Work in progress. This course has not yet received the author’s final audit. Chapters, exercises, and commands may still change before the course is marked complete.

Chapter 7: Implement Three-Valued Boolean Logic

SQL engines do not stop at two truth values. A missing value makes NULL AND FALSE false and NULL OR TRUE true, so nulls must flow into the Boolean scalar function instead of always short-circuiting.

Prerequisites: Chapter 6, the checked-expression boundary, and nullable Boolean columns.

By the end of this chapter, you will:

• distinguish strict null short-circuiting from SQL’s non-strict null semantics;
• implement AND, OR, and NOT over TRUE/FALSE/NULL; and
• publish one checked expression whose validation and row loop follow the same arity-before-type-before-length contract as the earlier shells.

cargo x copy-test --chapter 7
cargo test -p type-exercise-starter chapter_7 --locked

The first run should fail on the missing Boolean operator, truth table, or expression builder.

Two null policies

The Day 4–6 shells skip the scalar function for any strict null input: the row is null, and the function never sees it. That is the Strict policy. SQL’s three-valued logic needs more: a null operand can still decide the result when the other operand is absorbing (FALSE AND ..., TRUE OR ...). That is the NonStrict policy, where nulls are passed to the scalar function and the truth table decides.

Checkpoint 1: pin the truth table

• Target: type-exercise-starter/src/boolean_logic.rs::{NullEvaluationPolicy, BooleanOperator, BooleanTruthRow, BOOLEAN_TRUTH_TABLE}.
• Change: declare both policies, the three operators, and the 21 required nullable-Boolean rows (nine AND, nine OR, three NOT), with FALSE absorbing for AND, TRUE absorbing for OR, and NOT NULL staying null.
• Preserve: the row order and values match the supplied expected table exactly.
• Run: the Chapter 7 focused test.
• Passing means: the table rows are exactly the required three-valued truth table.

Checkpoint 2: evaluate one operator

• Target: type-exercise-starter/src/boolean_logic.rs::{BooleanExpression, build_boolean_expression}.
• Change: validate arity (two for AND/OR, one for NOT), physical types, and lengths before any row work, then build Boolean rows. build_boolean_expression selects the SQL NonStrict policy; BooleanExpression::new(operator, policy) exposes the strict variant for comparison.
• Preserve: the row error and null behavior stay inside the checked-expression contract; the error representation is your readable choice.
• Run: the focused and cumulative tests.
• Passing means: evaluation reproduces the full truth table, strict short-circuits before the truth table, and wrong arity/type/length fail closed.

Required and extension work

Both policies, all three operators, the exact truth table, and the checked builder are required. Nested expression trees and short-circuit execution plans are extensions outside this course.

cargo test -p type-exercise-starter chapter_7 --locked
cargo test -p type-exercise-starter --lib --locked

Next: Chapter 8 erases typed expressions behind one object-safe boundary.