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digitalmars.D.learn - Struct initializer in UDA

reply realhet <real_het hotmail.com> writes:
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;
Sep 26 2020
parent reply Anonymouse <zorael gmail.com> writes:
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.
Sep 26 2020
parent reply realhet <real_het hotmail.com> writes:
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
Sep 27 2020
parent reply Anonymouse <zorael gmail.com> writes:
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.
Sep 27 2020
parent realhet <real_het hotmail.com> writes:
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!
Sep 27 2020