# §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).

---

### 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**

```clojure
(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**

```clojure
(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**

```clojure
(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`.

---

## 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.