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 9: Bind and Coerce Logical Calls

Physical kernels exist, but a parsed call contains logical types and a name. The binder must choose one kernel, apply only approved widening promotion, and reject unsupported signatures before a batch runs.

Prerequisites: Chapters 2, 5, 7, and 8; HashMap; closures; logical versus physical types.

By the end of this chapter, you will:

• bind functions of any arity through slice-based metadata;
• keep logical metadata consistent with the chosen physical expression; and
• support correct numeric comparisons, string comparisons, contains, and existing concat.

cargo x copy-test --chapter 9
cargo test -p type-exercise-starter chapter_9 --locked

The first run should fail on slice-based binding, comparison semantics, or contains.

Separate four kinds of conversion

• Numeric promotion converts values to a planner-selected common type.
• Erased downcast checks whether a runtime enum contains the requested physical family.
• Trait-object downcast recovers one concrete expression type through Any.
• Lifetime shortening reborrows a value for a shorter valid lifetime.

Only the first is logical coercion. Do not call all four “casts” or restore an obsolete GAT upcast helper.

Checkpoint 1: generalize the registry

• Target: type-exercise-starter/src/binder.rs::{BindError, BoundExpression, FunctionRegistry::register, register_unary, register_binary, register_ternary, bind}.
• Change: store logical inputs as a slice/boxed slice and check requested arity before a factory indexes it.
• Preserve: unknown name, wrong arity, unsupported arguments, missing physical expression, and metadata mismatch remain distinct errors.
• Run: the Chapter 9 focused test.
• Passing means: neg, arithmetic, clamp, and custom slice factories share one planning boundary across unary, binary, and ternary arities.

BoundExpression::new maps logical inputs and output to physical types and compares them with the selected expression’s metadata. A valid bound expression records that proof once; evaluation then delegates without rebinding each batch.

Checkpoint 2: bind comparisons and strings

• Target: type-exercise-starter/src/operators.rs::{ComparisonOperator} and binder factories registered by FunctionRegistry::with_builtins.
• Change: support <, <=, >, >=, =, !=, contains, and concat for their approved logical signatures.
• Preserve: names match behavior. Any ordered float comparison with NaN is false; equality is false and inequality is true. Null input produces null before comparison.
• Run: focused and cumulative tests.
• Passing means: equal operands distinguish strict/inclusive operators and NaN never panics.

String Char and Varchar both use physical String, but their logical metadata stays distinct. This course does not enforce Char width.

Wire the binder into the starter crate root like the earlier chapters:

mod binder;
pub use binder::{BindError, BoundExpression, FunctionRegistry};

Checkpoint 3: bind three-valued Boolean functions

• Target: type-exercise-starter/src/binder.rs::bind_boolean and the builtin registry entries for boolean_and, boolean_or, and boolean_not.
• Change: register the Day 7 expressions through the same slice registry: boolean_and and boolean_or take two Boolean inputs, boolean_not takes one; the bound output is Boolean.
• Preserve: arity is checked before a factory indexes its slice; unsupported signatures are bind errors; evaluation keeps the SQL three-valued semantics from Day 7.
• Run: the focused and cumulative tests.
• Passing means: one-input boolean_not binds (never rejected by a two-arity signature) and bound Boolean evaluation matches the Day 7 truth table.

Checkpoint 4: keep promotion lossless

• Target: type-exercise-starter/src/promotion.rs::promote_numeric and its callers in type-exercise-starter/src/binder.rs::{bind_arithmetic, bind_comparison}.
• Change: apply the same approved common type in both paths and both operand orders.
• Preserve: unsupported or precision-losing pairs are bind errors; no silent narrowing.
• Run: the full Chapter 9 contract.
• Passing means: logical output metadata agrees with the chosen physical output.

Required and extension work

Slice-based binding, lossless promotion, six comparisons, contains, and concat are required. Narrowing casts, parsing casts, SQL-complete coercion, Decimal arithmetic, and overload selection from untyped NULL are extensions that need separate semantics.

cargo test -p type-exercise-starter chapter_9 --locked
cargo test -p type-exercise-starter --lib --locked

Next: Chapter 10 specializes one representative dense loop.