Opcode 8: Extend

Contents

Opcode 8: Extend#

Syntax#

*[a 8 b c]          *[[*[a b] a] c]

Explanation#

Opcode 8 pins a new value to the head of the subject, then evaluate the body. This is how Nock implements variable binding—the new value becomes accessible at address 2.

  1. Evaluate b against subject to produce a new value.

  2. Construct [new-value subject] as the extended subject.

  3. Evaluate c against the extended subject.

After opcode 8, addresses shift:

  • Address 2 → the new pinned value

  • Address 3 → the original subject

  • Address 6 → what was address 2

  • Address 7 → what was address 3

Opcode 8 extends the subject, while opcode 7 replaces it:

*[a 7 b c]  →  *[*[a b] c]       :: New subject is *[a b]
*[a 8 b c]  →  *[[*[a b] a] c]   :: New subject is [*[a b] a]

There are two primary patterns for using opcode 8. In the first, a local variable is bound for use in a computation:

*[data [8 [compute-value] [body-using-value-at-+2]]]

:subject 42

Subject set to: 42

[8 [1 41] [[0 2] [0 3]]]

[41 42]

The other, more complex, pattern is to pin a core and its arms to the subject for invocation later. We’ll see this pattern in detail in opcode 9’s discussion.

*[context [8 [1 battery] [9 2 [0 1]]]]
Contents