jolt/docs/spec/09-core-library.md

§9 The Core Library

Status: entry format fixed; exemplars for first, reduce, parse-uuid. The full portable surface (≈500 vars after classification, dashboard in coverage.md) is filled in chapter-by-chapter using this format.

Entries specify behavioral contracts, not implementations. Performance characteristics are specified only where the language community relies on them (e.g. vector nth is "effectively constant time" — SHOULD-level).

---

Collection return types & laziness (cross-cutting)

Two contracts hold across the sequence library and are not restated per entry.

Return-type fidelity. A function returns the same kind of collection the reference does — value equality is not enough, since (= [0 1] '(0 1)).

• Sequence transformations return seqs (lazy unless noted): map, filter, remove, keep, mapcat, take/drop and their -while forms, partition, partition-all, partition-by, interpose, dedupe, distinct, concat, reductions, cons, rest, sequence. The elements of partition / partition-all / partition-by are themselves seqs, not vectors.
• The vector variants return vectors: mapv, filterv, vec, subvec, partitionv, partitionv-all, splitv-at. split-at / split-with return a 2-vector [take drop]. A transducer applied eagerly (into [], the partition-all transducer's chunks) yields vectors.
• Type-preserving functions return the input's type: replace over a vector is a vector, over any other seqable a (lazy) seq; empty/into (empty coll) keep the collection kind; set/into #{} return sets; into {}/select-keys/zipmap/ frequencies/group-by/merge return maps (group-by values are vectors).

Laziness. The lazy sequence functions — including sequence, eduction, and mapcat — MUST consume their source incrementally and so terminate on an infinite or unbounded source when only a prefix is demanded: (first (sequence (map inc) (range))) and (take n (mapcat f (range))) return without realizing the whole source. (apply concat coll-of-colls) is likewise lazy in its argument seq. The eager consumers (reduce, into, count, vec, doall) realize the demanded portion fully.

These are exercised by the seq / lazy over infinite and the per-fn type-predicate rows in the conformance corpus.

---

first — since 1.0

(first coll)

Semantics

• S1. MUST return the first element of (seq coll).
• S2. If (seq coll) is nil (i.e. coll is empty or nil), MUST return nil.
• S3. MUST accept anything seqable (§5): seqs, lists, vectors, maps (yielding map entries), sets, strings (yielding characters), nil.
• S4. On a lazy sequence, MUST realize at most the first element (§5 laziness contract).

Edge cases

• E1. (first nil) ⇒ nil; (first []) ⇒ nil; (first "") ⇒ nil.
• E2. A nil or false first element is returned as-is — callers cannot distinguish "empty" from "first element is nil" via first alone (that is what seq is for).
• E3. On a map, the element is a map entry; on an unordered collection (map, set) which element is first is implementation-defined but MUST be consistent with that collection's seq order for the same collection value.

Errors

• X1. A non-seqable argument (e.g. a number) MUST throw.

Examples

(first [1 2 3])      ;=> 1
(first '())          ;=> nil
(first "ab")         ;=> \a
(first {:a 1})       ;=> [:a 1]
(first [nil 2])      ;=> nil

Conformance

S1–S3, E1–E2 → jolt sequences-spec "seq / access"; clojure-test-suite core_test/first.cljc. S4 → jolt lazy-seqs-spec counter cases. X1 → clojure-test-suite core_test/first.cljc (throwing cases).

---

reduce — since 1.0

(reduce f coll)
(reduce f init coll)

Semantics

• S1. With init: MUST return init if (seq coll) is nil; otherwise MUST return (f … (f (f init e₁) e₂) … eₙ), applying f left-to-right over the elements, exactly once each.
• S2. Without init: if coll is empty, MUST return (f) (f called with no arguments); if coll has one element, MUST return that element without calling f; otherwise as S1 with init = e₁ over e₂…eₙ.
• S3. Reduced short-circuit: if any intermediate result is a reduced value, iteration MUST stop and the dereferenced value MUST be returned immediately; f MUST NOT be called again.
• S4. reduce is eager: it MUST fully realize the consumed portion of a lazy coll (to the end, or to the reduced point).

Edge cases

• E1. (reduce f nil) ⇒ (f); (reduce f init nil) ⇒ init.
• E2. A reduced value as the initial init is NOT unwrapped before the first call in the reference — ⚠ under-documented; differential result to pin down and test before this entry is marked verified.
• E3. Visit order over maps is entry order of the map's seq; over vectors/lists/seqs it is sequential order (normative).

Errors

• X1. Without init, on an empty coll, if f has no zero-arg arity the call (f) MUST throw (arity error).

Examples

(reduce + [1 2 3 4])                                ;=> 10
(reduce + 10 [1 2 3 4])                             ;=> 20
(reduce + [])                                       ;=> 0    ; (+) is 0
(reduce + [5])                                      ;=> 5    ; f not called
(reduce (fn [a x] (if (> a 2) (reduced a) (+ a x))) 0 [1 2 3 4 5]) ;=> 3

Conformance

S1–S3, E1 → jolt sequences-spec "map filter reduce" group + transducers-spec "reduce honors reduced"; clojure-test-suite core_test/reduce.cljc. S2 (single-element, f-not-called) → jolt conformance "reduce single no init". E2 → UNVERIFIED (differential test to add). S4 → lazy-seqs-spec.

---

parse-uuid — since 1.11

(parse-uuid s)

Semantics

• S1. If s is a string in canonical UUID form — five groups of hex digits of lengths 8, 4, 4, 4, 12 separated by - — MUST return a UUID value u such that (uuid? u) is true and (str u) is the lowercase form of s.
• S2. Parsing MUST be case-insensitive and equality on the results case-insensitive: (= (parse-uuid s) (parse-uuid (upper-case s))) is true.
• S3. If s is a string not in canonical form, MUST return nil. ⚠ reference-divergence: reference Clojure (java.util.UUID) additionally accepts non-canonical forms like "0-0-0-0-0"; ClojureScript and other dialects are strict. This spec adopts strict (the cross-dialect behavior); the reference's permissiveness is recorded as host leniency.
• S4. UUID values MUST support value equality, hashing (usable as map keys and set members), str (lowercase canonical form), and print as the tagged literal #uuid "…" such that the printed form reads back equal (§2 tagged literals).

Edge cases

• E1. "", over-long, truncated, non-hex characters, and misplaced dashes ⇒ nil.

Errors

• X1. A non-string argument MUST throw.

Examples

(parse-uuid "b6883c0a-0342-4007-9966-bc2dfa6b109e")  ;=> #uuid "b6883c0a-…"
(uuid? *1)                                            ;=> true
(parse-uuid "df0993")                                 ;=> nil
(parse-uuid 1000)                                     ;; throws

Conformance

S1–S4, E1, X1 → jolt uuid-spec (30 cases) + 6 three-path conformance cases; clojure-test-suite core_test/parse_uuid.cljc, core_test/uuid_qmark.cljc, core_test/random_uuid.cljc.

---

clojure.template/apply-template, clojure.test/are — since 1.1

(apply-template argv expr values)
(are argv expr & args)

Semantics

• S1. apply-template MUST replace every occurrence of each argv symbol in expr with its corresponding value by structural walk (postwalk symbol substitution), not by lexical binding. Occurrences inside quote and at any nesting depth substitute: (apply-template '[x] '(f 'x) '[if]) ⇒ (f 'if).
• S2. do-template MUST partition args by (count argv) and expand to a do of one substituted expr per group.
• S3. clojure.test/are MUST expand through do-template with expr wrapped in is. Consequently (are [x] (special-symbol? 'x) if def) asserts (special-symbol? 'if) and (special-symbol? 'def) — a let-binding implementation is non-conforming (the quoted symbol would not substitute).

Errors

• X1. are MUST throw at macroexpansion when (count args) is not a positive multiple of a non-empty (count argv) (empty/empty is allowed).
• X2. apply-template MUST throw when argv is not a vector of symbols.

Conformance

S1–S3 → test/chez/clojure-test.clj (are with quoted template var); clojure-test-suite core_test/special_symbol_qmark.cljc and every are-based suite namespace.

---

Authoring notes

• Source examples from the ClojureDocs export (clojuredocs-export.edn, 648 core vars have community examples) — but every example is verified against the reference before inclusion.
• When writing an entry surfaces a behavior question, settle it by differential test first; if dialects split, that's a classification decision (host-dependent / divergence note), not a coin flip.
• An entry is Verified when no field carries UNVERIFIED; coverage.md tracks per-var status.