scheme: Phase 3 — if/define/set!/begin/lambda/closures + 24 tests

eval.sx grows: five new syntactic operators wired via the table-
driven dispatch from Phase 2. lambda creates closures
{:scm-tag :closure :params :rest :body :env} that capture the
static env; scheme-apply-closure binds formals + rest-arg, evaluates
multi-expression body in (extend static-env), returns last value.

Supports lambda formals shapes:
  ()            → no args
  (a b c)       → fixed arity
  args          → bare symbol; binds all call-args as a list

Dotted-pair tail (a b . rest) deferred until parser supports it.

define has both flavours:
  (define name expr)                 — direct binding
  (define (name . formals) body...)  — lambda sugar

set! walks the env chain via refl-env-find-frame, mutates at the
binding's source frame (no shadowing). Raises on unbound name.

24 new tests in lib/scheme/tests/syntax.sx, including:
- Factorial 5 → 120 and 10 → 3628800 (recursion + closures)
- make-counter via closed-over set! state
- Curried (((curry+ 1) 2) 3) → 6
- (lambda args args) rest-arg binding
- Multi-body lambdas with internal define

109 total Scheme tests (62 parse + 23 eval + 24 syntax).

lib/scheme/tests/syntax.sx

@@ -0,0 +1,237 @@
+;; lib/scheme/tests/syntax.sx — exercises Phase 3 syntactic operators.
+
+(define scm-syn-pass 0)
+(define scm-syn-fail 0)
+(define scm-syn-fails (list))
+
+(define
+  scm-syn-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! scm-syn-pass (+ scm-syn-pass 1))
+      (begin
+        (set! scm-syn-fail (+ scm-syn-fail 1))
+        (append! scm-syn-fails {:name name :actual actual :expected expected})))))
+
+(define scm-syn-eval (fn (src env) (scheme-eval (scheme-parse src) env)))
+
+(define
+  scm-syn-eval-all
+  (fn (src env) (scheme-eval-program (scheme-parse-all src) env)))
+
+;; Test env with arithmetic primitives.
+(define
+  scm-syn-env
+  (fn
+    ()
+    (let
+      ((env (scheme-make-env)))
+      (scheme-env-bind!
+        env
+        "+"
+        (fn (args) (+ (first args) (nth args 1))))
+      (scheme-env-bind!
+        env
+        "-"
+        (fn (args) (- (first args) (nth args 1))))
+      (scheme-env-bind!
+        env
+        "*"
+        (fn (args) (* (first args) (nth args 1))))
+      (scheme-env-bind!
+        env
+        "/"
+        (fn (args) (/ (first args) (nth args 1))))
+      (scheme-env-bind!
+        env
+        "<="
+        (fn (args) (<= (first args) (nth args 1))))
+      (scheme-env-bind!
+        env
+        "<"
+        (fn (args) (< (first args) (nth args 1))))
+      (scheme-env-bind!
+        env
+        "="
+        (fn (args) (= (first args) (nth args 1))))
+      (scheme-env-bind! env "list" (fn (args) args))
+      (scheme-env-bind!
+        env
+        "cons"
+        (fn (args) (cons (first args) (nth args 1))))
+      (scheme-env-bind! env "car" (fn (args) (first (first args))))
+      (scheme-env-bind! env "cdr" (fn (args) (rest (first args))))
+      env)))
+
+;; ── if ───────────────────────────────────────────────────────────
+(scm-syn-test
+  "if: true"
+  (scm-syn-eval "(if #t 1 2)" (scm-syn-env))
+  1)
+(scm-syn-test
+  "if: false"
+  (scm-syn-eval "(if #f 1 2)" (scm-syn-env))
+  2)
+(scm-syn-test
+  "if: predicate"
+  (scm-syn-eval "(if (<= 1 2) 99 nope)" (scm-syn-env))
+  99)
+(scm-syn-test
+  "if: no else returns nil"
+  (scm-syn-eval "(if #f 99)" (scm-syn-env))
+  nil)
+(scm-syn-test
+  "if: truthy non-#f"
+  (scm-syn-eval "(if 0 'yes 'no)" (scm-syn-env))
+  "yes")
+
+;; ── define ───────────────────────────────────────────────────────
+(scm-syn-test
+  "define: bind value"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval "(define x 42)" env)
+    (scm-syn-eval "x" env))
+  42)
+(scm-syn-test
+  "define: function sugar"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all "(define (double n) (+ n n)) (double 21)" env))
+  42)
+(scm-syn-test
+  "define: redefine"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all "(define x 1) (define x 2) x" env))
+  2)
+
+;; ── set! ─────────────────────────────────────────────────────────
+(scm-syn-test
+  "set!: mutate"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all "(define x 1) (set! x 99) x" env))
+  99)
+(scm-syn-test
+  "set!: walks parent"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all "(define x 1) ((lambda () (set! x 100))) x" env))
+  100)
+(scm-syn-test
+  "set!: errors on unbound"
+  (guard
+    (e (true :raised))
+    (scm-syn-eval-all "(set! never-defined 1)" (scm-syn-env)))
+  :raised)
+
+;; ── begin ────────────────────────────────────────────────────────
+(scm-syn-test
+  "begin: empty returns nil"
+  (scm-syn-eval "(begin)" (scm-syn-env))
+  nil)
+(scm-syn-test
+  "begin: returns last"
+  (scm-syn-eval "(begin 1 2 3)" (scm-syn-env))
+  3)
+(scm-syn-test
+  "begin: side effects in order"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define x 0) (begin (set! x 1) (set! x 2) (set! x 3)) x"
+      env))
+  3)
+
+;; ── lambda ───────────────────────────────────────────────────────
+(scm-syn-test
+  "lambda: identity"
+  (scm-syn-eval "((lambda (x) x) 42)" (scm-syn-env))
+  42)
+(scm-syn-test
+  "lambda: arithmetic"
+  (scm-syn-eval "((lambda (x y) (+ x y)) 3 4)" (scm-syn-env))
+  7)
+(scm-syn-test
+  "lambda: zero args"
+  (scm-syn-eval "((lambda () 99))" (scm-syn-env))
+  99)
+(scm-syn-test
+  "lambda: multi-body"
+  (scm-syn-eval "((lambda (x) (define t (+ x 1)) (+ t t)) 5)" (scm-syn-env))
+  12)
+(scm-syn-test
+  "lambda: rest-arg as bare symbol"
+  (scm-syn-eval "((lambda args args) 1 2 3)" (scm-syn-env))
+  (list 1 2 3))
+
+;; ── closures ─────────────────────────────────────────────────────
+(scm-syn-test
+  "closure: captures binding"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define (make-adder n) (lambda (x) (+ x n))) ((make-adder 10) 5)"
+      env))
+  15)
+(scm-syn-test
+  "closure: counter via set!"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define (make-counter) (define n 0) (lambda () (set! n (+ n 1)) n)) (define c (make-counter)) (c) (c) (c)"
+      env))
+  3)
+(scm-syn-test
+  "closure: curried"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define curry+ (lambda (a) (lambda (b) (lambda (c) (+ a (+ b c)))))) (((curry+ 1) 2) 3)"
+      env))
+  6)
+
+;; ── recursion ────────────────────────────────────────────────────
+(scm-syn-test
+  "recursive: factorial 5"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define (fact n) (if (<= n 1) 1 (* n (fact (- n 1))))) (fact 5)"
+      env))
+  120)
+(scm-syn-test
+  "recursive: factorial 10"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define (fact n) (if (<= n 1) 1 (* n (fact (- n 1))))) (fact 10)"
+      env))
+  3628800)
+(scm-syn-test
+  "recursive: list length"
+  (let
+    ((env (scm-syn-env)))
+    (scm-syn-eval-all
+      "(define (len xs) (if (= 0 (- 0 0)) (if (= xs (quote ())) 0 (+ 1 (len (cdr xs)))) 0)) (len '(a b c d))"
+      env))
+  4)
+
+;; ── quote vs eval distinction ────────────────────────────────────
+(scm-syn-test
+  "quote: list literal"
+  (scm-syn-eval "'(1 2 3)" (scm-syn-env))
+  (list 1 2 3))
+(scm-syn-test
+  "quote: nested"
+  (scm-syn-eval "'(a (b c) d)" (scm-syn-env))
+  (list "a" (list "b" "c") "d"))
+(scm-syn-test
+  "quote: symbol vs evaluated"
+  (let ((env (scm-syn-env))) (scm-syn-eval-all "(define x 42) 'x" env))
+  "x")
+
+(define scm-syn-tests-run! (fn () {:total (+ scm-syn-pass scm-syn-fail) :passed scm-syn-pass :failed scm-syn-fail :fails scm-syn-fails}))