Why UFCS doesn't work with "with" statement?

struct MyStruct1
{
  int a, b;
  int sum(){
    return a+b;
  }
}

int difference(ref MyStruct1 self)
{
  return self.a-self.b;

}

// in main()

MyStruct1 s = { 1, 2 };

s.difference().writeln; // works fine

with(s){
  sum().writeln;
  difference().writeln; // error: too few arguments, expected 1, got 0
}

On Monday, 9 June 2025 at 11:39:15 UTC, Lukanian wrote:

struct MyStruct1
{
  int a, b;
  int sum(){
    return a+b;
  }
}

int difference(ref MyStruct1 self)
{
  return self.a-self.b;

}

// in main()

MyStruct1 s = { 1, 2 };

s.difference().writeln; // works fine

with(s){
  sum().writeln;
  difference().writeln; // error: too few arguments, expected 1, got 0
}

It took me a while what you were trying to do, but I think I get it now. I'm not an expert on D, but from what I understand, the reason why it doesn't work is because the difference function is not a method of the struct. The "with" keyword works what's a member of the argument. So essentially yeah, you're calling difference without any argument.

On Monday, 9 June 2025 at 11:39:15 UTC, Lukanian wrote:

struct MyStruct1
{
  int a, b;
  int sum(){
    return a+b;
  }
}

int difference(ref MyStruct1 self)
{
  return self.a-self.b;

}

// in main()

MyStruct1 s = { 1, 2 };

s.difference().writeln; // works fine

with(s){
  sum().writeln;
  difference().writeln; // error: too few arguments, expected 1, got 0
}

The with block changes the lookup rule so that unqualified symbols (sum and difference in this case) are looked up in the scope of the (type of) the with argument first, only then normal lookup rules (innermost scope first) apply. What with(x) doesn’t do is try to resolve any unqualified symbol s as x.s. If it did that, your code would work.

UFCS doesn’t make members and non-members equivalent, it’s just a calling syntax that works in some cases.