Flexible Default Function Parameters via structs with Nullable Fields

[Simen Kjærås]

On Tuesday, 30 April 2019 at 13:10:54 UTC, Adam D. Ruppe wrote:
> On Tuesday, 30 April 2019 at 08:20:29 UTC, JN wrote:
>> It might be nifty by D standards, but for a person not 
>> familiar with D
>
> Or, as someone familiar with D, I wonder why not just use a 
> plain struct. D allows you to set initial values for struct 
> members plainly.

Yeah, though I can see some use cases for a struct with all 
nullable fields and a way to combine with a regular version of 
that struct. This could be made a lot easier than in the article:

import std.traits : FieldNameTuple;
import std.typecons : Nullable;

struct Partial(T) if (is(T == struct)) {
     static foreach (e; FieldNameTuple!T)
         mixin("Nullable!(typeof(__traits(getMember, T, e))) 
"~e~";");
}

auto combine(T, PT)(T t, PT pt) if (is(PT == Partial!T)) {
     T result;
     static foreach (e; FieldNameTuple!T)
         __traits(getMember, result, e) = __traits(getMember, pt, 
e).get(__traits(getMember, t, e));
     return result;
}

struct S {
     int x,y;
}

unittest {
     S a = S(1,2);
     Partial!S b;
     b.x = 3;
     assert(a.combine(b) == S(3,2));
}

Now, for the abomination that is 
callMemberFunctionWithParamsStruct!(t, "f")(combined)... It's 
just t.f(combined.tupleof) in a bad disguise, and I really can't 
see the benefit.

Lastly, the use of a mixin to define the struct ensures you can't 
put methods on the struct, thus drastically reducing usability.

All in all, it's a fun beginner's project, but the quality may 
not be good enough that it should be on the blog.

--
   Simen