On this fourth day of Nockmas we explore opcode 3, the first of our axiomatic functions.

Opcode 3: Cell Check

Syntax

Opcode 3 implements the idiomatic ? wut cell check operator, which tests whether a noun is a cell.

*[a 3 b]            ?*[a b]

Explanation

Opcode 3 tests whether the product of formula b is a cell or an atom. It returns 0 (yes) if it's a cell, 1 (no) if it's an atom.

1. Evaluate b against the subject to produce a noun.
2. Test if that noun is a cell.
3. Return 0 if cell, 1 if atom.

One implication of nouns is that an easy way to tell data apart is by their structure:

?[a b]              0
?a                  1

? wut is useful to ask whether a given noun is a cell or an atom. (Like C's int main() return type and error codes, 0 means TRUE and 1 means FALSE.)

It is worth noting here, that Nock takes the sometimes controversial stance of 'loobeans' or an inverse of the tradiational boolean where 1 is TRUE and 0 is FALSE. Much discussion has been had on this topic. Want to share your two cents? Join the [battery payload] group on Tlon Messenger.

(Opcode 5 is closely related, as it tests whether one noun is equal to another.)

Opcode 3 is used for type discrimination (by structure before pattern matching) and for validating noun data shapes.

Join us tomorrow when we cover Nock 4, Increment.

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.