On Saturday, 2 April 2022 at 14:49:15 UTC, Vijay Nayar wrote:
On Saturday, 2 April 2022 at 14:35:10 UTC, Vijay Nayar wrote:
The `tryMatch` method fails to compile, and instead I get the
following error:
```d
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(2004):
Error: static assert: "`handlers[0]` of type `int
function(ref ubyte[] _1, ref ubyte[] _2) pure nothrow @nogc
@safe` never matches"
```
Through sheer trial and error, I discovered that changing the
handler arguments from `ref ubyte[]` to `const ref ubyte[]` the
error goes away. However, I did not find any information in the
documentation or code that made this requirement clear. It was
basically a guess based on the fact that `string` has no
trouble but `ubyte[]` did, and string is basically just
`immutable(char[])`.
So far, I'm finding that learning to use `SumType` is
significantly more cryptic than `Variant`, by a large margin.
I'll still give it a shot of course, because I want to get past
this problem.
Try this:
```d
import std.sumtype;
import std.algorithm : cmp;
alias VarType = SumType!(double, ubyte[]);
int opCmp(const ref VarType v1, const ref VarType v2) {
//you need all combinations of types:
static int impl(A, B)(auto ref A a, auto ref B b){
// (double, ubyte[]) or (ubyte[], double):
static if(!is(immutable A == immutable B))
assert(0, "type missmatch");
// (ubyte[], ubyte[]):
else static if(is(immutable A == immutable ubyte[]))
return cmp(a, b);
// (double, double):
else
return (a < b) ? (-1) : (a < b ? 1 : 0);
}
return tryMatch!impl(v1, v2);
}
void main(){
VarType b1 = cast(ubyte[]) [0x01, 0x02, 0x03];
VarType b2 = cast(ubyte[]) [0x01, 0x02, 0x04];
assert(opCmp(b1, b2) == -1); //operator overloading work only if
opCmp is method.
}
```
