Opcode 2: Evaluate

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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.

:subject 42

Subject set to: 42

This simple formula computes the constant function that always returns 100, then returns its subject (now 100).

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

100

This formula acts as a dynamic formula selection, which sets the subject to 5 and then calculates its increment.

:subject [5 [4 0 1]]

Subject set to: [5 4 0 1]

[2 [0 2] [0 3]]

6
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