phel-lang - SKILL.md Agent Skill

name: phel-lang description: Write or verify Phel code (Lisp on PHP). Triggers on .phel files, phel-config.php, phel CLI commands, or Phel snippets in markdown docs. Verify any non-trivial snippet against the runtime before claiming it works. model: sonnet

Phel

Lisp dialect compiling to PHP. Persistent data structures. PHP interop via php/.

Verify before documenting

Any Phel snippet added to docs/blog/cookbook MUST be runtime-checked. Examples that "look right" silently rot. Two ways:

# One-shot eval
./vendor/bin/phel run -e '(println (map inc [1 2 3]))'

# REPL session for exploration
./vendor/bin/phel repl

Write the snippet, run it, paste real output. If output differs from what you assumed, fix the doc - not the runtime.

Gotchas (project-specific)

defprotocol cannot be implemented inline in defstruct. Use definterface for inline; defprotocol + extend-type per struct.
extends? works only on primitive type keywords (:string, :int). Returns false for struct types. Use satisfies? on instances instead.
CLI args: argv (vector of user args, excludes program name); *program* for the script path. Not *argv* or php/$argv.
Side effects: doseq / foreach. Build sequences: for. Mixing causes wrong return shape.
String module: phel\string (not phel\str).
Phel vectors print as @[...]. Clojure prints [...]. Match the runtime output in docs.

Core syntax

(defn greet [name] (str "Hello, " name))      ; function
(defrecord Todo [id text done])                ; record (positional + map ctor)
(definterface Showable (show [this]))          ; interface - inline-implementable
(defprotocol Renderable (render [this]))       ; protocol - extend-type only
(extend-type Todo Renderable (render [t] ...)) ; protocol impl per type

(defmulti area :shape)                          ; multimethod
(defmethod area :circle [{:radius r}] (* 3.14 r r))

(into [] (comp (filter odd?) (map inc)) [1 2 3 4 5]) ; transducer
(re-find #"\d+" "abc123")                       ; regex literal

PHP interop

(DateTimeImmutable. "2026-01-01")          ; constructor shorthand
(.format obj "Y-m-d")                       ; method shorthand
(.-prop obj)                                ; property shorthand
DateTimeImmutable/ATOM                      ; static constant
(DateTimeImmutable/createFromFormat ...)    ; static method
(php/strlen "x")                            ; any PHP function

This repo

Docs: content/documentation/, content/blog/
Phel scratch: local/main.phel
Build artifacts: build/phel-config.php, .phel/
CLI: ./vendor/bin/phel <run|repl|test|build>

For full language reference: content/documentation/language/ and content/documentation/reference/.