From 6ccef45ce439947bce21dd4f3c7716c542528a2d Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 24 Apr 2026 17:31:38 +0000
Subject: [PATCH] haskell: expression parser + precedence climbing (+42 tests,
 100/100)

---
 lib/haskell/parser.sx            | 511 +++++++++++++++++++++++++++++++
 lib/haskell/test.sh              |   2 +
 lib/haskell/tests/parser-expr.sx | 258 ++++++++++++++++
 plans/haskell-on-sx.md           |  28 +-
 4 files changed, 798 insertions(+), 1 deletion(-)
 create mode 100644 lib/haskell/parser.sx
 create mode 100644 lib/haskell/tests/parser-expr.sx

diff --git a/lib/haskell/parser.sx b/lib/haskell/parser.sx
new file mode 100644
index 00000000..ac17898a
--- /dev/null
+++ b/lib/haskell/parser.sx
@@ -0,0 +1,511 @@
+;; Haskell 98 expression parser.
+;;
+;; Input: the post-layout token list from (hk-layout (hk-tokenize src)).
+;; Output: an AST. Nodes are plain lists tagged by a keyword head
+;; (keywords evaluate to their string name, so `(list :var "x")` is
+;; indistinguishable from `(list "var" "x")` at runtime — this lets
+;; tests literally write `(list :var "x")` on both sides).
+;;
+;; Scope (this iteration — expressions only):
+;;   atoms         int/float/string/char/var/con, parens, tuple, list, range
+;;   application   left-associative, f x y z
+;;   prefix -      unary negation on an lexp
+;;   infix ops     precedence-climbing, full Haskell 98 default table
+;;   lambda        \x y -> body
+;;   if            if c then t else e
+;;   let           let { x = e ; y = e } in body   (uses layout braces)
+;;
+;; AST shapes:
+;;   (:int N)
+;;   (:float F)
+;;   (:string S)
+;;   (:char C)
+;;   (:var NAME)
+;;   (:con NAME)
+;;   (:app FN ARG)                 — binary, chain for multi-arg
+;;   (:op OP LHS RHS)              — binary infix
+;;   (:neg E)
+;;   (:tuple ITEMS)                — ITEMS is a list of AST nodes
+;;   (:list ITEMS)                 — enumerated list
+;;   (:range FROM TO)              — [from..to]
+;;   (:range-step FROM NEXT TO)    — [from,next..to]
+;;   (:if C T E)
+;;   (:lambda PARAMS BODY)         — PARAMS is list of varids
+;;   (:let BINDS BODY)             — BINDS is list of (:bind NAME EXPR)
+
+;; ── Operator precedence table (Haskell 98 defaults) ──────────────
+(define
+  hk-op-prec-table
+  (let
+    ((t (dict)))
+    (dict-set! t "!!" {:prec 9 :assoc "left"})
+    (dict-set! t "." {:prec 9 :assoc "right"})
+    (dict-set! t "^" {:prec 8 :assoc "right"})
+    (dict-set! t "^^" {:prec 8 :assoc "right"})
+    (dict-set! t "**" {:prec 8 :assoc "right"})
+    (dict-set! t "*" {:prec 7 :assoc "left"})
+    (dict-set! t "/" {:prec 7 :assoc "left"})
+    (dict-set! t "+" {:prec 6 :assoc "left"})
+    (dict-set! t "-" {:prec 6 :assoc "left"})
+    (dict-set! t ":" {:prec 5 :assoc "right"})
+    (dict-set! t "++" {:prec 5 :assoc "right"})
+    (dict-set! t "==" {:prec 4 :assoc "non"})
+    (dict-set! t "/=" {:prec 4 :assoc "non"})
+    (dict-set! t "<" {:prec 4 :assoc "non"})
+    (dict-set! t "<=" {:prec 4 :assoc "non"})
+    (dict-set! t ">" {:prec 4 :assoc "non"})
+    (dict-set! t ">=" {:prec 4 :assoc "non"})
+    (dict-set! t "&&" {:prec 3 :assoc "right"})
+    (dict-set! t "||" {:prec 2 :assoc "right"})
+    (dict-set! t ">>" {:prec 1 :assoc "left"})
+    (dict-set! t ">>=" {:prec 1 :assoc "left"})
+    (dict-set! t "=<<" {:prec 1 :assoc "right"})
+    (dict-set! t "$" {:prec 0 :assoc "right"})
+    (dict-set! t "$!" {:prec 0 :assoc "right"})
+    t))
+
+(define
+  hk-op-info
+  (fn
+    (op)
+    (if
+      (has-key? hk-op-prec-table op)
+      (get hk-op-prec-table op)
+      {:prec 9 :assoc "left"})))
+
+;; ── Atom-start predicate ─────────────────────────────────────────
+(define
+  hk-atom-start?
+  (fn
+    (tok)
+    (if
+      (nil? tok)
+      false
+      (let
+        ((ty (get tok "type")))
+        (or
+          (= ty "integer")
+          (= ty "float")
+          (= ty "string")
+          (= ty "char")
+          (= ty "varid")
+          (= ty "conid")
+          (= ty "qvarid")
+          (= ty "qconid")
+          (= ty "lparen")
+          (= ty "lbracket"))))))
+
+;; ── Main entry ───────────────────────────────────────────────────
+(define
+  hk-parse-expr
+  (fn
+    (tokens)
+    (let
+      ((toks tokens) (pos 0) (n (len tokens)))
+
+      (define hk-peek (fn () (if (< pos n) (nth toks pos) nil)))
+      (define
+        hk-peek-at
+        (fn
+          (offset)
+          (if (< (+ pos offset) n) (nth toks (+ pos offset)) nil)))
+      (define
+        hk-advance!
+        (fn () (let ((t (hk-peek))) (set! pos (+ pos 1)) t)))
+      (define
+        hk-peek-type
+        (fn () (let ((t (hk-peek))) (if (nil? t) "<eof>" (get t "type")))))
+      (define
+        hk-peek-value
+        (fn () (let ((t (hk-peek))) (if (nil? t) nil (get t "value")))))
+      (define
+        hk-match?
+        (fn
+          (ty v)
+          (let
+            ((t (hk-peek)))
+            (and
+              (not (nil? t))
+              (= (get t "type") ty)
+              (or (nil? v) (= (get t "value") v))))))
+      (define
+        hk-err
+        (fn
+          (msg)
+          (raise
+            (str
+              "parse error: "
+              msg
+              " (at "
+              (hk-peek-type)
+              (if (nil? (hk-peek-value)) "" (str " " (hk-peek-value)))
+              ")"))))
+      (define
+        hk-expect!
+        (fn
+          (ty v)
+          (if
+            (hk-match? ty v)
+            (hk-advance!)
+            (hk-err
+              (str "expected " ty (if (nil? v) "" (str " '" v "'")))))))
+
+      ;; ── Atoms ────────────────────────────────────────────────
+      (define
+        hk-parse-aexp
+        (fn
+          ()
+          (let
+            ((t (hk-peek)))
+            (cond
+              ((nil? t) (hk-err "unexpected end of input"))
+              ((= (get t "type") "integer")
+                (do (hk-advance!) (list :int (get t "value"))))
+              ((= (get t "type") "float")
+                (do (hk-advance!) (list :float (get t "value"))))
+              ((= (get t "type") "string")
+                (do (hk-advance!) (list :string (get t "value"))))
+              ((= (get t "type") "char")
+                (do (hk-advance!) (list :char (get t "value"))))
+              ((= (get t "type") "varid")
+                (do (hk-advance!) (list :var (get t "value"))))
+              ((= (get t "type") "conid")
+                (do (hk-advance!) (list :con (get t "value"))))
+              ((= (get t "type") "qvarid")
+                (do (hk-advance!) (list :var (get t "value"))))
+              ((= (get t "type") "qconid")
+                (do (hk-advance!) (list :con (get t "value"))))
+              ((= (get t "type") "lparen") (hk-parse-parens))
+              ((= (get t "type") "lbracket") (hk-parse-list-lit))
+              (:else (hk-err "unexpected token in expression"))))))
+
+      ;; ── Parens / tuple / unit ────────────────────────────────
+      (define
+        hk-parse-parens
+        (fn
+          ()
+          (hk-expect! "lparen" nil)
+          (cond
+            ((hk-match? "rparen" nil)
+              (do (hk-advance!) (list :con "()")))
+            (:else
+              (let
+                ((first-e (hk-parse-expr-inner))
+                  (items (list))
+                  (is-tuple false))
+                (append! items first-e)
+                (define
+                  hk-tup-loop
+                  (fn
+                    ()
+                    (when
+                      (hk-match? "comma" nil)
+                      (do
+                        (hk-advance!)
+                        (set! is-tuple true)
+                        (append! items (hk-parse-expr-inner))
+                        (hk-tup-loop)))))
+                (hk-tup-loop)
+                (hk-expect! "rparen" nil)
+                (if is-tuple (list :tuple items) first-e))))))
+
+      ;; ── List literal / range ─────────────────────────────────
+      (define
+        hk-parse-list-lit
+        (fn
+          ()
+          (hk-expect! "lbracket" nil)
+          (cond
+            ((hk-match? "rbracket" nil)
+              (do (hk-advance!) (list :list (list))))
+            (:else
+              (let
+                ((first-e (hk-parse-expr-inner)))
+                (cond
+                  ((hk-match? "reservedop" "..")
+                    (do
+                      (hk-advance!)
+                      (let
+                        ((end-e (hk-parse-expr-inner)))
+                        (hk-expect! "rbracket" nil)
+                        (list :range first-e end-e))))
+                  ((hk-match? "comma" nil)
+                    (do
+                      (hk-advance!)
+                      (let
+                        ((second-e (hk-parse-expr-inner)))
+                        (cond
+                          ((hk-match? "reservedop" "..")
+                            (do
+                              (hk-advance!)
+                              (let
+                                ((end-e (hk-parse-expr-inner)))
+                                (hk-expect! "rbracket" nil)
+                                (list
+                                  :range-step
+                                  first-e
+                                  second-e
+                                  end-e))))
+                          (:else
+                            (let
+                              ((items (list)))
+                              (append! items first-e)
+                              (append! items second-e)
+                              (define
+                                hk-list-loop
+                                (fn
+                                  ()
+                                  (when
+                                    (hk-match? "comma" nil)
+                                    (do
+                                      (hk-advance!)
+                                      (append!
+                                        items
+                                        (hk-parse-expr-inner))
+                                      (hk-list-loop)))))
+                              (hk-list-loop)
+                              (hk-expect! "rbracket" nil)
+                              (list :list items)))))))
+                  (:else
+                    (do
+                      (hk-expect! "rbracket" nil)
+                      (list :list (list first-e))))))))))
+
+      ;; ── Application: left-assoc aexp chain ───────────────────
+      (define
+        hk-parse-fexp
+        (fn
+          ()
+          (let
+            ((fn-e (hk-parse-aexp)))
+            (define
+              hk-app-loop
+              (fn
+                ()
+                (when
+                  (hk-atom-start? (hk-peek))
+                  (let
+                    ((arg (hk-parse-aexp)))
+                    (set! fn-e (list :app fn-e arg))
+                    (hk-app-loop)))))
+            (hk-app-loop)
+            fn-e)))
+
+      ;; ── Lambda: \ p1 p2 ... pn -> body ───────────────────────
+      (define
+        hk-parse-lambda
+        (fn
+          ()
+          (hk-expect! "reservedop" "\\")
+          (let
+            ((params (list)))
+            (when
+              (not (hk-match? "varid" nil))
+              (hk-err "lambda parameter must be a variable"))
+            (define
+              hk-lam-loop
+              (fn
+                ()
+                (when
+                  (hk-match? "varid" nil)
+                  (do
+                    (append! params (get (hk-advance!) "value"))
+                    (hk-lam-loop)))))
+            (hk-lam-loop)
+            (hk-expect! "reservedop" "->")
+            (list :lambda params (hk-parse-expr-inner)))))
+
+      ;; ── if-then-else ────────────────────────────────────────
+      (define
+        hk-parse-if
+        (fn
+          ()
+          (hk-expect! "reserved" "if")
+          (let
+            ((c (hk-parse-expr-inner)))
+            (hk-expect! "reserved" "then")
+            (let
+              ((th (hk-parse-expr-inner)))
+              (hk-expect! "reserved" "else")
+              (let
+                ((el (hk-parse-expr-inner)))
+                (list :if c th el))))))
+
+      ;; ── Let expression ──────────────────────────────────────
+      (define
+        hk-parse-let
+        (fn
+          ()
+          (hk-expect! "reserved" "let")
+          (let ((explicit (hk-match? "lbrace" nil)))
+            (if
+              explicit
+              (hk-advance!)
+              (hk-expect! "vlbrace" nil))
+            (let
+              ((binds (list)))
+              (when
+                (not
+                  (if
+                    explicit
+                    (hk-match? "rbrace" nil)
+                    (hk-match? "vrbrace" nil)))
+                (do
+                  (append! binds (hk-parse-bind))
+                  (define
+                    hk-let-loop
+                    (fn
+                      ()
+                      (when
+                        (or
+                          (hk-match? "semi" nil)
+                          (hk-match? "vsemi" nil))
+                        (do
+                          (hk-advance!)
+                          (when
+                            (not
+                              (if
+                                explicit
+                                (hk-match? "rbrace" nil)
+                                (hk-match? "vrbrace" nil)))
+                            (append! binds (hk-parse-bind)))
+                          (hk-let-loop)))))
+                  (hk-let-loop)))
+              (if
+                explicit
+                (hk-expect! "rbrace" nil)
+                (hk-expect! "vrbrace" nil))
+              (hk-expect! "reserved" "in")
+              (list :let binds (hk-parse-expr-inner))))))
+
+      (define
+        hk-parse-bind
+        (fn
+          ()
+          (when
+            (not (hk-match? "varid" nil))
+            (hk-err "binding must start with a variable"))
+          (let
+            ((name (get (hk-advance!) "value")))
+            (hk-expect! "reservedop" "=")
+            (list :bind name (hk-parse-expr-inner)))))
+
+      ;; ── lexp: lambda | if | let | fexp ──────────────────────
+      (define
+        hk-parse-lexp
+        (fn
+          ()
+          (cond
+            ((hk-match? "reservedop" "\\") (hk-parse-lambda))
+            ((hk-match? "reserved" "if") (hk-parse-if))
+            ((hk-match? "reserved" "let") (hk-parse-let))
+            (:else (hk-parse-fexp)))))
+
+      ;; ── Prefix: unary - ─────────────────────────────────────
+      (define
+        hk-parse-prefix
+        (fn
+          ()
+          (cond
+            ((and (hk-match? "varsym" "-"))
+              (do (hk-advance!) (list :neg (hk-parse-lexp))))
+            (:else (hk-parse-lexp)))))
+
+      ;; ── Infix: precedence climbing ──────────────────────────
+      (define
+        hk-is-infix-op?
+        (fn
+          (tok)
+          (if
+            (nil? tok)
+            false
+            (or
+              (= (get tok "type") "varsym")
+              (= (get tok "type") "consym")
+              (and
+                (= (get tok "type") "reservedop")
+                (= (get tok "value") ":"))
+              (= (get tok "type") "backtick")))))
+
+      (define
+        hk-consume-op!
+        (fn
+          ()
+          (let
+            ((t (hk-peek)))
+            (cond
+              ((= (get t "type") "backtick")
+                (do
+                  (hk-advance!)
+                  (let
+                    ((v (hk-expect! "varid" nil)))
+                    (hk-expect! "backtick" nil)
+                    (get v "value"))))
+              (:else (do (hk-advance!) (get t "value")))))))
+
+      (define
+        hk-parse-infix
+        (fn
+          (min-prec)
+          (let
+            ((left (hk-parse-prefix)))
+            (define
+              hk-inf-loop
+              (fn
+                ()
+                (when
+                  (hk-is-infix-op? (hk-peek))
+                  (let
+                    ((op-tok (hk-peek)))
+                    (let
+                      ((op-name
+                          (if
+                            (= (get op-tok "type") "backtick")
+                            (get (hk-peek-at 1) "value")
+                            (get op-tok "value"))))
+                      (let
+                        ((info (hk-op-info op-name)))
+                        (when
+                          (>= (get info "prec") min-prec)
+                          (do
+                            (hk-consume-op!)
+                            (let
+                              ((next-min
+                                  (cond
+                                    ((= (get info "assoc") "left")
+                                      (+ (get info "prec") 1))
+                                    ((= (get info "assoc") "right")
+                                      (get info "prec"))
+                                    (:else (+ (get info "prec") 1)))))
+                              (let
+                                ((right (hk-parse-infix next-min)))
+                                (set!
+                                  left
+                                  (list :op op-name left right))
+                                (hk-inf-loop)))))))))))
+            (hk-inf-loop)
+            left)))
+
+      (define hk-parse-expr-inner (fn () (hk-parse-infix 0)))
+
+      ;; ── Top-level: strip leading/trailing module-level braces ─
+      (let
+        ((start-brace
+            (or
+              (hk-match? "vlbrace" nil)
+              (hk-match? "lbrace" nil))))
+        (when start-brace (hk-advance!))
+        (let
+          ((result (hk-parse-expr-inner)))
+          (when start-brace
+            (when
+              (or
+                (hk-match? "vrbrace" nil)
+                (hk-match? "rbrace" nil))
+              (hk-advance!)))
+          result)))))
+
+;; ── Convenience: tokenize + layout + parse ───────────────────────
+(define
+  hk-parse
+  (fn (src) (hk-parse-expr (hk-layout (hk-tokenize src)))))
diff --git a/lib/haskell/test.sh b/lib/haskell/test.sh
index 2af17416..54a47fa4 100755
--- a/lib/haskell/test.sh
+++ b/lib/haskell/test.sh
@@ -47,6 +47,7 @@ for FILE in "${FILES[@]}"; do
 (epoch 1)
 (load "lib/haskell/tokenizer.sx")
 (load "lib/haskell/layout.sx")
+(load "lib/haskell/parser.sx")
 (load "lib/haskell/testlib.sx")
 (epoch 2)
 (load "$FILE")
@@ -84,6 +85,7 @@ EPOCHS
 (epoch 1)
 (load "lib/haskell/tokenizer.sx")
 (load "lib/haskell/layout.sx")
+(load "lib/haskell/parser.sx")
 (load "lib/haskell/testlib.sx")
 (epoch 2)
 (load "$FILE")
diff --git a/lib/haskell/tests/parser-expr.sx b/lib/haskell/tests/parser-expr.sx
new file mode 100644
index 00000000..e9d4d67b
--- /dev/null
+++ b/lib/haskell/tests/parser-expr.sx
@@ -0,0 +1,258 @@
+;; Haskell expression parser tests.
+;; hk-parse tokenises, runs layout, then parses. Output is an AST
+;; whose head is a keyword tag (evaluates to its string name).
+
+;; ── 1. Literals ──
+(hk-test "integer" (hk-parse "42") (list :int 42))
+(hk-test "float" (hk-parse "3.14") (list :float 3.14))
+(hk-test "string" (hk-parse "\"hi\"") (list :string "hi"))
+(hk-test "char" (hk-parse "'a'") (list :char "a"))
+
+;; ── 2. Variables and constructors ──
+(hk-test "varid" (hk-parse "foo") (list :var "foo"))
+(hk-test "conid" (hk-parse "Nothing") (list :con "Nothing"))
+(hk-test "qvarid" (hk-parse "Data.Map.lookup") (list :var "Data.Map.lookup"))
+(hk-test "qconid" (hk-parse "Data.Map") (list :con "Data.Map"))
+
+;; ── 3. Parens / unit / tuple ──
+(hk-test "parens strip" (hk-parse "(42)") (list :int 42))
+(hk-test "unit" (hk-parse "()") (list :con "()"))
+(hk-test
+  "2-tuple"
+  (hk-parse "(1, 2)")
+  (list :tuple (list (list :int 1) (list :int 2))))
+(hk-test
+  "3-tuple"
+  (hk-parse "(x, y, z)")
+  (list
+    :tuple
+    (list (list :var "x") (list :var "y") (list :var "z"))))
+
+;; ── 4. Lists ──
+(hk-test "empty list" (hk-parse "[]") (list :list (list)))
+(hk-test
+  "singleton list"
+  (hk-parse "[1]")
+  (list :list (list (list :int 1))))
+(hk-test
+  "list of ints"
+  (hk-parse "[1, 2, 3]")
+  (list
+    :list
+    (list (list :int 1) (list :int 2) (list :int 3))))
+(hk-test
+  "range"
+  (hk-parse "[1..10]")
+  (list :range (list :int 1) (list :int 10)))
+(hk-test
+  "range with step"
+  (hk-parse "[1, 3..10]")
+  (list
+    :range-step
+    (list :int 1)
+    (list :int 3)
+    (list :int 10)))
+
+;; ── 5. Application ──
+(hk-test
+  "one-arg app"
+  (hk-parse "f x")
+  (list :app (list :var "f") (list :var "x")))
+(hk-test
+  "multi-arg app is left-assoc"
+  (hk-parse "f x y z")
+  (list
+    :app
+    (list
+      :app
+      (list :app (list :var "f") (list :var "x"))
+      (list :var "y"))
+    (list :var "z")))
+(hk-test
+  "app with con"
+  (hk-parse "Just 5")
+  (list :app (list :con "Just") (list :int 5)))
+
+;; ── 6. Infix operators ──
+(hk-test
+  "simple +"
+  (hk-parse "1 + 2")
+  (list :op "+" (list :int 1) (list :int 2)))
+(hk-test
+  "precedence: * binds tighter than +"
+  (hk-parse "1 + 2 * 3")
+  (list
+    :op
+    "+"
+    (list :int 1)
+    (list :op "*" (list :int 2) (list :int 3))))
+(hk-test
+  "- is left-assoc"
+  (hk-parse "10 - 3 - 2")
+  (list
+    :op
+    "-"
+    (list :op "-" (list :int 10) (list :int 3))
+    (list :int 2)))
+(hk-test
+  ": is right-assoc"
+  (hk-parse "a : b : c")
+  (list
+    :op
+    ":"
+    (list :var "a")
+    (list :op ":" (list :var "b") (list :var "c"))))
+(hk-test
+  "app binds tighter than op"
+  (hk-parse "f x + g y")
+  (list
+    :op
+    "+"
+    (list :app (list :var "f") (list :var "x"))
+    (list :app (list :var "g") (list :var "y"))))
+(hk-test
+  "$ is lowest precedence, right-assoc"
+  (hk-parse "f $ g x")
+  (list
+    :op
+    "$"
+    (list :var "f")
+    (list :app (list :var "g") (list :var "x"))))
+
+;; ── 7. Backticks (varid-as-operator) ──
+(hk-test
+  "backtick operator"
+  (hk-parse "x `mod` 3")
+  (list :op "mod" (list :var "x") (list :int 3)))
+
+;; ── 8. Unary negation ──
+(hk-test
+  "unary -"
+  (hk-parse "- 5")
+  (list :neg (list :int 5)))
+(hk-test
+  "unary - on application"
+  (hk-parse "- f x")
+  (list :neg (list :app (list :var "f") (list :var "x"))))
+(hk-test
+  "- n + m → (- n) + m"
+  (hk-parse "- 1 + 2")
+  (list
+    :op
+    "+"
+    (list :neg (list :int 1))
+    (list :int 2)))
+
+;; ── 9. Lambda ──
+(hk-test
+  "lambda single param"
+  (hk-parse "\\x -> x")
+  (list :lambda (list "x") (list :var "x")))
+(hk-test
+  "lambda multi-param"
+  (hk-parse "\\x y -> x + y")
+  (list
+    :lambda
+    (list "x" "y")
+    (list :op "+" (list :var "x") (list :var "y"))))
+(hk-test
+  "lambda body is full expression"
+  (hk-parse "\\f -> f 1 + f 2")
+  (list
+    :lambda
+    (list "f")
+    (list
+      :op
+      "+"
+      (list :app (list :var "f") (list :int 1))
+      (list :app (list :var "f") (list :int 2)))))
+
+;; ── 10. if-then-else ──
+(hk-test
+  "if basic"
+  (hk-parse "if x then 1 else 2")
+  (list :if (list :var "x") (list :int 1) (list :int 2)))
+(hk-test
+  "if with infix cond"
+  (hk-parse "if x == 0 then y else z")
+  (list
+    :if
+    (list :op "==" (list :var "x") (list :int 0))
+    (list :var "y")
+    (list :var "z")))
+
+;; ── 11. let-in ──
+(hk-test
+  "let single binding"
+  (hk-parse "let x = 1 in x")
+  (list
+    :let
+    (list (list :bind "x" (list :int 1)))
+    (list :var "x")))
+(hk-test
+  "let two bindings (multi-line)"
+  (hk-parse "let x = 1\n    y = 2\nin x + y")
+  (list
+    :let
+    (list
+      (list :bind "x" (list :int 1))
+      (list :bind "y" (list :int 2)))
+    (list :op "+" (list :var "x") (list :var "y"))))
+(hk-test
+  "let with explicit braces"
+  (hk-parse "let { x = 1 ; y = 2 } in x + y")
+  (list
+    :let
+    (list
+      (list :bind "x" (list :int 1))
+      (list :bind "y" (list :int 2)))
+    (list :op "+" (list :var "x") (list :var "y"))))
+
+;; ── 12. Mixed / nesting ──
+(hk-test
+  "nested application"
+  (hk-parse "f (g x) y")
+  (list
+    :app
+    (list
+      :app
+      (list :var "f")
+      (list :app (list :var "g") (list :var "x")))
+    (list :var "y")))
+(hk-test
+  "lambda applied"
+  (hk-parse "(\\x -> x + 1) 5")
+  (list
+    :app
+    (list
+      :lambda
+      (list "x")
+      (list :op "+" (list :var "x") (list :int 1)))
+    (list :int 5)))
+(hk-test
+  "lambda + if"
+  (hk-parse "\\n -> if n == 0 then 1 else n")
+  (list
+    :lambda
+    (list "n")
+    (list
+      :if
+      (list :op "==" (list :var "n") (list :int 0))
+      (list :int 1)
+      (list :var "n"))))
+
+;; ── 13. Precedence corners ──
+(hk-test
+  ". is right-assoc (prec 9)"
+  (hk-parse "f . g . h")
+  (list
+    :op
+    "."
+    (list :var "f")
+    (list :op "." (list :var "g") (list :var "h"))))
+(hk-test
+  "== is non-associative (single use)"
+  (hk-parse "x == y")
+  (list :op "==" (list :var "x") (list :var "y")))
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}
diff --git a/plans/haskell-on-sx.md b/plans/haskell-on-sx.md
index b87ad274..ea0142b5 100644
--- a/plans/haskell-on-sx.md
+++ b/plans/haskell-on-sx.md
@@ -56,7 +56,14 @@ Key mappings:
 ### Phase 1 — tokenizer + parser + layout rule
 - [x] Tokenizer: reserved words, qualified names, operators, numbers (int, float, Rational later), chars/strings, comments (`--` and `{-` nested)
 - [x] Layout algorithm: turn indentation into virtual `{`, `;`, `}` tokens per Haskell 98 §10.3
-- [ ] Parser: modules, imports (stub), top-level decls, type sigs, function clauses with patterns + guards + where-clauses, expressions with operator precedence, lambdas, `let`, `if`, `case`, `do`, list comp, sections
+- Parser (split into sub-items — implement one per iteration):
+  - [x] Expressions: atoms, parens, tuples, lists, ranges, application, infix with full Haskell-98 precedence table, unary `-`, backtick operators, lambdas, `if`, `let`
+  - [ ] `case … of` and `do`-notation expressions
+  - [ ] Patterns (var, wildcard, literal, constructor, as, nested) — consumed by lambdas, let, case, and function clauses
+  - [ ] Top-level decls: function clauses, type signatures, `data`, `type`, `newtype`, fixity decls
+  - [ ] `where` clauses + guards
+  - [ ] Module header + imports (stub)
+  - [ ] List comprehensions + operator sections
 - [ ] AST design modelled on GHC's HsSyn at a surface level
 - [x] Unit tests in `lib/haskell/tests/parse.sx` (43 tokenizer tests, all green)
 
@@ -107,6 +114,25 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-24** — Phase 1: expression parser (`lib/haskell/parser.sx`, ~380 lines).
+  Pratt-style precedence climbing against a Haskell-98-default op table (24
+  operators across precedence 0–9, left/right/non assoc, default infixl 9 for
+  anything unlisted). Supports literals (int/float/string/char), varid/conid
+  (qualified variants folded into `:var` / `:con`), parens / unit / tuples,
+  list literals, ranges `[a..b]` and `[a,b..c]`, left-associative application,
+  unary `-`, backtick operators (`x \`mod\` 3`), lambdas, `if-then-else`, and
+  `let … in` consuming both virtual and explicit braces. AST uses keyword
+  tags (`:var`, `:op`, `:lambda`, `:let`, `:bind`, `:tuple`, `:range`,
+  `:range-step`, `:app`, `:neg`, `:if`, `:list`, `:int`, `:float`, `:string`,
+  `:char`, `:con`). The parser skips a leading `vlbrace` / `lbrace` so it can
+  be called on full post-layout output, and uses a `raise`-based error channel
+  with location-lite messages. 42 new tests in `lib/haskell/tests/parser-expr.sx`
+  cover literals, identifiers, parens/tuple/unit, list + range, app associativity,
+  operator precedence (mul over add, cons right-assoc, function-composition
+  right-assoc, `$` lowest), backtick ops, unary `-`, lambda multi-param,
+  `if` with infix condition, single- and multi-binding `let` (both implicit
+  and explicit braces), plus a few mixed nestings. 100/100 green.
+
 - **2026-04-24** — Phase 1: layout algorithm (`lib/haskell/layout.sx`, ~260 lines)
   implementing Haskell 98 §10.3. Two-pass design: a pre-pass augments the raw
   token stream with explicit `layout-open` / `layout-indent` markers (suppressing