This is the payoff the whole project built toward, and like the recursion lesson it needs no new engine feature - only the glue that runs the pipeline end to end. Take the module’s raw bytes, decode its sections with the chapter-one and chapter-two decoders, instantiate it (evaluate globals, apply element and data segments, allocate memory), look up the export fact, and invoke it with an argument. The exact same loop that ran i32.const 42 back in chapter three now runs a real function’s control flow, arithmetic, and locals, and hands back a number.

Trace what the fact body does and you will recognize every piece you built: it sets a local acc to 1, then a block around a loop where br_if exits when n reaches zero (i32.eqz), the body multiplies acc by n and decrements n, and br jumps back to the loop’s top - the exact block-versus-loop branching from chapter five, over the i32 arithmetic and locals from chapters three and four. So fact(5) walks 120 and fact(0) short-circuits to 1. That a runtime assembled one lesson at a time can take bytes a compiler emitted and produce the right answer is the whole point: you have built a real, if teaching-grade, WebAssembly runtime. The finalize pass turns this into a command-line tool with the module embedded, completes the instruction families the lessons sampled, and writes down honestly what it does and does not yet do.