Chapter 1: Connect One Type Family by Hand

An i32 can be copied out of an array. A String should be read as an &str that borrows the array. This chapter connects both cases without forcing one into the other’s ownership model.

Prerequisites: enums, traits, references, associated types, and Option.

By the end of this chapter, you will:

• connect owned values, borrowed values, arrays, and builders for i32 and String;
• use a generic associated type for the borrowed member of each family; and
• upcast typed values with From and downcast erased enums with checked TryFrom conversions.

See the missing connections

Copy the cumulative contract and run it once:

cargo x copy-test --chapter 1
cargo test -p type-exercise-starter chapter_1 --locked

The untouched starter compiles because it declares the Day 1 target shapes, but the focused test should fail at the Day 1 todo! boundary. It should not fail on an unresolved target. Do not edit the copied test.

The family you are building has reciprocal arrows:

Scalar ──RefType<'a>──> ScalarRef<'a>
  │                         │
ArrayType                ArrayType
  ▼                         ▼
Array ─────Builder─────> ArrayBuilder

For i32, the borrowed scalar is another i32. For String, it is &'a str. Array::get therefore returns Option<i32> for I32Array and Option<&str> for StringArray without an allocation on the string read.

Checkpoint 1: describe the two physical families

• Target: type-exercise-starter/src/physical_type.rs::{PhysicalType, TypeMismatch} and type-exercise-starter/src/scalar.rs::{Scalar, ScalarRef, ScalarImpl, ScalarRefImpl}.
• Change: add only the Int32 and String physical rows and their reciprocal associated types.
• Preserve: the original two ScalarImpl variants and safe Rust.
• Run: the Chapter 1 focused test.
• Passing means: owned and borrowed values point to the correct array family.

for<'a> on a bound means the relationship holds for every caller-chosen borrow lifetime. The integer implementation may ignore that lifetime; the string implementation cannot.

Checkpoint 2: store nullable rows

• Target: type-exercise-starter/src/array.rs::{Array, ArrayBuilder, ArrayImpl}, type-exercise-starter/src/array/primitive_array.rs::{PrimitiveArray, PrimitiveArrayBuilder}, and type-exercise-starter/src/array/string_array.rs::{StringArray, StringArrayBuilder}.
• Change: implement get, len, iter, from_slice, builder push, and finish for the two families. Expose read-only values/validity accessors on PrimitiveArray and data/offsets/validity accessors on StringArray so the buffer contract is visible.
• Preserve: row count and null positions; returned strings must borrow array storage; offsets count UTF-8 bytes rather than characters.
• Run: the same focused test.
• Passing means: normal, null, and empty arrays read through one generic contract.

Use the Arrow-like layout required by the supplied test. A fixed-width array stores one flat Vec<T> with exactly one slot per row. A string array stores all UTF-8 bytes in one flat Vec<u8> and uses rows + 1 monotone offsets: the first is zero, the last is the byte-buffer length, and a null or empty row repeats an offset. Both arrays store row validity in the packed bitvec::vec::BitVec already declared in the starter. A null row returns None, but it is never an Option<T> payload slot; strings are not stored as one owned String per row.

Checkpoint 3: erase and recover values

• Target: From/TryFrom implementations in type-exercise-starter/src/scalar.rs and type-exercise-starter/src/array.rs, plus exports in type-exercise-starter/src/lib.rs.
• Change: upcast typed values into erased enums and recover the requested type.
• Preserve: wrong variants return TypeMismatch { expected, actual }; they do not panic.
• Run: the focused and cumulative starter tests.
• Passing means: correct variants round-trip and wrong variants fail at the boundary.

Required and extension work

Required work is exactly the explicit i32 and String rows. Additional physical types, macros, columns, and expressions belong to later chapters. As an extension, sketch a third family on paper and mark every enum arm and conversion it would require; do not implement it yet.

cargo test -p type-exercise-starter chapter_1 --locked
cargo test -p type-exercise-starter --lib --locked

Before continuing, explain why StringArray::get needs a lifetime-indexed associated type and why an erased downcast is fallible even when the compile-time family is consistent.

Next: Chapter 2 scales the family without copying every connection.

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