I have been wondering about the following problem:
//////////////
open class Y {
type INT = "int";
ctor INT: int = "$1";
instance Eq[INT] { fun == : INT * INT -> bool = "$1==$2"; }
}
open Eq[INT];
var a = INT 1;
var b = INT 2;
println$ a == b;
///////////
This works fine. But now consider this one:
////////////////
header "enum X {A,B,C};";
cenum X = A,B,C;
println$ A == B;
//////////////////
I have just implemented "cenum" in response to the hugely ugly problem:
type X = "X";
const A:X = "A";
const B:X = "B";
const C:X = "C";
It's long winded, the cenum is better. But I want to go further:
since these are C enumerations, they're just integers.
So we should at least be able to compare them. This is implemented too,
but the comparison is just like:
fun == : X * X -> bool = "$1*$2";
There's nothing wrong with this definition, the problem is that it isn't
an instance of Eq, so there's no virtual dispatch. Ok so lets suppose I
generate this:
instance Eq[X] {
fun == : X * X -> bool = "$1*$2";
}
Now we have the dispatch. Of course we have to
open Eq[X];
But what happens if the definitions are in a class C?
No problem if we write the open Eq[C::X] after the class ..
the problem is that the code generator (Scheme code) is in the
parser, and it can only put code locally.
If we just emit open Eq[X] inside the class, it is all fine inside
the class .. but the view will not be visible outside the class C
even if we open the class C.
class C {
type X = "X";
open Eq[X];
}
open C;
We can't see the == operator here, only in the class C.
SOLUTION:
inherit Eq[X];
Lol. Does the same as open except it also exports the view (when C is
open, or itself inherited).
The effect of this is profound. The whole standard library is wrong:
I regularly open all the classes like Eq, Str etc in global scope
to expose them (even if the class itself is not open by default).
--
john skaller
[email protected]
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