On Friday, 5 February 2016 at 19:48:45 UTC, Robert M. Münch wrote:
I thought about it too, but I need it to work with more then
one parameter, so I tried this which doesn't work:
Value nativePlus(Value a, Value b){
// @@ not working, runtime exception
castSwitch!(
(IntV a) {
castSwitch!(
(IntV b) {return new IntV(a.num + b.num);}
)(b);
},
(StringV a) {return new StringV("string plus");},
)(a);
// to keep compiler happy when using castSwitch (has no return
statement)
return new UnsetV();
}
I don't see why this wouldn't work, if you've in fact covered all
combinations. One thing that tripped my up was a null references
- these are never castable to anything, not even Object. If not
all combinations are valid, you can add a lambda that accept
Value (or even Object), which is called as a "default" branch.
and ended with this, which works and is straight forward but
maybe not that elegant:
Value nativePlus(Value a, Value b){
if(cast(IntV) a && cast(IntV) b)
return new IntV((cast(IntV)a).num + (cast(IntV)b).num);
if(cast(StringV) a && cast(StringV) b)
return new StringV((cast(StringV)a).str ~
(cast(StringV)b).str);
// if no case was hit (could throw)
return new UnsetV();
}
Can this be written simpler or more elegant?
It's similar to how castSwitch is implemented, though the double
casts are inefficient. You could use:
if(auto inta = cast(IntV) a) {
if(auto intb = cast(IntV) b) {
return new IntV(inta.num + intb.num);
}
}
(Again, this can be automated.)
I read this here:
https://github.com/D-Programming-Language/phobos/pull/1266#issuecomment-53507509 (functional pattern matching) but it seems it won't be implemented... at the end of the day what I simulate are poor-mans-multimethods
As I read the discussion, it was just decided to defer the more
complex version of castSwitch for later, but it wasn't rejected.
