We're not abusing guard :: syntax, are you trying to get my goat? :-P
Andrea Giammarchi wrote:
recap ...
in class point3d
```javascript
function + (a :: point2d, b :: point3d) {
return point3d(a.x + b.x, a.y + b.y, b.z);
}
```
in class point2d
```javascript
function + (a :: point2d, b :: point3d) {
return point2d(a.x + b.x, a.y + b.y); // ignore b.z
// or do something else with z
}
```
which one would be the result and why? what if both defines a and b
arguments differently too ?
Those are distinct multimethods. One returns a point3d, one a point2d.
Declaring both as you show (with the bogo-syntax changed to avoid
guard-::) results in ambiguous method calls on instances: we get the
left+ symbol's set from point2d.prototype, and the right+ symbol's set
from point3d.prototype, for an expression of the form
var presult = p2 + p3.
We interset the sets. The result is not a singleton, but a set with two
functions. Since there's no return-type dispatch, or indeed any clue
what type is wanted for presult, the call is ambiguous and an error is
thrown.
This points out another benefit of putting multimethod pattern-based
definitions in value class declarations: we can raise an early error
when the second value class is processed.
/be
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