typed: add missing file

racket/typed/syndicate/sugar.rkt

@@ -0,0 +1,26 @@
+#lang turnstile
+
+(provide Observe★)
+
+(require "core-types.rkt")
+(require turnstile/typedefs)
+
+(define-syntax (Observe★ stx)
+  (define star (type-eval #'★/t))
+  (syntax-parse stx
+    [(_ TyCons:id)
+     #:do [(define arity? (get-type-arity #'TyCons))]
+     #:when arity?
+     (mk-Observe- (list (reassemble-type #'TyCons (make-list (arity-min arity?) star))))]
+    [(_ (~Any/new TyCons τ ...))
+     #:when (reassemblable? #'TyCons)
+     (mk-Observe- (list (reassemble-type #'TyCons (stx-map (lambda (_) star) #'(τ ...)))))]
+    [_
+     (raise-syntax-error #f "Not a type that can automatically be subscribed to" stx)]))
+
+(begin-for-syntax
+  ;; Arity -> Nat
+  (define (arity-min a)
+    (match a
+      [(arity-eq n) n]
+      [(arity-ge n) n])))