On this sixth day of Nockmas, we visit opcode 5, Equality Check. The last of the axiomatic operators, nock 5 different but closely related to opcode 3 which we discussed previously.

Opcode 5: Equality Check

Syntax

Opcode 5 implements the idiomatic = tis equality operator, which tests for deep equality between two nouns.

*[a 5 b c]          =[*[a b] *[a c]]

Explanation

Opcode 5 tests whether the products of formulas b and c are structurally identical. It returns 0 if equal, 1 if not.

1. Evaluate b against the subject to produce noun l.
2. Evaluate c against the subject to produce noun r.
3. Return 0 if l and r are identical nouns, 1 otherwise.

=[a a]              0
=[a b]              1

Like opcode 3, = tis checks by structure, and also by value. Equality is deep structural comparison: two cells are equal iff their heads are equal and their tails are equal.

Taking our basic subject:

:subject 42

Output:

Subject set to: 42

We can call a nock 5 on two cells:

[5 [0 1] [1 42]]

Output:

0

If we expand our subject:

:subject [42 43]

Output:

Subject set to: [42 43]

Two common patterns recur. The first checks whether a computed noun is equal to a specific target constant noun:

[5 [0 2] [1 42]]

Output:

0

The second compares two slots in the subject to see if they hold equal values:

[5 [0 2] [0 3]]

Output:

1

Of course we can use this in conjunction with the computed results of any other noun:

[5 [4 0 2] [0 3]]

Output:

0

Join us tomorrow when we cover Nock 6, Conditional.

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.