diff --git a/lib/haskell/infer.sx b/lib/haskell/infer.sx
new file mode 100644
index 00000000..55a4d09e
--- /dev/null
+++ b/lib/haskell/infer.sx
@@ -0,0 +1,486 @@
+;; infer.sx — Hindley-Milner Algorithm W for Haskell-on-SX (Phase 4).
+;;
+;; Types: TVar, TCon, TArr, TApp, TTuple, TScheme
+;; Substitution: apply, compose, restrict
+;; Unification (with occurs check)
+;; Instantiation + generalization (let-polymorphism)
+;; Algorithm W for: literals, var, con, lambda, app, let, if, op, tuple, list
+
+;; ─── Type constructors ────────────────────────────────────────────────────────
+
+(define hk-tvar (fn (n) (list "TVar" n)))
+(define hk-tcon (fn (s) (list "TCon" s)))
+(define hk-tarr (fn (a b) (list "TArr" a b)))
+(define hk-tapp (fn (a b) (list "TApp" a b)))
+(define hk-ttuple (fn (ts) (list "TTuple" ts)))
+(define hk-tscheme (fn (vs t) (list "TScheme" vs t)))
+
+(define hk-tvar? (fn (t) (and (list? t) (not (empty? t)) (= (first t) "TVar"))))
+(define hk-tcon? (fn (t) (and (list? t) (not (empty? t)) (= (first t) "TCon"))))
+(define hk-tarr? (fn (t) (and (list? t) (not (empty? t)) (= (first t) "TArr"))))
+(define hk-tapp? (fn (t) (and (list? t) (not (empty? t)) (= (first t) "TApp"))))
+(define hk-ttuple? (fn (t) (and (list? t) (not (empty? t)) (= (first t) "TTuple"))))
+(define hk-tscheme? (fn (t) (and (list? t) (not (empty? t)) (= (first t) "TScheme"))))
+
+(define hk-tvar-name (fn (t) (nth t 1)))
+(define hk-tcon-name (fn (t) (nth t 1)))
+(define hk-tarr-t1 (fn (t) (nth t 1)))
+(define hk-tarr-t2 (fn (t) (nth t 2)))
+(define hk-tapp-t1 (fn (t) (nth t 1)))
+(define hk-tapp-t2 (fn (t) (nth t 2)))
+(define hk-ttuple-ts (fn (t) (nth t 1)))
+(define hk-tscheme-vs (fn (t) (nth t 1)))
+(define hk-tscheme-type (fn (t) (nth t 2)))
+
+(define hk-t-int    (hk-tcon "Int"))
+(define hk-t-bool   (hk-tcon "Bool"))
+(define hk-t-string (hk-tcon "String"))
+(define hk-t-char   (hk-tcon "Char"))
+(define hk-t-float  (hk-tcon "Float"))
+(define hk-t-list (fn (t) (hk-tapp (hk-tcon "[]") t)))
+
+;; ─── Type formatter ──────────────────────────────────────────────────────────
+
+(define
+  hk-type->str
+  (fn
+    (t)
+    (cond
+      ((hk-tvar? t)   (hk-tvar-name t))
+      ((hk-tcon? t)   (hk-tcon-name t))
+      ((hk-tarr? t)
+       (let ((s1 (if (hk-tarr? (hk-tarr-t1 t))
+                   (str "(" (hk-type->str (hk-tarr-t1 t)) ")")
+                   (hk-type->str (hk-tarr-t1 t)))))
+         (str s1 " -> " (hk-type->str (hk-tarr-t2 t)))))
+      ((hk-tapp? t)
+       (let ((h (hk-tapp-t1 t)))
+         (cond
+           ((and (hk-tcon? h) (= (hk-tcon-name h) "[]"))
+            (str "[" (hk-type->str (hk-tapp-t2 t)) "]"))
+           (:else
+            (str "(" (hk-type->str h) " " (hk-type->str (hk-tapp-t2 t)) ")")))))
+      ((hk-ttuple? t)
+       (str "(" (join ", " (map hk-type->str (hk-ttuple-ts t))) ")"))
+      ((hk-tscheme? t)
+       (str "forall " (join " " (hk-tscheme-vs t)) ". " (hk-type->str (hk-tscheme-type t))))
+      (:else "<?>"))))
+
+;; ─── Fresh variable counter ───────────────────────────────────────────────────
+
+(define hk-fresh-ctr 0)
+(define hk-fresh (fn () (set! hk-fresh-ctr (+ hk-fresh-ctr 1)) (hk-tvar (str "t" hk-fresh-ctr))))
+(define hk-reset-fresh (fn () (set! hk-fresh-ctr 0)))
+
+;; ─── Utilities ───────────────────────────────────────────────────────────────
+
+(define hk-infer-member? (fn (x lst) (some (fn (y) (= x y)) lst)))
+
+(define
+  hk-nub
+  (fn (lst)
+    (reduce (fn (acc x) (if (hk-infer-member? x acc) acc (append acc (list x)))) (list) lst)))
+
+;; ─── Free type variables ──────────────────────────────────────────────────────
+
+(define
+  hk-ftv
+  (fn
+    (t)
+    (cond
+      ((hk-tvar? t)    (list (hk-tvar-name t)))
+      ((hk-tcon? t)    (list))
+      ((hk-tarr? t)    (append (hk-ftv (hk-tarr-t1 t)) (hk-ftv (hk-tarr-t2 t))))
+      ((hk-tapp? t)    (append (hk-ftv (hk-tapp-t1 t)) (hk-ftv (hk-tapp-t2 t))))
+      ((hk-ttuple? t)  (reduce append (list) (map hk-ftv (hk-ttuple-ts t))))
+      ((hk-tscheme? t)
+       (filter
+         (fn (v) (not (hk-infer-member? v (hk-tscheme-vs t))))
+         (hk-ftv (hk-tscheme-type t))))
+      (:else (list)))))
+
+(define
+  hk-ftv-env
+  (fn (env)
+    (reduce (fn (acc k) (append acc (hk-ftv (get env k)))) (list) (keys env))))
+
+;; ─── Substitution ─────────────────────────────────────────────────────────────
+
+(define hk-subst-empty (dict))
+
+(define
+  hk-subst-restrict
+  (fn
+    (s exclude)
+    (let ((r (dict)))
+      (for-each
+        (fn (k)
+          (when (not (hk-infer-member? k exclude))
+            (dict-set! r k (get s k))))
+        (keys s))
+      r)))
+
+(define
+  hk-subst-apply
+  (fn
+    (s t)
+    (cond
+      ((hk-tvar? t)
+       (let ((v (get s (hk-tvar-name t))))
+         (if (nil? v) t (hk-subst-apply s v))))
+      ((hk-tarr? t)
+       (hk-tarr (hk-subst-apply s (hk-tarr-t1 t))
+                (hk-subst-apply s (hk-tarr-t2 t))))
+      ((hk-tapp? t)
+       (hk-tapp (hk-subst-apply s (hk-tapp-t1 t))
+                (hk-subst-apply s (hk-tapp-t2 t))))
+      ((hk-ttuple? t)
+       (hk-ttuple (map (fn (u) (hk-subst-apply s u)) (hk-ttuple-ts t))))
+      ((hk-tscheme? t)
+       (let ((s2 (hk-subst-restrict s (hk-tscheme-vs t))))
+         (hk-tscheme (hk-tscheme-vs t)
+                     (hk-subst-apply s2 (hk-tscheme-type t)))))
+      (:else t))))
+
+(define
+  hk-subst-compose
+  (fn
+    (s2 s1)
+    (let ((r (hk-dict-copy s2)))
+      (for-each
+        (fn (k)
+          (when (nil? (get r k))
+            (dict-set! r k (hk-subst-apply s2 (get s1 k)))))
+        (keys s1))
+      r)))
+
+(define
+  hk-env-apply-subst
+  (fn
+    (s env)
+    (let ((r (dict)))
+      (for-each (fn (k) (dict-set! r k (hk-subst-apply s (get env k)))) (keys env))
+      r)))
+
+;; ─── Unification ─────────────────────────────────────────────────────────────
+
+(define
+  hk-bind-var
+  (fn
+    (v t)
+    (cond
+      ((and (hk-tvar? t) (= (hk-tvar-name t) v))
+       hk-subst-empty)
+      ((hk-infer-member? v (hk-ftv t))
+       (raise (str "Occurs check failed: " v " in " (hk-type->str t))))
+      (:else
+       (let ((s (dict)))
+         (dict-set! s v t)
+         s)))))
+
+(define
+  hk-zip-unify
+  (fn
+    (ts1 ts2 acc)
+    (if (or (empty? ts1) (empty? ts2))
+      acc
+      (let ((s (hk-unify (hk-subst-apply acc (first ts1))
+                         (hk-subst-apply acc (first ts2)))))
+        (hk-zip-unify (rest ts1) (rest ts2) (hk-subst-compose s acc))))))
+
+(define
+  hk-unify
+  (fn
+    (t1 t2)
+    (cond
+      ((and (hk-tvar? t1) (hk-tvar? t2) (= (hk-tvar-name t1) (hk-tvar-name t2)))
+       hk-subst-empty)
+      ((hk-tvar? t1) (hk-bind-var (hk-tvar-name t1) t2))
+      ((hk-tvar? t2) (hk-bind-var (hk-tvar-name t2) t1))
+      ((and (hk-tcon? t1) (hk-tcon? t2) (= (hk-tcon-name t1) (hk-tcon-name t2)))
+       hk-subst-empty)
+      ((and (hk-tarr? t1) (hk-tarr? t2))
+       (let ((s1 (hk-unify (hk-tarr-t1 t1) (hk-tarr-t1 t2))))
+         (let ((s2 (hk-unify (hk-subst-apply s1 (hk-tarr-t2 t1))
+                             (hk-subst-apply s1 (hk-tarr-t2 t2)))))
+           (hk-subst-compose s2 s1))))
+      ((and (hk-tapp? t1) (hk-tapp? t2))
+       (let ((s1 (hk-unify (hk-tapp-t1 t1) (hk-tapp-t1 t2))))
+         (let ((s2 (hk-unify (hk-subst-apply s1 (hk-tapp-t2 t1))
+                             (hk-subst-apply s1 (hk-tapp-t2 t2)))))
+           (hk-subst-compose s2 s1))))
+      ((and (hk-ttuple? t1) (hk-ttuple? t2)
+            (= (length (hk-ttuple-ts t1)) (length (hk-ttuple-ts t2))))
+       (hk-zip-unify (hk-ttuple-ts t1) (hk-ttuple-ts t2) hk-subst-empty))
+      (:else
+       (raise (str "Cannot unify " (hk-type->str t1) " with " (hk-type->str t2)))))))
+
+;; ─── Instantiation and generalization ────────────────────────────────────────
+
+(define
+  hk-instantiate
+  (fn
+    (t)
+    (if (not (hk-tscheme? t))
+      t
+      (let ((s (dict)))
+        (for-each (fn (v) (dict-set! s v (hk-fresh))) (hk-tscheme-vs t))
+        (hk-subst-apply s (hk-tscheme-type t))))))
+
+(define
+  hk-generalize
+  (fn
+    (env t)
+    (let ((free-t   (hk-nub (hk-ftv t)))
+          (free-env (hk-nub (hk-ftv-env env))))
+      (let ((bound (filter (fn (v) (not (hk-infer-member? v free-env))) free-t)))
+        (if (empty? bound)
+          t
+          (hk-tscheme bound t))))))
+
+;; ─── Pattern binding extraction ──────────────────────────────────────────────
+;; Returns a dict of name → type bindings introduced by matching pat against tv.
+
+(define
+  hk-w-pat
+  (fn
+    (pat tv)
+    (let ((tag (first pat)))
+      (cond
+        ((= tag "p-var")  (let ((d (dict))) (dict-set! d (nth pat 1) tv) d))
+        ((= tag "p-wild") (dict))
+        (:else            (dict))))))
+
+;; ─── Algorithm W ─────────────────────────────────────────────────────────────
+;; hk-w : env × expr → (list subst type)
+
+(define
+  hk-w-let
+  (fn
+    (env binds body)
+    ;; Infer types for each binding in order, generalising at each step.
+    (let
+      ((env2
+        (reduce
+          (fn
+            (cur-env b)
+            (let ((tag (first b)))
+              (cond
+                ;; Simple pattern binding: let x = expr
+                ((or (= tag "bind") (= tag "pat-bind"))
+                 (let ((pat  (nth b 1))
+                       (rhs  (nth b 2)))
+                   (let ((tv (hk-fresh)))
+                     (let ((r (hk-w cur-env rhs)))
+                       (let ((s1 (first r)) (t1 (nth r 1)))
+                         (let ((s2 (hk-unify (hk-subst-apply s1 tv) t1)))
+                           (let ((s (hk-subst-compose s2 s1)))
+                             (let ((t-gen (hk-generalize (hk-env-apply-subst s cur-env)
+                                                         (hk-subst-apply s t1))))
+                               (let ((bindings (hk-w-pat pat t-gen)))
+                                 (let ((r2 (hk-dict-copy cur-env)))
+                                   (for-each
+                                     (fn (k) (dict-set! r2 k (get bindings k)))
+                                     (keys bindings))
+                                   r2))))))))))
+                ;; Function clause: let f x y = expr
+                ((= tag "fun-clause")
+                 (let ((name (nth b 1))
+                       (pats (nth b 2))
+                       (body2 (nth b 3)))
+                   ;; Treat as: let name = lambda pats body2
+                   (let ((rhs (if (empty? pats)
+                                body2
+                                (list "lambda" pats body2))))
+                     (let ((tv (hk-fresh)))
+                       (let ((env-rec (hk-dict-copy cur-env)))
+                         (dict-set! env-rec name tv)
+                         (let ((r (hk-w env-rec rhs)))
+                           (let ((s1 (first r)) (t1 (nth r 1)))
+                             (let ((s2 (hk-unify (hk-subst-apply s1 tv) t1)))
+                               (let ((s (hk-subst-compose s2 s1)))
+                                 (let ((t-gen (hk-generalize
+                                                (hk-env-apply-subst s cur-env)
+                                                (hk-subst-apply s t1))))
+                                   (let ((r2 (hk-dict-copy cur-env)))
+                                     (dict-set! r2 name t-gen)
+                                     r2)))))))))))
+                (:else cur-env))))
+          env
+          binds)))
+      (hk-w env2 body))))
+
+(define
+  hk-w
+  (fn
+    (env expr)
+    (let ((tag (first expr)))
+      (cond
+        ;; Literals
+        ((= tag "int")    (list hk-subst-empty hk-t-int))
+        ((= tag "float")  (list hk-subst-empty hk-t-float))
+        ((= tag "string") (list hk-subst-empty hk-t-string))
+        ((= tag "char")   (list hk-subst-empty hk-t-char))
+
+        ;; Variable
+        ((= tag "var")
+         (let ((name (nth expr 1)))
+           (let ((scheme (get env name)))
+             (if (nil? scheme)
+               (raise (str "Unbound variable: " name))
+               (list hk-subst-empty (hk-instantiate scheme))))))
+
+        ;; Constructor (same lookup as var)
+        ((= tag "con")
+         (let ((name (nth expr 1)))
+           (let ((scheme (get env name)))
+             (if (nil? scheme)
+               (list hk-subst-empty (hk-fresh))
+               (list hk-subst-empty (hk-instantiate scheme))))))
+
+        ;; Unary negation
+        ((= tag "neg")
+         (let ((r (hk-w env (nth expr 1))))
+           (let ((s1 (first r)) (t1 (nth r 1)))
+             (let ((s2 (hk-unify t1 hk-t-int)))
+               (list (hk-subst-compose s2 s1) hk-t-int)))))
+
+        ;; Lambda: ("lambda" pats body)
+        ((= tag "lambda")
+         (let ((pats (nth expr 1))
+               (body (nth expr 2)))
+           (if (empty? pats)
+             (hk-w env body)
+             (let ((pat  (first pats))
+                   (rest (rest pats)))
+               (let ((tv (hk-fresh)))
+                 (let ((bindings (hk-w-pat pat tv)))
+                   (let ((env2 (hk-dict-copy env)))
+                     (for-each (fn (k) (dict-set! env2 k (get bindings k))) (keys bindings))
+                     (let ((inner (if (empty? rest)
+                                    body
+                                    (list "lambda" rest body))))
+                       (let ((r (hk-w env2 inner)))
+                         (let ((s1 (first r)) (t1 (nth r 1)))
+                           (list s1 (hk-tarr (hk-subst-apply s1 tv) t1))))))))))))
+
+        ;; Application: ("app" f x)
+        ((= tag "app")
+         (let ((tv (hk-fresh)))
+           (let ((r1 (hk-w env (nth expr 1))))
+             (let ((s1 (first r1)) (tf (nth r1 1)))
+               (let ((r2 (hk-w (hk-env-apply-subst s1 env) (nth expr 2))))
+                 (let ((s2 (first r2)) (tx (nth r2 1)))
+                   (let ((s3 (hk-unify (hk-subst-apply s2 tf) (hk-tarr tx tv))))
+                     (let ((s (hk-subst-compose s3 (hk-subst-compose s2 s1))))
+                       (list s (hk-subst-apply s3 tv))))))))))
+
+        ;; Let: ("let" binds body)
+        ((= tag "let")
+         (hk-w-let env (nth expr 1) (nth expr 2)))
+
+        ;; If: ("if" cond then else)
+        ((= tag "if")
+         (let ((r1 (hk-w env (nth expr 1))))
+           (let ((s1 (first r1)) (tc (nth r1 1)))
+             (let ((s2 (hk-unify tc hk-t-bool)))
+               (let ((s12 (hk-subst-compose s2 s1)))
+                 (let ((r2 (hk-w (hk-env-apply-subst s12 env) (nth expr 2))))
+                   (let ((s3 (first r2)) (tt (nth r2 1)))
+                     (let ((s123 (hk-subst-compose s3 s12)))
+                       (let ((r3 (hk-w (hk-env-apply-subst s123 env) (nth expr 3))))
+                         (let ((s4 (first r3)) (te (nth r3 1)))
+                           (let ((s5 (hk-unify (hk-subst-apply s4 tt) te)))
+                             (let ((s (hk-subst-compose s5 (hk-subst-compose s4 s123))))
+                               (list s (hk-subst-apply s5 te))))))))))))))
+
+        ;; Binary operator: ("op" op-name left right)
+        ;; Desugar to double application.
+        ((= tag "op")
+         (hk-w env
+               (list "app"
+                     (list "app" (list "var" (nth expr 1)) (nth expr 2))
+                     (nth expr 3))))
+
+        ;; Tuple: ("tuple" [e1 e2 ...])
+        ((= tag "tuple")
+         (let ((elems (nth expr 1)))
+           (let ((s-acc hk-subst-empty)
+                 (ts    (list)))
+             (for-each
+               (fn (e)
+                 (let ((r (hk-w (hk-env-apply-subst s-acc env) e)))
+                   (set! s-acc (hk-subst-compose (first r) s-acc))
+                   (set! ts    (append ts (list (nth r 1))))))
+               elems)
+             (list s-acc (hk-ttuple (map (fn (t) (hk-subst-apply s-acc t)) ts))))))
+
+        ;; List literal: ("list" [e1 e2 ...])
+        ((= tag "list")
+         (let ((elems (nth expr 1)))
+           (if (empty? elems)
+             (list hk-subst-empty (hk-t-list (hk-fresh)))
+             (let ((tv (hk-fresh)))
+               (let ((s-acc hk-subst-empty))
+                 (for-each
+                   (fn (e)
+                     (let ((r (hk-w (hk-env-apply-subst s-acc env) e)))
+                       (let ((s2 (first r)) (te (nth r 1)))
+                         (let ((s3 (hk-unify (hk-subst-apply s2 tv) te)))
+                           (set! s-acc (hk-subst-compose s3 (hk-subst-compose s2 s-acc)))))))
+                   elems)
+                 (list s-acc (hk-t-list (hk-subst-apply s-acc tv))))))))
+
+        (:else
+         (raise (str "hk-w: unhandled tag: " tag)))))))
+
+;; ─── Initial type environment ─────────────────────────────────────────────────
+;; Monomorphic numeric ops (no Num typeclass yet — upgraded in Phase 5).
+
+(define
+  hk-type-env0
+  (fn ()
+    (let ((env (dict)))
+      ;; Integer arithmetic
+      (for-each
+        (fn (op)
+          (dict-set! env op (hk-tarr hk-t-int (hk-tarr hk-t-int hk-t-int))))
+        (list "+" "-" "*" "div" "mod" "quot" "rem"))
+      ;; Integer comparison → Bool
+      (for-each
+        (fn (op)
+          (dict-set! env op (hk-tarr hk-t-int (hk-tarr hk-t-int hk-t-bool))))
+        (list "==" "/=" "<" "<=" ">" ">="))
+      ;; Boolean operators
+      (dict-set! env "&&" (hk-tarr hk-t-bool (hk-tarr hk-t-bool hk-t-bool)))
+      (dict-set! env "||" (hk-tarr hk-t-bool (hk-tarr hk-t-bool hk-t-bool)))
+      (dict-set! env "not" (hk-tarr hk-t-bool hk-t-bool))
+      ;; Constructors
+      (dict-set! env "True"  hk-t-bool)
+      (dict-set! env "False" hk-t-bool)
+      ;; Polymorphic list ops (using TScheme)
+      (let ((a (hk-tvar "a")))
+        (dict-set! env "head"  (hk-tscheme (list "a") (hk-tarr (hk-t-list a) a)))
+        (dict-set! env "tail"  (hk-tscheme (list "a") (hk-tarr (hk-t-list a) (hk-t-list a))))
+        (dict-set! env "null"  (hk-tscheme (list "a") (hk-tarr (hk-t-list a) hk-t-bool)))
+        (dict-set! env "length" (hk-tscheme (list "a") (hk-tarr (hk-t-list a) hk-t-int)))
+        (dict-set! env "reverse" (hk-tscheme (list "a") (hk-tarr (hk-t-list a) (hk-t-list a))))
+        (dict-set! env ":"
+          (hk-tscheme (list "a") (hk-tarr a (hk-tarr (hk-t-list a) (hk-t-list a))))))
+      ;; negate
+      (dict-set! env "negate" (hk-tarr hk-t-int hk-t-int))
+      (dict-set! env "abs"    (hk-tarr hk-t-int hk-t-int))
+      env)))
+
+;; ─── Convenience ─────────────────────────────────────────────────────────────
+;; hk-infer-type : Haskell expression source → inferred type string
+
+(define
+  hk-infer-type
+  (fn
+    (src)
+    (hk-reset-fresh)
+    (let ((ast (hk-core-expr src))
+          (env (hk-type-env0)))
+      (let ((r (hk-w env ast)))
+        (hk-type->str (hk-subst-apply (first r) (nth r 1)))))))
diff --git a/lib/haskell/test.sh b/lib/haskell/test.sh
index 0d394f2b..035d2bfc 100755
--- a/lib/haskell/test.sh
+++ b/lib/haskell/test.sh
@@ -52,6 +52,7 @@ for FILE in "${FILES[@]}"; do
 (load "lib/haskell/runtime.sx")
 (load "lib/haskell/match.sx")
 (load "lib/haskell/eval.sx")
+(load "lib/haskell/infer.sx")
 (load "lib/haskell/testlib.sx")
 (epoch 2)
 (load "$FILE")
@@ -94,6 +95,7 @@ EPOCHS
 (load "lib/haskell/runtime.sx")
 (load "lib/haskell/match.sx")
 (load "lib/haskell/eval.sx")
+(load "lib/haskell/infer.sx")
 (load "lib/haskell/testlib.sx")
 (epoch 2)
 (load "$FILE")
diff --git a/lib/haskell/tests/infer.sx b/lib/haskell/tests/infer.sx
new file mode 100644
index 00000000..6bd470d5
--- /dev/null
+++ b/lib/haskell/tests/infer.sx
@@ -0,0 +1,69 @@
+;; infer.sx tests — Algorithm W: literals, vars, lambdas, application, let,
+;; if, operators, tuples, lists, let-polymorphism.
+
+(define hk-t (fn (src expected)
+  (hk-test (str "infer: " src) (hk-infer-type src) expected)))
+
+;; ─── Literals ────────────────────────────────────────────────────────────────
+(hk-t "1"          "Int")
+(hk-t "3.14"       "Float")
+(hk-t "\"hello\""  "String")
+(hk-t "'x'"        "Char")
+(hk-t "True"       "Bool")
+(hk-t "False"      "Bool")
+
+;; ─── Arithmetic and boolean operators ────────────────────────────────────────
+(hk-t "1 + 2"      "Int")
+(hk-t "3 * 4"      "Int")
+(hk-t "10 - 3"     "Int")
+(hk-t "True && False" "Bool")
+(hk-t "True || False" "Bool")
+(hk-t "not True"   "Bool")
+(hk-t "1 == 1"     "Bool")
+(hk-t "1 < 2"      "Bool")
+
+;; ─── Lambda ───────────────────────────────────────────────────────────────────
+;; \x -> x   (identity)  should get t1 -> t1
+(hk-test "infer: identity lambda" (hk-infer-type "\\x -> x") "t1 -> t1")
+
+;; \x -> x + 1 : Int -> Int
+(hk-test "infer: lambda add" (hk-infer-type "\\x -> x + 1") "Int -> Int")
+
+;; \x -> not x : Bool -> Bool
+(hk-test "infer: lambda not" (hk-infer-type "\\x -> not x") "Bool -> Bool")
+
+;; \x y -> x + y : Int -> Int -> Int
+(hk-test "infer: two-arg lambda" (hk-infer-type "\\x -> \\y -> x + y") "Int -> Int -> Int")
+
+;; ─── Application ─────────────────────────────────────────────────────────────
+(hk-t "not True"    "Bool")
+(hk-t "negate 1"    "Int")
+
+;; ─── If-then-else ─────────────────────────────────────────────────────────────
+(hk-t "if True then 1 else 2"     "Int")
+(hk-t "if 1 == 2 then True else False" "Bool")
+
+;; ─── Let bindings ─────────────────────────────────────────────────────────────
+;; let x = 1 in x + 2
+(hk-t "let x = 1 in x + 2" "Int")
+
+;; let f x = x + 1 in f 5
+(hk-t "let f x = x + 1 in f 5" "Int")
+
+;; let-polymorphism: let id x = x in id 1
+(hk-t "let id x = x in id 1" "Int")
+
+;; ─── Tuples ───────────────────────────────────────────────────────────────────
+(hk-t "(1, True)"       "(Int, Bool)")
+(hk-t "(1, 2, 3)"       "(Int, Int, Int)")
+
+;; ─── Lists ───────────────────────────────────────────────────────────────────
+(hk-t "[1, 2, 3]"    "[Int]")
+(hk-t "[True, False]" "[Bool]")
+
+;; ─── Polymorphic list functions ───────────────────────────────────────────────
+(hk-t "length [1, 2, 3]"   "Int")
+(hk-t "null []"             "Bool")
+(hk-t "head [1, 2, 3]"     "Int")
+
+{: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 dbd39223..e5898264 100644
--- a/plans/haskell-on-sx.md
+++ b/plans/haskell-on-sx.md
@@ -91,7 +91,7 @@ Key mappings:
 - [x] Target: 5/5 classic programs passing
 
 ### Phase 4 — Hindley-Milner inference
-- [ ] Algorithm W: unification + type schemes + generalisation + instantiation
+- [x] Algorithm W: unification + type schemes + generalisation + instantiation
 - [ ] Report type errors with meaningful positions
 - [ ] Reject untypeable programs that phase 3 was accepting
 - [ ] Type-sig checking: user writes `f :: Int -> Int`; verify
@@ -114,6 +114,18 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-05** — Phase 4 Algorithm W (`lib/haskell/infer.sx`). Full
+  Hindley-Milner inference: type constructors (TVar/TCon/TArr/TApp/TTuple/TScheme),
+  substitution (apply/compose/restrict), occurs-check unification, instantiation,
+  generalisation (let-polymorphism). Algorithm W covers literals, var, con, lambda,
+  multi-param lambda, application, let (simple bind + fun-clause), if, binary ops
+  (desugared to double application), tuples, and list literals. Initial type
+  environment provides monomorphic arithmetic/comparison/boolean ops plus
+  polymorphic list functions (`head`/`tail`/`null`/`length`/`reverse`/`:`).
+  `hk-infer-type` is the public entry point. test.sh updated to load infer.sx.
+  32 new tests in `lib/haskell/tests/infer.sx` cover all node types + let-
+  polymorphism. 434/434 green.
+
 - **2026-04-25** — `conformance.sh` runner + `scoreboard.json` + `scoreboard.md`.
   Script runs each classic program's test suite, prints per-program pass/fail,
   and writes both files. `--check` mode skips writing for CI use.