flan/spec-memory.md

Spec 1 — Ownership, containers, and copies

Status: frozen. Closes plan.org open decisions #6 and #10, and resolves the contradiction between "value structs copy on assignment" and owning containers. Everything else in the design references this vocabulary.

The four container types

Notation	Layout	Assignment	Owns storage	Allocator
[n T]	n contiguous T	copies	no (inline)	—
[T]	ptr + len	copies the view	no	—
(Vec T)	ptr + len + cap	moves	yes	stored
(Map K V)	open-addressed, flat key/value arrays	moves	yes	stored

• [n T] is a value. It lives wherever it is declared, copies on assignment and on pass-by-value, and is what defconst colors [4 u32] ... and (defvar grid [rows [cols u32]] ...) are.
• [T] is a non-owning slice: a borrowed window into a [n T], a (Vec T), or a literal in read-only memory. Copying a slice copies ptr+len, never the elements. A slice may be const-qualified; freeing through one is not possible because a slice has no allocator and no cap.
• (Vec T) and (Map K V) are move-only. Binding, passing, or returning one transfers ownership; the source binding is dead afterwards and using it is a compile error. There is no shallow copy, so there is no double free.

Maps — first implementation

Every map is homogeneous: (Map K V) has one key type and one value type. The first implementation accepts only built-in structural key types: integers, enums, strings, fixed arrays, and value structs composed recursively from those types. Tuples and triples join that set when they are introduced. Ptr, slices, Vec, and Map are not map keys yet.

Equality and hashing for those keys are compiler-provided structural operations, not type classes and not operations available to an unconstrained type variable. An empty map takes its type from its context:

(defvar enemies (Map string Enemy) (map-new))

(get m k) returns (Option V): absence is None, not an untyped nil. (put m k v) is the upsert operation and returns Unit; it either inserts or replaces. (set (get m k) v) is not map syntax.

The first Map implementation admits copyable keys and values only, so get returns a copy. Move-aware lookup, removal, and owned entries are deferred until Vec/Map values are supported in maps; the map itself remains an owning, move-only container.

Copying is always explicit

(clone x) produces an independent deep copy of a Vec/Map using the current allocator; (clone x alloc) names one. Value types ([n T], structs of value types, primitives) need no clone — assignment already copies them.

A struct containing a Vec field is itself move-only. Ownership is structural, not declared: a type is a value type iff all of its fields are.

Borrowing

• (as-slice v) / (as-slice v lo hi) view a Vec or fixed array as [T].
• The first implementation follows Zig/Odin's explicit model, not Rust's borrow checker. A slice is invalidated by any operation that may reallocate its owner (push, put, reserve); its user is responsible for respecting that contract. Dev builds carry a generation word on Vec and trap on use of a stale slice. Ptr is the explicit lower-level escape hatch and has the same lifetime contract.
• A future lightweight provenance pass may reject the obvious mistakes (a borrow of a local escaping, use after an owner moves, and reallocation with a live borrow). It must not require Rust-style lifetime annotations or dictate an ECS-shaped object model. Long-lived graph links use (Handle a); temporary graphs may use explicitly managed, stable region storage.
• Cross-referencing long-lived objects uses (Handle a) into a pool, never a raw pointer or slice. A stale handle is detectable.

Taking an address

(addr x) yields (Ptr T) for any assignable place x — a local, a global, a field, an element. The pointer is non-owning and does not extend anything's lifetime, so addr of a local is only valid while that frame lives. This is the same escape question as case 3 below. The first implementation leaves it as an explicit lifetime contract; a future provenance pass can check it.

addr is how a value struct is shared mutably without an allocator — recursive descent over a cursor, an entity passed down a call chain — and it is why milestone 2 needs no heap at all. In the first implementation its non-escape rule is an explicit programmer contract, aided by dev checks; the future provenance pass above may enforce it.

Places — what set accepts

A fixed set of assignable forms, not a setf-style extensible place mechanism:

(set x v)              ; a local or a defvar
(set (.field x) v)     ; struct field; x may be a struct, (Ptr S) or (Handle S)
(set (at a i ...) v)   ; fixed array, slice, or Vec element
(set (deref p) v)      ; whole-object store through a pointer

.field and at auto-deref exactly one pointer or handle level, which is what makes (set (.hp e) ...) legal when e : (Ptr Enemy) and illegal when e : Enemy bound by value.

Mutating something you matched. Pattern bindings bind values, so a matched struct is a copy. To mutate in place, obtain a pointer first — the pointer is visible in the type:

(match (resolve w h)          ; (Option (Ptr Enemy))
  (Some e) (set (.hp e) ...)  ; e : (Ptr Enemy), field access derefs
  None     ...)

deref yields a value; resolve yields a pointer. Both are overloaded on (Ptr a) and (Handle a) and resolve at compile time.

Generics

Parametric polymorphism is monomorphisation, with no type classes and no constraints. The consequence is a hard rule:

A type variable a supports only what every type supports: move, clone, field-free storage. It does not support =, <, +, or hash.

Anything else is passed in explicitly as a function value:

(defn largest [xs [a]  gt (Fn [a a] bool)] (Option a) ...)

Ordered/arithmetic operators over a are therefore rejected, not silently instantiated. The alternatives — compile-time interfaces, or intrinsics restricted to primitives — are deliberately deferred until the base checker is stable (build sequence milestone 4).

println is the deliberate exception. It is a compiler-provided, type-directed intrinsic: monomorphisation selects or emits a structural printer for each concrete instantiation, so (println x) is legal for x : a without introducing a Printable type class. Structs, fixed arrays, options and, eventually, Vecs and Maps print structurally. Ptr and Handle print their address or identity rather than recursively dereferencing, and collection printers impose depth and length limits. any and Error use their runtime type metadata. User generic code still passes an explicit function for every other operation that depends on a type's structure.

Type arguments are inferred at call sites from the argument types; there is no explicit instantiation syntax in the first implementation. A type variable that appears only in the return type is therefore an error.

Function values

Three cases, split by whether the value escapes the frame that made it.

1. (Fn [T1 T2] R) — a plain, stable function pointer. No captured environment or allocation. In a dev build, a reference to a top-level defn is the address of a stable trampoline that loads that function version's indirection cell and calls its current body; it is never the address of a particular body. Thus stored callbacks and ordinary calls observe a later body redefinition, as in Common Lisp. Release builds may call the body directly because it cannot be redefined.

A signature-changing redefinition makes a new internal function version and a new trampoline ABI. Newly compiled code resolves the source name to that new version. Existing callers and stored Fn values keep their old trampoline and therefore safely call the old version. The session immediately warns at every tracked caller source location that still targets the old signature; recompiling one either retargets it successfully or reports an ordinary type error. This is what raylib callbacks, hot-reload cells, and function parameters use. A top-level defn is one, so (largest hps >) passes > at i32 directly. This is the only function type that may cross an FFI boundary or sit in a reload cell.

2. Non-escaping fn — captures by value into a stack environment. A fn whose value provably does not outlive the frame that created it gets an environment allocated in that frame and captures the named locals by value at the point of creation. No heap, no allocator, no lifetime question. This covers essentially every lambda in practice:

• callbacks to reduce / filter / each / map, which consume them and return
• comparators passed to a function that does not store them
• handler-bind handler bodies

That last one is not a convenience. A handler must be able to see the enclosing locals — (fn [c] (push errors c) (invoke-restart 'skip-form)) capturing a local (Vec ParseError) is the accumulation pattern, and conditions are not worth building without it. Handlers are strictly non-escaping: the handler-bind frame outlives every call to them.

Captured Vec/Map are captured by pointer, not moved, since the capture does not outlive the owner. A non-escaping fn is therefore not itself an owner.

3. Escaping closures — deferred. A fn stored in a struct, pushed into a container, or returned needs a heap environment and an answer to "which allocator owns it, and what happens when the frame arena resets". Do not settle this until a concrete use case requires it; revisit it with the optional lightweight provenance work.

Early exit inside a fn. try, some, and return in a fn body exit the fn, not the enclosing function — a fn is a function. Code that wants to propagate out of a loop uses an imperative loop form, not a callback.

Allocators

The allocator is part of the calling convention (context/allocator, context/temp). Vec and Map record the allocator they were created with, so free and clone never need it named again. Allocation uses the current implicit allocator by default, as in Odin; an operation never falls back to a hidden global allocator, and an explicit allocator can override the context.