utils: Add extract-symbols.

utils-tests.rkt

@@ -29,3 +29,7 @@
     (check-equal? (let ([ht #hash((a . 1) (b . 1))])
                     (eval-with1 ht '(+ b a 1)))
                   3)))
+
+(test-case "Analysis of quoted expressions"
+  (check-equal? (extract-symbols '(1 (2 3) x (y z 3)))
+                '(x y z)))

utils.rkt

@@ -7,7 +7,8 @@
 (require (for-syntax syntax/parse)
          (for-syntax racket/list))
 
-(provide auto-hash-ref/explicit auto-hash-ref/: eval-with eval-with1)
+(provide auto-hash-ref/explicit auto-hash-ref/: eval-with eval-with1
+         extract-symbols)
 
 ;;; ===================
 ;;; HashTable Injection
@@ -109,3 +110,21 @@
   (let ([vals (call-with-values (λ () (eval-with ht expr))
                                 (λ vals vals))])
     (car vals)))
+
+
+;;; ==============================
+;;; Analysis of quoted expressions
+;;; ==============================
+
+;;; Produces a list of symbols appearing in the quoted expression
+;;; passed in the first argument.
+(: extract-symbols (-> Any (Listof Symbol)))
+(define (extract-symbols form)
+  (cond
+    [(symbol? form)
+     (list (cast form Symbol))]
+    [(list? form)
+     (cast (flatten (for/list : (Listof (Listof Symbol))
+                        ([x (cast form (Listof Any))])
+                      (extract-symbols x))) (Listof Symbol))]
+    [else '()]))