Here on the second day of Nockmas, our focus is on opcode 1, Constant.

Opcode 1: Constant

Syntax

*[a 1 b]            b

Explanation

In many respects, opcode 1 is the simplest conceivable Nock operation: it simply returns its argument b, ignoring its subject a entirely.

Opcode 1 is used to store data for use in later computations. This can either be conventional data as atoms (including byte arrays) and structured data in a cell format; or it can be Nock code itself, which can be retrieved and executed later using opcode 9.

Imagine we have the following subject:

:subject 42

Output:

Subject set to: 42

And then we call Nock 1:

[1 1000]

Output:

1000

While you haven't seen the full Nock specification for the other opcodes yet, observe how evaluating a stored noun with opcode 1 results in an executable Nock expression.

[1 4 0 1]

Output:

[4 0 1]

In the following expression, we use opcode 2 to evaluate the noun stored in the subject [0 1], then we retrieve the constant code stored using opcode 1, and finally we execute that code with the result of the first evaluation as its subject. A bit roundabout, in this case, but it demonstrates how stored code can be executed later.

[2 [0 1] [1 4 4 4 0 1]]

Output:

45

Join us tomorrow when we cover Nock 2, Evaluate.

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.