collect() Builds What the Place It Goes Into DeclaresStatus: Superseded by ADR-293 — its D1-D2 are D25-D26 there
Date: September 26, 2026
Target Version: Nikaia 0.0.210
Answers: #208’s collect() into a declared map passes the
checker and fails in rustc, with the owner’s choice: collect builds what
the target declares
Related: ADR-105 (Seq, and collect as its eager walk),
ADR-152 D4 and ADR-207 D2 (a use answers a
literal), ADR-223 and ADR-224 (text positions)
let m: collections::HashMap[String, i64] = text.lines().map(fn(l) { (l, 1) }).collect()passed the checker and failed in rustc: the emitter wrote
.collect::<Vec<_>>() into a map. The ledger says Seq::collect hands back
Vec[$T], so the checker should have refused the let, but the items were
not typed (ADR-225) and a Vec of unknowns fit. With the items typed, the
program was refused, which is a wall. The owner chose the other answer: build
the map.
collect() builds a list, or what the let, field or result it goes into declares.Where nothing declares anything, collect() builds a Vec, as before. Where a
let, a struct field or a function’s result declares a map whose key and
value the items fit as pairs, a set whose element they fit, or String from
characters or pieces of text, that is what it builds, and the value has that
type. It is the same move the checker already makes for a literal: the place
it is used says what it is.
Below, the language’s own collect builds the type the place declares, so the
emitter leaves off the ::<Vec<_>> it writes everywhere else. Nothing is
built twice: there is no list in between.
What the items do not fit is refused as before (NK1103): pairs are not a set
of text.
A map is collected from pairs, and its key and its value are two positions
(ADR-223 D1). Where one of them is mixed, the pairs go through
either_keys(), either_values() or either_pairs() before collect, as a
list’s items go through either_items() (ADR-224 D3).
For that to be decided, the pass that decides tiers reads what map makes as
what its lambda hands back, with the lambda’s parameter bound to what came in:
lines.map(fn(l) { (l, 1) }) is pairs keyed by views. filter, rev,
skip, take and step_by hand on what they were given. A number, a truth
value and null are no text of either kind; they used to count as text of
its own, which made a pair of a view and a number mixed.
VecDeque. std has no deque a program can name.collect() as an argument, where the parameter declares the container.
An argument is not one of the places the checker answers a literal by use
either; it builds a list there, as before.#208’s entry closes. Found on the way and recorded there: a
filter lambda that compares its item to a value fails in rustc (&char
against char), with or without this change.
All of it, in 0.0.210: Checker::collect_by_use and Checked::collected_into;
the emitter’s collect without the turbofish there; EitherPairs in
nikaia_std::either_text, in the prelude; the pair hands, map’s lambda and
the non-text literals in crates/nikaia/src/text_tiers.rs.
crates/nikaia/tests/collect_into.rs runs a map, a set, an ordered set, text
and a list, and a map both kinds of key are collected into and a result
declares, at both settings of user_parallelism, and holds the refusal.