jolt/.dirge/skills/jolt-dev/SKILL.md

name	description
jolt-dev	Jolt development workflow — build, test, special form patterns, Janet gotchas

jolt-dev

Jolt development workflow — build, test, special form patterns, Janet gotchas

Jolt Development

Build & Test

cd /Users/yogthos/src/jolt
jpm build           # produces build/jolt
jpm test            # runs all tests
janet test/foo.janet  # run a single test file from project root

Special Form Checklist

To add a new special form to the evaluator:

1. Add the name to special-symbol? in src/jolt/evaluator.janet
2. Add a match arm in eval-list (the match on name)
3. Add tests

The match arm receives ctx, bindings, and form (the full list). Use (in form 1) for first arg, etc.

Non-symbol heads (keywords, etc.): eval-list first checks (and (struct? first-form) (= :symbol (...))) before extracting name. If not a symbol, falls through to default function application.

Current special forms (37):

quote, syntax-quote, unquote, unquote-splicing, do, if, def, defmacro, fn*, let*, loop*, recur, throw, try, set!, var, locking, instance?, defmulti, defmethod, deftype, new, ., var-get, var-set, var?, alter-var-root, find-var, intern, alter-meta!, reset-meta!, disj, set?, protocol-dispatch, register-method, make-reified, satisfies?

Compiler Architecture

Two-phase: analyze-form [form bindings ctx] → emit-ast (string) or emit-expr (data structures).

Why data structures: Janet's eval can't see use-imported symbols. Embed function VALUES directly via core-fn-values table.

eval-string dispatch (compile mode): stateful forms → interpreter; everything else → compile-and-eval. Macros expand at analyze time.

Protocol System

Protocols are maps with :jolt/type :jolt/protocol and :methods map. Type registry in context env (:type-registry) maps type-tag → proto-name → method-name → fn.

Special forms:

• protocol-dispatch [proto-sym method-sym obj rest-args] — resolves method via type registry or reified methods
• register-method [type-sym proto-sym method-sym fn-form] — stores impl in type registry
• make-reified [proto-sym methods-map] — creates anonymous object with :jolt/protocol-methods

Critical rule: fn* form inside extend-type/extend-protocol MUST be @[...] (array) to trigger eval-list's special form dispatch. Tuples [...] hit (tuple? form) branch instead. Same for register-method, protocol-dispatch calls.

REPL Collection Rendering (Buffer-Based)

Use write-value + write-collection with a StringBuffer (@"") rather than prin/print directly. Build the entire output string in a buffer, then atomically (print (string buf)). Prevents Janet's C runtime (in jpm build executables) from interleaving its native <tuple 0x...> printer between incremental prin calls.

(var write-value nil)  ; forward declaration

(defn- write-collection [v buf]
  (cond (tuple? v) (do (buffer/push-string buf "[") ...)
        (array? v) (do (buffer/push-string buf "(") ...) ...))

(set write-value (fn [v buf]
  (cond (nil? v) (buffer/push-string buf "nil")
        (number? v) (buffer/push-string buf (string v))
        (tuple? v) (write-collection v buf)
        true (buffer/push-string buf (string v)))))  ; true REQUIRED

(defn print-value [v] (def buf @"") (write-value v buf) (print (string buf)))

Critical: Janet's cond treats a bare expression in the last position as a test clause, not a catch-all body. Use true as the guard.

PersistentHashMap Gotchas

• core-map?: (if (and (table? x) (get x :jolt/deftype)) true false) — and returns last truthy, not boolean
• core-count: subtract 1 for deftype tables (skip :jolt/deftype key)
• Equality: convert via phm-to-struct before deep=

defrecord / deftype Patterns

• defrecord emits (deftype TypeName [fields]) + arrow factory
• Records are tables with :jolt/deftype = type name string
• set! field mutation: (set! (.-x obj) val) parses as array with .-x symbol head

Binding Macro

Uses array-map (plain Janet struct) not hash-map (PHM) to avoid PHM get() incompatibility with var-get.

Tagged Literals (#inst, #uuid)

Use dynamic table construction: (let [dr @{}] (put dr (keyword "#inst") fn) dr)

LazySeq Patterns

• Use indexed? not tuple? for realized sequences (may be arrays from cons/concat)
• Avoid val' (apostrophe in symbol names) — use vf instead

Janet Gotchas

• def creates constants; use (var x nil) for mutable locals
• Bare tuples in eval are function calls: [1 2 3] tries to call 1
• try format: (try body ([err] handler)) NOT (try body (catch sym handler))
• core-renames MUST match actual fn names: "-" → "core-sub" (not "core--")
• (break val) breaks from loop returning val — useful in bucket search patterns
• boolean doesn't exist — use (if x true false)
• Janet doesn't support Clojure-style multi-arity defn — use [& args] with case (length args)
• Janet's cond treats bare expression in last position as test, not catch-all — use true guard