019 proc reflects own output union

// PIN — a |zig proc can reflect on its own declared answer space.
// Inside a proc body @This().Output names the event's declared return
// union; @typeInfo enumerates its branch names and payload types at
// comptime, and @unionInit builds a branch by reflected name. This is
// the hook koru/odds' Jev backend stands on: f64-payload branches are
// the category set, every other branch is control. Renaming a branch
// renames the reflected key — the answer space is declared once, in
// the signature, and no proc ever spells it again.
const std = @import("std");
~import std/io

~tor pick {}
| bug f64
| feature f64
| other f64
| exhausted
~proc pick|zig {
    // Enumerate the category set — only f64-payload branches. stdout,
    // one stream — stderr/stdout merge order is not pinnable.
    const out = std.fs.File.stdout();
    const fields = @typeInfo(@This().Output).@"union".fields;
    inline for (fields) |f| {
        if (f.type == f64) {
            out.writeAll(f.name) catch unreachable;
            out.writeAll("\n") catch unreachable;
        }
    }
    // Build the winner by reflected name — the same call the backend's
    // argmax/sampler makes on real probabilities.
    inline for (fields) |f| {
        if (f.type == f64) {
            return @unionInit(@This().Output, f.name, 1.0);
        }
    }
    unreachable;
}

~pick()
| bug p |> std/io:print.ln("bug {{ p:f }}")
| feature p |> std/io:print.ln("feature {{ p:f }}")
| other p |> std/io:print.ln("other {{ p:f }}")
| exhausted |> std/io:print.ln("exhausted")