Nikaia
Menu
v0.0.1343pre-alpha GitHub

ADR-collections-2: An Array Is Read as a List Is: Array[T, N] and Its Run Have iter(), and the Sequence That Starts There; So Has Every Set

Status: Accepted Date: October 29, 2026 (owner’s decision, option A; #955); D3 October 30, 2026 (owner’s decision, option B of #1006) Answers: whether Array[T, N] gets iter() and the sequence surface of Vec::iter(), which was NK1171 (#955, found by #953 at ty.nika’s bind_code); D3: whether BTreeSet and HashSet get it too (#1006) Supersedes / Related: ADR-288 D11 (what std offers is decided by the coherence of its types); ADR-282 D7 (no copy the program did not write); ADR-collections-1 D5 (what one set kind has, the other has, apart from hashing and order; applied by D3); Part I 3.3 (a for lends its list), 4.5 (Array[T, N])

1. Question

Does an array have iter()? std.contracts declares Array::len, is_empty and contains and nothing that walks it, so a.iter().zip(b.iter()) over two arrays is refused, and the only way to a sequence is .to_vec(), a copy.

2. How others do it

Language Its answer Source
Rust [T; N] and [T] have iter(), the slice’s iterator, so every Iterator method follows std docs, from memory, not checked
Swift a fixed array is a Sequence like any other from memory, not checked
C++ std::array has begin/end as std::vector does from memory, not checked
Go range walks an array as it walks a slice from memory, not checked

3. Options, weighed

  D1 the special case pays D2 the compiler works D3 no cost for the common case D4 consistent If it is wrong
A nobody one contract entry; the lowering is Rust’s .iter() none an array reads as a list of fixed length -
C every walk over an array the developer writes the copy an allocation per walk an array is indexed as a list but not walked as one -

4. Decision

D1 — a.iter() on an array is Vec::iter()

On Array[T, N] and on a run ref Array[T] (Part I 3.3), a.iter() hands back the elements one at a time, lent, as the sequence Vec::iter() hands back (Seq[ref(self) T] sync ends sized): it can be walked from either end, its length is known, and every method of a sequence follows (zip, map, filter, enumerate, rev, take, skip, fold, collect). It is lowered to Rust’s .iter() on [T; N] and on [T].

D2 — A for lends an array as it lends a list

for x in a walks the array without taking it, and x is a view of its element, under the rules of Part I 3.3.

D3 — Every set has iter(), in its order

s.iter() on a BTreeSet[T] hands back the elements one at a time, lent, smallest first, as Vec::iter() hands back a list’s (Seq[ref(self) T] sync ends sized); on a HashSet[T] it hands back the same sequence in the set’s order, which is none, as HashMap::keys’s (Seq[ref(self) T] sync sized: it does not run backwards). Every method of a sequence follows (any, all, filter, map, count, fold, collect). It is lowered to Rust’s .iter().

Why: a set is already walked by for (Part I 3.3), and Vec, arrays and both maps start a pipeline with iter(), keys() or values(); a set without it answers the same walk two ways (ADR-288 D11). Rejected: no iter, with a for loop and a flag for each any, or a copy into a Vec first (a copy the program does not need, ADR-282 D7); and the sequence methods on the set itself (s.any(f), s.filter(f), Swift’s model), which says each thing two ways, doubles the ledger per set kind, leaves open whether s.filter gives a set or a sequence, and has no counterpart in Rust to lower to. Rust, Java (TreeSet.iterator()), C++ (std::set::begin) walk an ordered set in its order through the iterator their list has, and put no pipeline on the set.

5. Consequences

6. What this does not decide

This site is open source. Improve this page.