On this third day of Nockmas, we will explore how opcode 2 is used to evaluate a subject and formula.

Opcode 2: Evaluate

Syntax

Opcode 2 implements the * tar evaluate operator, which dynamically computes a new subject and formula, then evaluates the formula against the subject.

*[a 2 b c]          *[*[a b] *[a c]]

Explanation

The eval of Nock, opcode 2 computes a new subject from formula b, compute a new formula from formula c, then nock the results together. This enables dynamic code execution.

1. Evaluate b against the subject to produce a noun (new subject).
2. Evaluate c against the subject to produce a noun (new formula).
3. Evaluate the new formula against the new subject to yield the final product.

Opcode 2 is the foundation for function calls, code generation, and metaprogramming in Nock. However, it is not used directly by the Hoon and Jock compiler, which instead use opcode 2 via the opcode 9 macro.

Again let's assume the most basic subject:

:subject 42

Output:

Subject set to: 42

Then take the following simple formula to compute a constant function that always returns 100, then returns its subject (now 100).

[2 [1 100] [1 [0 1]]]

Output:

100

This formula acts as a dynamic formula selection, which sets the subject to 5 and then calculates its increment using opcode 4 (which we will discuss in two days):

:subject [5 [4 0 1]]

Output:

Subject set to: [5 4 0 1]

[2 [0 2] [0 3]]

Output:

6

Join us tomorrow when we cover Nock 3, Cell Check.

12 Days of Nockmas is an exploration of Nock, Urbit's instruction set architecture. This ISA is used by both Urbit and Nockchain, has interpreters written in many languages, with production versions in both C and Rust. The content of this series is drawn from the Nock language site. Visit the site for interactive code examples and more Nock related content.