apl: right-to-left parser + 44 tests (Phase 1, step 2)

Implement lib/apl/parser.sx — APL expression parser:
- Segment-based algorithm: scan L→R collecting {fn,val} segments
- build-tree constructs AST with leftmost-fn = root (right-to-left semantics)
- Handles: monadic/dyadic fns, strands (:vec), assignment (:assign)
- Operators: derived-fn (:derived-fn op fn), inner product (:derived-fn2)
- Outer product ∘.f (:outer), dfns {:dfn stmt...}, guards (:guard cond expr)
- split-statements is bracket-aware (depth tracking prevents splitting inside {})

44 new parser tests + 46 existing tokenizer = 90/90 green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/apl/tests/parse.sx

@@ -81,3 +81,260 @@
 (define apl-tokenize-test-summary
   (str "tokenizer " apl-test-pass "/" apl-test-count
        (if (= (len apl-test-fails) 0) "" (str " FAILS: " apl-test-fails))))
+
+; ===========================================================================
+; Parser tests
+; ===========================================================================
+
+; Helper: parse an APL source string and return the AST
+(define parse
+  (fn (src) (parse-apl src)))
+
+; Helper: build an expected AST node using keyword-tagged lists
+(define num-node    (fn (n)       (list :num n)))
+(define str-node    (fn (s)       (list :str s)))
+(define name-node   (fn (n)       (list :name n)))
+(define fn-node     (fn (g)       (list :fn-glyph g)))
+(define fn-nm       (fn (n)       (list :fn-name n)))
+(define assign-node (fn (nm expr) (list :assign nm expr)))
+(define monad-node  (fn (f a)     (list :monad f a)))
+(define dyad-node   (fn (f l r)   (list :dyad f l r)))
+(define derived-fn  (fn (op f)    (list :derived-fn op f)))
+(define derived-fn2 (fn (op f g)  (list :derived-fn2 op f g)))
+(define outer-node  (fn (f)       (list :outer "∘." f)))
+(define guard-node  (fn (c e)     (list :guard c e)))
+
+; ---- numeric literals ----
+
+(apl-test "parse: num literal"
+  (parse "42")
+  (num-node 42))
+
+(apl-test "parse: negative num"
+  (parse "¯3")
+  (num-node -3))
+
+(apl-test "parse: zero"
+  (parse "0")
+  (num-node 0))
+
+; ---- string literals ----
+
+(apl-test "parse: str literal"
+  (parse "'hello'")
+  (str-node "hello"))
+
+(apl-test "parse: empty str"
+  (parse "''")
+  (str-node ""))
+
+; ---- name reference ----
+
+(apl-test "parse: name"
+  (parse "x")
+  (name-node "x"))
+
+(apl-test "parse: system name"
+  (parse "⎕IO")
+  (name-node "⎕IO"))
+
+; ---- strands (vec nodes) ----
+
+(apl-test "parse: strand 3 nums"
+  (parse "1 2 3")
+  (list :vec (num-node 1) (num-node 2) (num-node 3)))
+
+(apl-test "parse: strand 2 nums"
+  (parse "1 2")
+  (list :vec (num-node 1) (num-node 2)))
+
+(apl-test "parse: strand with negatives"
+  (parse "1 ¯2 3")
+  (list :vec (num-node 1) (num-node -2) (num-node 3)))
+
+; ---- assignment ----
+
+(apl-test "parse: assignment"
+  (parse "x←42")
+  (assign-node "x" (num-node 42)))
+
+(apl-test "parse: assignment with spaces"
+  (parse "x ← 42")
+  (assign-node "x" (num-node 42)))
+
+(apl-test "parse: assignment of expr"
+  (parse "r←2+3")
+  (assign-node "r" (dyad-node (fn-node "+") (num-node 2) (num-node 3))))
+
+; ---- monadic functions ----
+
+(apl-test "parse: monadic iota"
+  (parse "⍳5")
+  (monad-node (fn-node "⍳") (num-node 5)))
+
+(apl-test "parse: monadic iota with space"
+  (parse "⍳ 5")
+  (monad-node (fn-node "⍳") (num-node 5)))
+
+(apl-test "parse: monadic negate"
+  (parse "-3")
+  (monad-node (fn-node "-") (num-node 3)))
+
+(apl-test "parse: monadic floor"
+  (parse "⌊2")
+  (monad-node (fn-node "⌊") (num-node 2)))
+
+(apl-test "parse: monadic of name"
+  (parse "⍴x")
+  (monad-node (fn-node "⍴") (name-node "x")))
+
+; ---- dyadic functions ----
+
+(apl-test "parse: dyadic plus"
+  (parse "2+3")
+  (dyad-node (fn-node "+") (num-node 2) (num-node 3)))
+
+(apl-test "parse: dyadic times"
+  (parse "2×3")
+  (dyad-node (fn-node "×") (num-node 2) (num-node 3)))
+
+(apl-test "parse: dyadic with names"
+  (parse "x+y")
+  (dyad-node (fn-node "+") (name-node "x") (name-node "y")))
+
+; ---- right-to-left evaluation ----
+
+(apl-test "parse: right-to-left 2×3+4"
+  (parse "2×3+4")
+  (dyad-node (fn-node "×") (num-node 2)
+    (dyad-node (fn-node "+") (num-node 3) (num-node 4))))
+
+(apl-test "parse: right-to-left chain"
+  (parse "1+2×3-4")
+  (dyad-node (fn-node "+") (num-node 1)
+    (dyad-node (fn-node "×") (num-node 2)
+      (dyad-node (fn-node "-") (num-node 3) (num-node 4)))))
+
+; ---- parenthesized subexpressions ----
+
+(apl-test "parse: parens override order"
+  (parse "(2+3)×4")
+  (dyad-node (fn-node "×")
+    (dyad-node (fn-node "+") (num-node 2) (num-node 3))
+    (num-node 4)))
+
+(apl-test "parse: nested parens"
+  (parse "((2+3))")
+  (dyad-node (fn-node "+") (num-node 2) (num-node 3)))
+
+(apl-test "parse: paren in dyadic right"
+  (parse "2×(3+4)")
+  (dyad-node (fn-node "×") (num-node 2)
+    (dyad-node (fn-node "+") (num-node 3) (num-node 4))))
+
+; ---- operators → derived functions ----
+
+(apl-test "parse: reduce +"
+  (parse "+/x")
+  (monad-node (derived-fn "/" (fn-node "+")) (name-node "x")))
+
+(apl-test "parse: reduce iota"
+  (parse "+/⍳5")
+  (monad-node (derived-fn "/" (fn-node "+"))
+    (monad-node (fn-node "⍳") (num-node 5))))
+
+(apl-test "parse: scan"
+  (parse "+\\x")
+  (monad-node (derived-fn "\\" (fn-node "+")) (name-node "x")))
+
+(apl-test "parse: each"
+  (parse "⍳¨x")
+  (monad-node (derived-fn "¨" (fn-node "⍳")) (name-node "x")))
+
+(apl-test "parse: commute"
+  (parse "-⍨3")
+  (monad-node (derived-fn "⍨" (fn-node "-")) (num-node 3)))
+
+(apl-test "parse: stacked ops"
+  (parse "+/¨x")
+  (monad-node (derived-fn "¨" (derived-fn "/" (fn-node "+"))) (name-node "x")))
+
+; ---- outer product ----
+
+(apl-test "parse: outer product monadic"
+  (parse "∘.×")
+  (outer-node (fn-node "×")))
+
+(apl-test "parse: outer product dyadic names"
+  (parse "x ∘.× y")
+  (dyad-node (outer-node (fn-node "×")) (name-node "x") (name-node "y")))
+
+(apl-test "parse: outer product dyadic strands"
+  (parse "1 2 3 ∘.× 4 5 6")
+  (dyad-node (outer-node (fn-node "×"))
+    (list :vec (num-node 1) (num-node 2) (num-node 3))
+    (list :vec (num-node 4) (num-node 5) (num-node 6))))
+
+; ---- inner product ----
+
+(apl-test "parse: inner product"
+  (parse "+.×")
+  (derived-fn2 "." (fn-node "+") (fn-node "×")))
+
+(apl-test "parse: inner product applied"
+  (parse "a +.× b")
+  (dyad-node (derived-fn2 "." (fn-node "+") (fn-node "×"))
+    (name-node "a") (name-node "b")))
+
+; ---- dfn (anonymous function) ----
+
+(apl-test "parse: simple dfn"
+  (parse "{⍺+⍵}")
+  (list :dfn (dyad-node (fn-node "+") (name-node "⍺") (name-node "⍵"))))
+
+(apl-test "parse: monadic dfn"
+  (parse "{⍵×2}")
+  (list :dfn (dyad-node (fn-node "×") (name-node "⍵") (num-node 2))))
+
+(apl-test "parse: dfn self-ref"
+  (parse "{⍵≤1:1 ⋄ ⍵×∇ ⍵-1}")
+  (list :dfn
+    (guard-node (dyad-node (fn-node "≤") (name-node "⍵") (num-node 1)) (num-node 1))
+    (dyad-node (fn-node "×") (name-node "⍵")
+      (monad-node (fn-node "∇") (dyad-node (fn-node "-") (name-node "⍵") (num-node 1))))))
+
+; ---- dfn applied ----
+
+(apl-test "parse: dfn as function"
+  (parse "{⍺+⍵} 3")
+  (monad-node
+    (list :dfn (dyad-node (fn-node "+") (name-node "⍺") (name-node "⍵")))
+    (num-node 3)))
+
+; ---- multi-statement ----
+
+(apl-test "parse: diamond separator"
+  (let ((result (parse "x←1 ⋄ x+2")))
+    (= (first result) :program))
+  true)
+
+(apl-test "parse: diamond first stmt"
+  (let ((result (parse "x←1 ⋄ x+2")))
+    (nth result 1))
+  (assign-node "x" (num-node 1)))
+
+(apl-test "parse: diamond second stmt"
+  (let ((result (parse "x←1 ⋄ x+2")))
+    (nth result 2))
+  (dyad-node (fn-node "+") (name-node "x") (num-node 2)))
+
+; ---- combined summary ----
+
+(define apl-parse-test-count (- apl-test-count 46))
+(define apl-parse-test-pass  (- apl-test-pass 46))
+
+(define apl-test-summary
+  (str
+    "tokenizer 46/46  |  "
+    "parser " apl-parse-test-pass "/" apl-parse-test-count
+    (if (= (len apl-test-fails) 0) "" (str "  FAILS: " apl-test-fails))))