jolt/README.md

Jolt

A Clojure interpreter running on Janet. Jolt reads Clojure source, evaluates it with an interpreter written in pure Janet, and ships a Clojure-compatible standard library. The goal is a Janet-hosted SCI runtime — a minimal bootstrap that loads SCI's Clojure source as its standard library.

Build

git clone https://github.com/yogthos/jolt.git
cd jolt
git submodule update --init   # pulls vendor/sci
jpm build                     # compiles build/jolt

Requires Janet ≥ 1.36 and jpm.

Run

build/jolt                 # start a REPL
build/jolt file.clj [args] # run a file (binds *command-line-args* and *file*)
build/jolt -e EXPR [args]  # evaluate EXPR and print the result
build/jolt -h              # help

The REPL accumulates multi-line forms until they balance:

user=> (defn fib [n] (if (< n 2) n (+ (fib (- n 1)) (fib (- n 2)))))
#'user/fib
user=> (map fib (range 10))
(0 1 1 2 3 5 8 13 21 34)

Running a file evaluates its top-level forms:

$ echo '(println "hello" (* 6 7))' > hello.clj
$ build/jolt hello.clj
hello 42

Use as a library

(use jolt/api)

(def ctx (init))
(eval-string ctx "(+ 1 2)")            # → 3
(eval-string ctx "(map inc [1 2 3])")  # → [2 3 4]

(init) returns a context with clojure.core loaded. Each context is isolated; use separate contexts for separate environments.

Host interop

Jolt exposes CLJS-style host interop through . on any Janet table or struct — a field holding a function is called with the receiver as the first argument:

(def obj {:greet (fn [self name] (str "Hello " name))})
(. obj greet "Alice")   ; → "Hello Alice"
(.-greet obj)           ; field access (reader sugar for (. obj :greet))

Janet's standard library is reachable through jolt.interop (and the jolt.shell / jolt.http helpers built on it):

(require '[jolt.interop :as j])
(j/janet-type [1 2])              ; → :tuple
(j/janet-table-keys {:a 1 :b 2})  ; → [:b :a]

Differences from Clojure

Jolt targets Clojure semantics but runs on Janet, not the JVM. The notable divergences:

• Host platform. No JVM and no Java interop — import, gen-class, proxy of Java classes, and java.* are unavailable. instance? recognizes a small set of built-in types (clojure.lang.Atom, Number, String, …).
• Numbers. Janet integers and doubles. (/ 1 3) is 0.3333… and large products lose precision. No ratios or BigDecimal (ratio? is always false, bigdec falls back to a double); bigint/biginteger use Janet's 64-bit int/s64, not arbitrary precision. The auto-promoting +'/-'/*'/inc'/dec' are aliases for the plain ops, since Janet numbers don't overflow. quot/rem/mod follow Clojure's sign rules.
• Collections. By default Jolt uses immutable persistent data structures: vectors are 32-way branching tries (structural-sharing persistent vectors with O(log₃₂ n) conj/assoc/nth), lists are persistent singly-linked cons cells (O(1) conj/cons prepend with structural sharing), and maps/sets are persistent hash structures. Value equality and sequence operations are Clojure-compatible, but hash-map/hash-set iteration order is unspecified and differs from Clojure — use sorted-map/sorted-set when order matters.
• Mutable build mode. Jolt can be compiled to use fast Janet-native mutable collections instead, via a build-time flag: JOLT_MUTABLE=1 jpm build (default jpm build is immutable). In mutable mode vectors and lists share one mutable array representation (so conj mutates in place and appends, and vector?/list? no longer distinguish them) — a performance/looseness trade-off. The default immutable build has full Clojure value semantics.
• Concurrency / STM. Single-threaded. No refs, dosync, agents, or send; locking evaluates its body without real locking. Atoms, volatiles, and delays are supported.
• Regex. Compiled to Janet's PEG engine (Janet has no regex). Supported: capturing groups ([whole g1 …]), greedy and lazy quantifiers with backtracking, (?:…), lookahead (?=…)/(?!…), alternation, anchors ^ $ \b \B, character classes, and the (?i) flag. Not supported: lookbehind, backreferences (\1), and named groups ((?<name>…)).
• Arrays. Java-style arrays map onto Janet's native types: byte-array is a Janet buffer (contiguous, C-backed); object-array/int-array/double-array/etc. are Janet arrays. aget/aset/alength/aclone work over both.
• Transients. transient/conj!/assoc!/dissoc!/disj!/pop!/persistent! are real mutable scratch collections backed by Janet's native arrays and tables (vectors → arrays, maps/sets → tables), so building a collection with them avoids the per-step copying of the persistent path (notably for maps/sets). persistent! freezes back to a persistent value.
• Not implemented. JVM reflection, proxy, and the clojure.repl/clojure.template namespaces.

Supported and Clojure-compatible: chars as a distinct type, lazy/infinite sequences, transducers, destructuring, multimethods with hierarchies, protocols/records (deftype/defrecord/reify/extend-protocol), metadata, namespaces, and the reader (#(), #_, #?, tagged literals, #"…").

Test

jpm test                                    # full suite (recurses test/)
janet test/spec/sequences-spec.janet        # a single spec
janet test/integration/conformance-test.janet

Tests are organized in three layers:

• test/spec/ — the contract. Black-box, behavior-defining tables (one file per public API area) that collectively pin down Jolt's defined behavior. This is the authoritative description of what Jolt promises.
• test/integration/ — cross-cutting and regression batteries: the Clojure conformance suite, SCI bootstrap/runtime loading, jank conformance, compile-mode tests, the library API, and a broad systematic-coverage net.
• test/unit/ — white-box tests for individual components (reader, evaluator, types, persistent collections, regex, compiler).

test/support/harness.janet provides the shared defspec table runner (cases are ["label" expected actual], compared with Jolt's own =) plus expect=/expect-throws for unit tests.

The syntactic half of the contract — the surface syntax the reader accepts — is specified as an EBNF grammar in doc/grammar.ebnf, with Jolt-vs-Clojure deviations noted inline. test/spec/reader-syntax-spec.janet exercises it.

License

Eclipse Public License 1.0