Part II — The Disciplines on

Chapter 3 — Types: Making Illegal Values Unrepresentable

Mon, 01 Jan 0001 00:00:00 +0000

Every value is a claim about the world. A type is the claim written where the machine can read it.

— workshop wall

# 3.1 The Practice

The Practice. Every value has a type, and the type says which values make sense and which words may touch them. The checker rejects programs that break the promise before they exist — and when a value breaks it at run time anyway, the runtime traps with a name, not a shrug.

Chapter 4 — Contracts: Promises the Machine Reads

Mon, 01 Jan 0001 00:00:00 +0000

A type says what a value is. A contract says what must be true. Only one of them fits in a comment.

— workshop wall

# 4.1 The Practice

The Practice. A contract is a checked program about a claim: the checker verifies the claim’s shape at compile time, the runtime verifies the claim’s truth at the boundary, and both verifications apply to every caller, present and future.

Chapter 5 — Effects and Capabilities: The Two-Sided Law

Mon, 01 Jan 0001 00:00:00 +0000

A signature can say what arrives and what leaves. The interesting question is what happens in between.

— workshop wall

# 5.1 The Practice

The Practice. Declare what a word does; the caller decides. A word’s effects are checked against the context that calls it, and a context’s capabilities belong to the code lexically inside it — so the reviewer reads the label on the tin, and the compiler checks that the contents match.

Chapter 6 — The Stack Has a Shape

Mon, 01 Jan 0001 00:00:00 +0000

Every phrase has a height. Most languages measure it at 3 a.m., in the field, with a broken device. Tyu measures it at compile time, on paper.

— workshop wall

# 6.1 The Practice

The Practice. Every phrase has a shape — what it adds, what it removes, and how high it reaches. The checker computes the reach for every word, composes it through every call, and refuses programs whose reach cannot be bounded.

Chapter 7 — Places, Borrowing, Ownership

Mon, 01 Jan 0001 00:00:00 +0000

Copying is a decision someone made. Ownership is a decision someone has to answer for.

— workshop wall

# 7.1 The Practice

The Practice. A place has one address and a rule: one writer or many readers, never both. A pointer borrows a place and lives exactly as long as its block. Custody moves — it is never cloned. The checker keeps the ledger, and the ledger has no rounding errors.

Chapter 8 — Touching Hardware: MMIO

Mon, 01 Jan 0001 00:00:00 +0000

The datasheet is a specification. Most languages make you retype it as comments. Tyu makes you retype it as a contract.

— workshop wall

# 8.1 The Practice

The Practice. Hardware is described once, in a platform descriptor — devices named, registers placed, access rules stated. The source’s register-map is checked against that description row by row, and every access the source writes is checked against the rules. The datasheet becomes a compiler input.