add attrs-have-key? function

main.rkt

@@ -153,6 +153,11 @@
   (hash? . -> . txexpr-attrs?)
   (hash-map hash list))
 
+(define+provide+safe (attrs-have-key? x key)
+  ((or/c txexpr-attrs? txexpr?) can-be-txexpr-attr-key? . -> . boolean?)
+  (define attrs (if (txexpr-attrs? x) x (get-attrs x)))
+  (hash-has-key? (attrs->hash attrs) (->txexpr-attr-key key)))
+
 (define+provide+safe (attr-set tx key value)
   (txexpr? can-be-txexpr-attr-key? can-be-txexpr-attr-value? . -> . txexpr?)
   (define new-attrs 

scribblings/txexpr.scrbl

@@ -270,6 +270,19 @@ Convert @racket[_attrs] to an immutable hash, and back again.
 (hash->attrs '#hash((class . "red") (id . "top")))
 ]
 
+@defproc[
+(attrs-have-key?
+[attrs (or/c txexpr-attrs? txexpr?)]
+[key can-be-txexpr-attr-key?])
+boolean?]
+Returns @racket[#t] if the @racket[_attrs] contain a value for the given @racket[_key], @racket[#f] otherwise.
+
+@examples[#:eval my-eval
+(define tx '(div [[id "top"][class "red"]] "Hello" (p "World")))
+(attrs-have-key? tx 'id)
+(attrs-have-key? tx 'grackle)
+]
+
 @defproc[
 (attr-ref
 [tx txexpr?]

tests.rkt

@@ -88,6 +88,11 @@
 (check-equal? (attr-ref '(p ((foo "bar"))) 'foo) "bar")
 (check-equal? (attr-set '(p ((foo "bar"))) 'foo "fraw") '(p ((foo "fraw"))))
 
+(check-true (attrs-have-key? '(p ((color "red")(shape "circle"))) 'color))
+(check-true (attrs-have-key? '(p ((color "red")(shape "circle"))) "color"))
+(check-false (attrs-have-key? '((color "red")(shape "circle")) 'nonexistent))
+
+
 (check-equal? (merge-attrs 'foo "bar") '((foo "bar")))
 (check-equal? (merge-attrs '(foo "bar")) '((foo "bar")))
 (check-equal? (merge-attrs '((foo "bar"))) '((foo "bar")))