated
by the constructor function in this case (but, you could return a pointer
if you wanted a variable, or a value if you didn't).
Now the line
var b = a:>> (sum: 1-> int);
is an upcast (coercion) to a type with only the sum field. So we have
subclassing based on "forgetting some fields". There's a more
anonymous way forget fields using a match. This will work too:
match a with
| (sum= ?pp)=> println$ pp();
endmatch;
you should note that record coercions have nothing to do with
objects (coercions and pattern matches work with all records).
At present the overloading system requires exact matches so you
must explicitly coerce a record to strip fields to match a function
taking a record. [It may be possible to relax that].
Clearly, you can easily implement extensions:
fun Y(a:X, b:int) { .... }
The most important thing to note about this style of OO is that
the subtyping notion is NOT based on extension. You can
upcast to a "base" consisting of ANY subset of fields without
pre-declaring any base types. This is similar to Ocaml's object
system and is conceptually VASTLY superior to conventional
inheritance: a type with a set of fields Y "isa" X for X with every subset
of the fields of Y.
I wonder if this is worth sugaring ..
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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