Substructural Types in Turmeric

Turmeric supports three substructural type disciplines. Each discipline restricts which structural rules a value may use:

These disciplines are enabled by default; no flag is required.

When to use each discipline

^linear -- use exactly once

Use ^linear for values that represent exclusive ownership of a resource, where both dropping the resource unintentionally and aliasing it are errors.

(defn open-file  [path : cstr]             : ^linear FileHandle)
(defn close-file [^linear fh : FileHandle] : unit)

(defn copy-file [src dst : cstr] : unit
  (let [fh (open-file src)]
    ;; fh must be consumed exactly once
    (close-file fh)))

defn open-file  [path : cstr]             : ^linear FileHandle
defn close-file [^linear fh : FileHandle] : unit

defn copy-file [src dst : cstr] : unit
  let [fh open-file(src)]
    ;; fh must be consumed exactly once
    close-file(fh)

ref<T> bindings are automatically inferred as ^linear unless another annotation is present. You must explicitly (drop! r) or consume the ref -- it will not be silently freed.

^affine -- use at most once

Use ^affine when a value may be discarded but must not be aliased. This is appropriate for one-shot tokens, single-use callbacks, or initialization keys.

(defn initialize [^affine key : EncryptionKey] : unit
  ...)

;; OK: use once
(let [^affine k (generate-key)]
  (initialize k))

;; OK: discard without use (weakening allowed)
(let [^affine k (generate-key)]
  0)

;; ERROR TUR-E0150: affine value used more than once
(let [^affine k (generate-key)]
  (initialize k)
  (initialize k))

defn initialize [^affine key : EncryptionKey] : unit
  ...

;; OK: use once
let [^affine k generate-key()]
  initialize(k)

;; OK: discard without use (weakening allowed)
let [^affine k generate-key()]
  0

;; ERROR TUR-E0150: affine value used more than once
let [^affine k generate-key()]
  initialize(k)
  initialize(k)

^relevant -- use at least once

Use ^relevant for values that must be observed but may be inspected multiple times. This is appropriate for audit logs, mandatory acknowledgements, or results that must not be silently discarded.

(defn log-and-store [^relevant msg : str] : unit
  (log msg)     ;; first use (duplication OK)
  (store msg))  ;; second use

;; ERROR TUR-E0151: relevant value dropped without use
(let [^relevant msg "important event"]
  0)

defn log-and-store [^relevant msg : str] : unit
  log(msg)     ;; first use (duplication OK)
  store(msg)   ;; second use

;; ERROR TUR-E0151: relevant value dropped without use
let [^relevant msg "important event"]
  0

Borrowing a linear handle: ^borrow parameters

A ^linear (or ^affine) handle must be consumed exactly once, but resources typically expose non-consuming accessors -- reads that observe the handle and hand it back for further use (fs/tmpfile-path, mutex-lock, mutex-unlock, ...). Marking such a parameter ^borrow lets the function read its argument without consuming it: the single-consumption obligation stays with the caller for a later consuming op.

;; mutex-lock / mutex-unlock borrow; mutex-free consumes.
(defn mutex-lock [^borrow m : Mutex] : nil ...)
(defn mutex-free [m : Mutex]         : nil ...)   ;; consumes

(let [m (mutex-new)]
  (mutex-lock m)      ;; borrow -- m not consumed
  (mutex-unlock m)    ;; borrow again -- still fine
  (mutex-free m))     ;; the single legal consumption

(let [r (must-use (acquire-resource))]
  (process r)   ;; OK -- ^relevant allows duplication
  (process r))

;; ERROR TUR-E0151: relevant value 'r' dropped without use
(let [r (must-use (acquire-resource))]
  0)

let [r must-use(acquire-resource())]
  process(r)   ;; OK -- ^relevant allows duplication
  process(r)

;; ERROR TUR-E0151: relevant value 'r' dropped without use
let [r must-use(acquire-resource())]
  0

(with-resource [name init] body...)

Scoped resource binding -- equivalent to (let [name init] body...). Useful to signal intent that name is a resource to be consumed within body. With a ref<T> init, the binding is inferred as ^linear and must be explicitly consumed (via (drop! name) or a consuming call).

(with-resource [r (ref 42)]
  (drop! r))    ;; must consume the linear ref

with-resource [r ref(42)]
  drop!(r)    ;; must consume the linear ref

Error codes

Use tur --explain TUR-E0150 (or any other code) for a detailed explanation and fix suggestions.

Interaction with other features

ref<T> and linear inference

All ref<T> let bindings and :ref parameters are automatically inferred as ^linear. You do not need an explicit ^linear annotation -- but you must explicitly consume the ref.

(defn consume [r : ref] : int
  (drop! r)
  0)

defn consume [r :ref] :int
  drop!(r)
  0

Affine vs. move semantics

Turmeric's ownership model is affine by default for move types (CK_UNIQUE): values can be dropped implicitly but cannot be aliased. The explicit ^affine annotation makes this restriction visible in the type signature, enabling the elaborator to catch double-use at compile time.

Pattern matching

Substructural annotations are checked at each match arm independently. A ^linear value bound inside a match arm must be consumed within that arm.

(match opt
  (some v) (let [^linear r (ref v)]
             (drop! r))   ;; must consume before arm exits
  none     0)

match opt
  (some v)
  let [^linear r ref(v)]
    drop!(r)   ;; must consume before arm exits
  none
  0

Borrows and Lifetimes (TY4)

A borrow ((& x) / (&mut x)) is a non-owning reference whose validity is tied to the value it points at. Turmeric enforces, at compile time, that a borrow never outlives the value it borrows from (TUR-E0105):

;; ERROR (TUR-E0105): `r` borrows `x`, which lives only in the inner let.
(defn bad [] : int
  (let [r (let [x 42] (& x))]   ;; x dies when the inner let ends ...
    (deref r)))                 ;; ... but r still points at it

;; OK: the borrow does not outlive its referent.
(defn deref-outer [] : int
  (let [x 42]
    (let [r (& x)]              ;; x outlives r
      (deref r))))

The check is flow-sensitive through do, let, and both if branches, so a borrow produced inside a nested block is still attributed to its referent. It fires at two positions: a let binding whose initializer is an escaping borrow, and a function whose result is a borrow of one of its own locals. A borrow of an equal-or-outer-scope binding is always fine.

Borrow types and explicit 'a lifetimes

Borrows are first-class type annotations, written Rust-style with the lifetime immediately after the borrow sigil:

;; &'a int        -- immutable borrow with lifetime 'a
;; &mut 'a int    -- mutable borrow with lifetime 'a
;; &int           -- borrow with an elided (implicitly fresh) lifetime

Lifetimes are implicitly quantified: every distinct 'a in a signature is a fresh lifetime parameter, and a repeated 'a refers to the same one (just like type variables). A borrow may also be a return type, which is how an output borrow is tied back to an input:

;; The result borrows from x and y (both 'a), so it is valid for as long as
;; the shorter of the two arguments.
(defn longer [x : &'a int y : &'a int] : &'a int
  (if (> @x @y) x y))

Inter-procedural borrow checking

Because the return lifetime is tied to a parameter, the borrow checker follows the relationship across calls. A borrow returned from a call is treated as a borrow of the corresponding argument, so it cannot outlive that argument's referent:

(defn idb [x : &'a int] : &'a int x)

;; ERROR (TUR-E0105): (idb (& a)) borrows the local `a`; returning it lets that
;; borrow escape `bad`, where `a` no longer exists.
(defn bad [] : &int
  (let [a 5]
    (idb (& a))))

This reuses the same TUR-E0105 escape machinery as the direct case, so a single diagnostic is reported (no double-reporting).

Lifetime elision and outlives constraints

A borrow parameter or return that omits a lifetime gets one by the classic elision rules -- each borrow parameter gets its own lifetime (rule 1), a sole input lifetime flows to an elided borrow return (rule 2), and a receiver-style first borrow parameter lends its lifetime to the return (rule 3). Explicit 'a annotations override elision for the borrows that carry them.

Nested borrows imply outlives constraints: &'a &'b T requires the inner reference to outlive the outer, i.e. 'b outlives 'a. The resulting constraints are solved with cycle detection, so a contradictory, cyclic signature -- e.g. one parameter &'a &'b int paired with another &'b &'a int -- is rejected (TUR-E0106) rather than looping. The elision rules, the name-to-id interning, and the cycle-safe solver are unit-tested in tests/lifetime_unit.c; end-to-end fixtures live in tests/fixtures/lifetime-* and tests/fixtures/errors/lifetime-*.

See also

• Uniqueness Types Guide -- ^unique (at-most-one-reference discipline)
• tur --explain TUR-E0100 -- linear drop without consume
• tur --explain TUR-E0150 -- affine used twice
• tur --explain TUR-E0151 -- relevant dropped without use