haskell: value-level pattern matcher (+31 tests, 281/281)

lib/haskell/match.sx

@@ -0,0 +1,190 @@
+;; Value-level pattern matching.
+;;
+;; Constructor values are tagged lists whose first element is the
+;; constructor name (a string). Tuples use the special tag "Tuple".
+;; Lists use the spine of `:` cons and `[]` nil.
+;;
+;;   Just 5         →  ("Just" 5)
+;;   Nothing        →  ("Nothing")
+;;   (1, 2)         →  ("Tuple" 1 2)
+;;   [1, 2]         →  (":" 1 (":" 2 ("[]")))
+;;   ()             →  ("()")
+;;
+;; Primitive values (numbers, strings, chars) are stored raw.
+;;
+;; The matcher takes a pattern AST node, a value, and an environment
+;; dict; it returns an extended dict on success, or `nil` on failure.
+
+;; ── Value builders ──────────────────────────────────────────
+(define
+  hk-mk-con
+  (fn
+    (cname args)
+    (let ((result (list cname)))
+      (for-each (fn (a) (append! result a)) args)
+      result)))
+
+(define
+  hk-mk-tuple
+  (fn
+    (items)
+    (let ((result (list "Tuple")))
+      (for-each (fn (x) (append! result x)) items)
+      result)))
+
+(define hk-mk-nil (fn () (list "[]")))
+
+(define hk-mk-cons (fn (h t) (list ":" h t)))
+
+(define
+  hk-mk-list
+  (fn
+    (items)
+    (cond
+      ((empty? items) (hk-mk-nil))
+      (:else
+        (hk-mk-cons (first items) (hk-mk-list (rest items)))))))
+
+;; ── Predicates / accessors on constructor values ───────────
+(define
+  hk-is-con-val?
+  (fn
+    (v)
+    (and
+      (list? v)
+      (not (empty? v))
+      (string? (first v)))))
+
+(define hk-val-con-name (fn (v) (first v)))
+
+(define hk-val-con-args (fn (v) (rest v)))
+
+;; ── The matcher ────────────────────────────────────────────
+(define
+  hk-match
+  (fn
+    (pat val env)
+    (cond
+      ((not (list? pat)) nil)
+      ((empty? pat) nil)
+      (:else
+        (let
+          ((tag (first pat)))
+          (cond
+            ((= tag "p-wild") env)
+            ((= tag "p-var") (assoc env (nth pat 1) val))
+            ((= tag "p-int")
+              (if
+                (and (number? val) (= val (nth pat 1)))
+                env
+                nil))
+            ((= tag "p-float")
+              (if
+                (and (number? val) (= val (nth pat 1)))
+                env
+                nil))
+            ((= tag "p-string")
+              (if
+                (and (string? val) (= val (nth pat 1)))
+                env
+                nil))
+            ((= tag "p-char")
+              (if
+                (and (string? val) (= val (nth pat 1)))
+                env
+                nil))
+            ((= tag "p-as")
+              (let
+                ((res (hk-match (nth pat 2) val env)))
+                (cond
+                  ((nil? res) nil)
+                  (:else (assoc res (nth pat 1) val)))))
+            ((= tag "p-lazy")
+              ;; Eager match for now; phase 3 wires laziness back in.
+              (hk-match (nth pat 1) val env))
+            ((= tag "p-con")
+              (let
+                ((pat-name (nth pat 1)) (pat-args (nth pat 2)))
+                (cond
+                  ((not (hk-is-con-val? val)) nil)
+                  ((not (= (hk-val-con-name val) pat-name)) nil)
+                  (:else
+                    (let
+                      ((val-args (hk-val-con-args val)))
+                      (cond
+                        ((not (= (len pat-args) (len val-args)))
+                          nil)
+                        (:else
+                          (hk-match-all pat-args val-args env))))))))
+            ((= tag "p-tuple")
+              (let
+                ((items (nth pat 1)))
+                (cond
+                  ((not (hk-is-con-val? val)) nil)
+                  ((not (= (hk-val-con-name val) "Tuple")) nil)
+                  ((not (= (len (hk-val-con-args val)) (len items)))
+                    nil)
+                  (:else
+                    (hk-match-all
+                      items
+                      (hk-val-con-args val)
+                      env)))))
+            ((= tag "p-list")
+              (hk-match-list-pat (nth pat 1) val env))
+            (:else nil)))))))
+
+(define
+  hk-match-all
+  (fn
+    (pats vals env)
+    (cond
+      ((empty? pats) env)
+      (:else
+        (let
+          ((res (hk-match (first pats) (first vals) env)))
+          (cond
+            ((nil? res) nil)
+            (:else
+              (hk-match-all (rest pats) (rest vals) res))))))))
+
+(define
+  hk-match-list-pat
+  (fn
+    (items val env)
+    (cond
+      ((empty? items)
+        (if
+          (and
+            (hk-is-con-val? val)
+            (= (hk-val-con-name val) "[]"))
+          env
+          nil))
+      (:else
+        (cond
+          ((not (hk-is-con-val? val)) nil)
+          ((not (= (hk-val-con-name val) ":")) nil)
+          (:else
+            (let
+              ((args (hk-val-con-args val)))
+              (let
+                ((h (first args)) (t (first (rest args))))
+                (let
+                  ((res (hk-match (first items) h env)))
+                  (cond
+                    ((nil? res) nil)
+                    (:else
+                      (hk-match-list-pat
+                        (rest items)
+                        t
+                        res))))))))))))
+
+;; ── Convenience: parse a pattern from source for tests ─────
+;; (Uses the parser's case-alt entry — `case _ of pat -> 0` —
+;; to extract a pattern AST.)
+(define
+  hk-parse-pat-source
+  (fn
+    (src)
+    (let
+      ((expr (hk-parse (str "case 0 of " src " -> 0"))))
+      (nth (nth (nth expr 2) 0) 1))))

lib/haskell/test.sh

@@ -50,6 +50,7 @@ for FILE in "${FILES[@]}"; do
 (load "lib/haskell/parser.sx")
 (load "lib/haskell/desugar.sx")
 (load "lib/haskell/runtime.sx")
+(load "lib/haskell/match.sx")
 (load "lib/haskell/testlib.sx")
 (epoch 2)
 (load "$FILE")
@@ -90,6 +91,7 @@ EPOCHS
 (load "lib/haskell/parser.sx")
 (load "lib/haskell/desugar.sx")
 (load "lib/haskell/runtime.sx")
+(load "lib/haskell/match.sx")
 (load "lib/haskell/testlib.sx")
 (epoch 2)
 (load "$FILE")

lib/haskell/tests/match.sx

@@ -0,0 +1,256 @@
+;; Pattern-matcher tests. The matcher takes (pat val env) and returns
+;; an extended env dict on success, or `nil` on failure. Constructor
+;; values are tagged lists (con-name first); tuples use the "Tuple"
+;; tag; lists use chained `:` cons with `[]` nil.
+
+;; ── Atomic patterns ──
+(hk-test
+  "wildcard always matches"
+  (hk-match (list :p-wild) 42 (dict))
+  (dict))
+
+(hk-test
+  "var binds value"
+  (hk-match (list :p-var "x") 42 (dict))
+  {:x 42})
+
+(hk-test
+  "var preserves prior env"
+  (hk-match (list :p-var "y") 7 {:x 1})
+  {:x 1 :y 7})
+
+(hk-test
+  "int literal matches equal"
+  (hk-match (list :p-int 5) 5 (dict))
+  (dict))
+
+(hk-test
+  "int literal fails on mismatch"
+  (hk-match (list :p-int 5) 6 (dict))
+  nil)
+
+(hk-test
+  "negative int literal matches"
+  (hk-match (list :p-int -3) -3 (dict))
+  (dict))
+
+(hk-test
+  "string literal matches"
+  (hk-match (list :p-string "hi") "hi" (dict))
+  (dict))
+
+(hk-test
+  "string literal fails"
+  (hk-match (list :p-string "hi") "bye" (dict))
+  nil)
+
+(hk-test
+  "char literal matches"
+  (hk-match (list :p-char "a") "a" (dict))
+  (dict))
+
+;; ── Constructor patterns ──
+(hk-test
+  "0-arity con matches"
+  (hk-match
+    (list :p-con "Nothing" (list))
+    (hk-mk-con "Nothing" (list))
+    (dict))
+  (dict))
+
+(hk-test
+  "1-arity con matches and binds"
+  (hk-match
+    (list :p-con "Just" (list (list :p-var "y")))
+    (hk-mk-con "Just" (list 9))
+    (dict))
+  {:y 9})
+
+(hk-test
+  "con name mismatch fails"
+  (hk-match
+    (list :p-con "Just" (list (list :p-var "y")))
+    (hk-mk-con "Nothing" (list))
+    (dict))
+  nil)
+
+(hk-test
+  "con arity mismatch fails"
+  (hk-match
+    (list :p-con "Pair" (list (list :p-var "a") (list :p-var "b")))
+    (hk-mk-con "Pair" (list 1))
+    (dict))
+  nil)
+
+(hk-test
+  "nested con: Just (Just x)"
+  (hk-match
+    (list
+      :p-con
+      "Just"
+      (list
+        (list
+          :p-con
+          "Just"
+          (list (list :p-var "x")))))
+    (hk-mk-con "Just" (list (hk-mk-con "Just" (list 42))))
+    (dict))
+  {:x 42})
+
+;; ── Tuple patterns ──
+(hk-test
+  "2-tuple matches and binds"
+  (hk-match
+    (list
+      :p-tuple
+      (list (list :p-var "a") (list :p-var "b")))
+    (hk-mk-tuple (list 10 20))
+    (dict))
+  {:a 10 :b 20})
+
+(hk-test
+  "tuple arity mismatch fails"
+  (hk-match
+    (list
+      :p-tuple
+      (list (list :p-var "a") (list :p-var "b")))
+    (hk-mk-tuple (list 10 20 30))
+    (dict))
+  nil)
+
+;; ── List patterns ──
+(hk-test
+  "[] pattern matches empty list"
+  (hk-match (list :p-list (list)) (hk-mk-nil) (dict))
+  (dict))
+
+(hk-test
+  "[] pattern fails on non-empty"
+  (hk-match (list :p-list (list)) (hk-mk-list (list 1)) (dict))
+  nil)
+
+(hk-test
+  "[a] pattern matches singleton"
+  (hk-match
+    (list :p-list (list (list :p-var "a")))
+    (hk-mk-list (list 7))
+    (dict))
+  {:a 7})
+
+(hk-test
+  "[a, b] pattern matches pair-list and binds"
+  (hk-match
+    (list
+      :p-list
+      (list (list :p-var "a") (list :p-var "b")))
+    (hk-mk-list (list 1 2))
+    (dict))
+  {:a 1 :b 2})
+
+(hk-test
+  "[a, b] fails on too-long list"
+  (hk-match
+    (list
+      :p-list
+      (list (list :p-var "a") (list :p-var "b")))
+    (hk-mk-list (list 1 2 3))
+    (dict))
+  nil)
+
+;; Cons-style infix pattern (which the parser produces as :p-con ":")
+(hk-test
+  "cons (h:t) on non-empty list"
+  (hk-match
+    (list
+      :p-con
+      ":"
+      (list (list :p-var "h") (list :p-var "t")))
+    (hk-mk-list (list 1 2 3))
+    (dict))
+  {:h 1 :t (list ":" 2 (list ":" 3 (list "[]")))})
+
+(hk-test
+  "cons fails on empty list"
+  (hk-match
+    (list
+      :p-con
+      ":"
+      (list (list :p-var "h") (list :p-var "t")))
+    (hk-mk-nil)
+    (dict))
+  nil)
+
+;; ── as patterns ──
+(hk-test
+  "as binds whole + sub-pattern"
+  (hk-match
+    (list
+      :p-as
+      "all"
+      (list :p-con "Just" (list (list :p-var "x"))))
+    (hk-mk-con "Just" (list 99))
+    (dict))
+  {:all (list "Just" 99) :x 99})
+
+(hk-test
+  "as on wildcard binds whole"
+  (hk-match
+    (list :p-as "v" (list :p-wild))
+    "anything"
+    (dict))
+  {:v "anything"})
+
+(hk-test
+  "as fails when sub-pattern fails"
+  (hk-match
+    (list
+      :p-as
+      "n"
+      (list :p-con "Just" (list (list :p-var "x"))))
+    (hk-mk-con "Nothing" (list))
+    (dict))
+  nil)
+
+;; ── lazy ~ pattern (eager equivalent for now) ──
+(hk-test
+  "lazy pattern eager-matches its inner"
+  (hk-match
+    (list :p-lazy (list :p-var "y"))
+    42
+    (dict))
+  {:y 42})
+
+;; ── Source-driven: parse a real Haskell pattern, match a value ──
+(hk-test
+  "parsed pattern: Just x against Just 5"
+  (hk-match
+    (hk-parse-pat-source "Just x")
+    (hk-mk-con "Just" (list 5))
+    (dict))
+  {:x 5})
+
+(hk-test
+  "parsed pattern: x : xs against [10, 20, 30]"
+  (hk-match
+    (hk-parse-pat-source "x : xs")
+    (hk-mk-list (list 10 20 30))
+    (dict))
+  {:x 10 :xs (list ":" 20 (list ":" 30 (list "[]")))})
+
+(hk-test
+  "parsed pattern: (a, b) against (1, 2)"
+  (hk-match
+    (hk-parse-pat-source "(a, b)")
+    (hk-mk-tuple (list 1 2))
+    (dict))
+  {:a 1 :b 2})
+
+(hk-test
+  "parsed pattern: n@(Just x) against Just 7"
+  (hk-match
+    (hk-parse-pat-source "n@(Just x)")
+    (hk-mk-con "Just" (list 7))
+    (dict))
+  {:n (list "Just" 7) :x 7})
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}

plans/haskell-on-sx.md

@@ -70,7 +70,7 @@ Key mappings:
 ### Phase 2 — desugar + eager-ish eval + ADTs (untyped)
 - [x] Desugar: guards → nested `if`s; `where` → `let`; list comp → `concatMap`-based; do-notation stays for now (desugared in phase 3)
 - [x] `data` declarations register constructors in runtime
-- [ ] Pattern match (tag-based, value-level): atoms, vars, wildcards, constructor patterns, `as` patterns, nested
+- [x] Pattern match (tag-based, value-level): atoms, vars, wildcards, constructor patterns, `as` patterns, nested
 - [ ] Evaluator (still strict internally — laziness in phase 3): `let`, `lambda`, application, `case`, literals, constructors
 - [ ] 30+ eval tests in `lib/haskell/tests/eval.sx`
 
@@ -114,6 +114,33 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-24** — Phase 2: value-level pattern matcher
+  (`lib/haskell/match.sx`). Core entry `hk-match pat val env` returns
+  an extended env dict on success or `nil` on failure (uses `assoc`
+  rather than `dict-set!` so failed branches never pollute the
+  caller's env). Constructor values are tagged lists with the
+  constructor name as the first element; tuples use the tag `"Tuple"`,
+  lists are chained `(":" h t)` cons cells terminated by `("[]")`.
+  Value builders `hk-mk-con` / `hk-mk-tuple` / `hk-mk-nil` /
+  `hk-mk-cons` / `hk-mk-list` keep tests readable. The matcher
+  handles every pattern node the parser emits:
+  - `:p-wild` (always matches), `:p-var` (binds), `:p-int` /
+    `:p-float` / `:p-string` / `:p-char` (literal equality)
+  - `:p-as` (sub-match then bind whole), `:p-lazy` (eager for now;
+    laziness wired in phase 3)
+  - `:p-con` with arity check + recursive arg matching, including
+    deeply nested patterns and infix `:` cons (uses the same
+    code path as named constructors)
+  - `:p-tuple` against `"Tuple"` values, `:p-list` against an
+    exact-length cons spine.
+  Helper `hk-parse-pat-source` lifts a real Haskell pattern out of
+  `case _ of <pat> -> 0`, letting tests drive against parser output.
+  31 new tests in `lib/haskell/tests/match.sx` cover atomic
+  patterns, success/failure for each con/tuple/list shape, nested
+  `Just (Just x)`, cons-vs-empty, `as` over con / wildcard /
+  failing-sub, `~` lazy, plus four parser-driven cases (`Just x`,
+  `x : xs`, `(a, b)`, `n@(Just x)`). 281/281 green.
+
 - **2026-04-24** — Phase 2: runtime constructor registry
   (`lib/haskell/runtime.sx`). A mutable dict `hk-constructors` keyed
   by constructor name, each entry carrying arity and owning type.