[Issue 12732] Add an Appender-like template that recursively builds a structure of Appender fields

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12732

--- Comment #4 from Andrej Mitrovic <andrej.mitrovich at gmail.com> ---
There we go:

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import std.array;
import std.string;
import std.typetuple;

alias ApplyAppender(T : E[], E) = Appender!T;
alias ApplyAppender(T) = T;

enum Identifier(alias S) = __traits(identifier, S);

string generateAliases(T)()
{
    string[] res;

    foreach (idx, str; staticMap!(Identifier, T.tupleof))
        res ~= format("alias %s = fields[%s];", str, idx);

    return res.join("\n");
}

struct AppenderWrapper(T)
{
    alias Fields = staticMap!(ApplyAppender, typeof(T.tupleof));
    Fields fields;

    mixin(generateAliases!T);

    @property T data()
    {
        T res;

        foreach (idx, field; fields)
        {
            static if (is(typeof(res.tupleof[idx]) : E[], E))
                res.tupleof[idx] = field.data;
            else
                res.tupleof[idx] = field;
        }

        return res;
    }
}

alias vec3 = int[3];

struct Model
{
    vec3[] indices;
    vec3[] vertices;
    vec3[] normals;
    int other;
}

Model loadModel(string path)
{
    AppenderWrapper!(typeof(return)) result;

    vec3 vec;
    result.indices ~= vec;
    result.vertices ~= vec;
    result.normals ~= vec;
    result.other = 1;

    return result.data;
}

void main() { }
-----

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