Thread overview
Struct initializer in UDA
Sep 26
realhet
Sep 27
realhet
Sep 28
realhet
September 26
Hi,

  struct S{int a, b, c=9, d, e, f;}

Is there a way or a trick to declare an UDA by using a nice struct initializer?

It would be nice to be able to use the form:

  @S{f:42} int a;  //or something similar to this.

instead of this longer and error-prone way:

  @S(0, 0, 0, 9, 0, 42) int a;
September 26
On Saturday, 26 September 2020 at 16:05:58 UTC, realhet wrote:
> Hi,
>
>   struct S{int a, b, c=9, d, e, f;}
>
> Is there a way or a trick to declare an UDA by using a nice struct initializer?
>
> It would be nice to be able to use the form:
>
>   @S{f:42} int a;  //or something similar to this.
>
> instead of this longer and error-prone way:
>
>   @S(0, 0, 0, 9, 0, 42) int a;

I don't think you can currently, no, but I'd be happy to be proven wrong.

The closest I can get is @(S.init.c(9).f(42)) with use of opDispatch, which is easier to read but still ugly.
September 27
On Saturday, 26 September 2020 at 17:13:17 UTC, Anonymouse wrote:
> On Saturday, 26 September 2020 at 16:05:58 UTC, realhet wrote:
> The closest I can get is @(S.init.c(9).f(42)) with use of opDispatch, which is easier to read but still ugly.

All I can get is that the
- an identifier of a member is stronger than the opDispatch. -> Error: function expected before (), not S(0, 0).c of type int
- and if I prefix it with '_' it ruins toString. -> Error: no property toString for type onlineapp.S


import std.stdio, std.range, std.algorithm, std.traits, std.meta, std.conv, std.string, std.uni, std.meta, std.functional, std.exception;

struct S{
    int a, b;

    auto opDispatch(string name, T)(T value)
    if(name.startsWith("_"))
    {
        mixin(name[1..$], "= value;");
        return this;
    }
}

void main(){
    S.init._a(5).writeln;
}


Now I'm more confused, as the compiler completely ignores the if(name.startsWith("_")) constraint o.O
September 27
On Sunday, 27 September 2020 at 10:17:39 UTC, realhet wrote:
> On Saturday, 26 September 2020 at 17:13:17 UTC, Anonymouse wrote:
>> On Saturday, 26 September 2020 at 16:05:58 UTC, realhet wrote:
>> The closest I can get is @(S.init.c(9).f(42)) with use of opDispatch, which is easier to read but still ugly.
>
> All I can get is that the
> - an identifier of a member is stronger than the opDispatch. -> Error: function expected before (), not S(0, 0).c of type int
> - and if I prefix it with '_' it ruins toString. -> Error: no property toString for type onlineapp.S
>
>
> import std.stdio, std.range, std.algorithm, std.traits, std.meta, std.conv, std.string, std.uni, std.meta, std.functional, std.exception;
>
> struct S{
>     int a, b;
>
>     auto opDispatch(string name, T)(T value)
>     if(name.startsWith("_"))
>     {
>         mixin(name[1..$], "= value;");
>         return this;
>     }
> }
>
> void main(){
>     S.init._a(5).writeln;
> }
>
>
> Now I'm more confused, as the compiler completely ignores the if(name.startsWith("_")) constraint o.O

It works if you specialise opDispatch to take an int parameter instead of a type T. It smells like a bug but I don't know enough to say.

I used two opDispatches to be able to avoid having to use _a and _b, and std.algorithm.comparison.among to constrain them.

struct S{
    private int _a, _b;

    auto opDispatch(string name)(int value)
    if (name.among("a", "b"))
    {
        mixin("_", name, "= value;");
        return this;
    }

    auto opDispatch(string name)()
    if (name.among("a", "b"))
    {
       	mixin("return _", name, ";");
    }
}

void main(){
    S.init.a(123).b(456).writeln;
    S().b(456).a(123).writeln;  // Alternative syntax, may not work if opCall is defined
}

It's brittle in that you have to update and sync the two among("a", "b") constraints every time you add or remove a field, but I can't seem to get the names by introspection without it endlessly recursing over opDispatch again.
September 28
On Sunday, 27 September 2020 at 11:59:49 UTC, Anonymouse wrote:
> On Sunday, 27 September 2020 at 10:17:39 UTC, realhet wrote:
>> On Saturday, 26 September 2020 at 17:13:17 UTC, Anonymouse wrote:
>>> On Saturday, 26 September 2020 at 16:05:58 UTC, realhet wrote:

That looks the closes to the python named parameters or the struct initializer.

For my use case this opDispatch trick seems to be more flexible than the named-parameters thing:

@(FieldProps().range(-360, 360).format("%.2f").caption("Turret rotation").unit("deg")) float alpha = 0;

for example if I use the name: "logRange" it can also set the isLogarithmic flag as a side effect to true inside the FieldProps struct. Just by choosing a slightly different name.

With this idealized format it would be not possible:
@FieldProps{ range: {-360, 360}, format:"%.2f", caption:"Turret rotation", unit:"deg"} float alpha = 0;

The more work inside the struct is not a problem, because I'm willing to use it from 1000 places. Also __traits(allMembers) can help.

Thank you!