utils: Add hash-pred and use it in variable-mapping?

utils.rkt

@@ -12,7 +12,7 @@
  (contract-out [eval-with (-> variable-mapping? any/c any)]
                [extract-symbols (-> any/c list?)])
  ;; Functions without contracts
- variable-mapping?
+ variable-mapping? hash-pred
  ;; Syntax
  auto-hash-ref/explicit auto-hash-ref/:)
 
@@ -24,12 +24,22 @@
 ;;; usage of hash tables mapping symbols to values.  The goal is
 ;;; essentially to avoid having to write explicit hash-ref calls.
 
-;;; A variable mapping is a hash table mapping symbols to values.
-(define (variable-mapping? dict)
+;;; Checks whether dict is a hash table and whether its keys satisfy
+;;; key-pred and its values satisfy val-pred.  If one of the
+;;; predicates is #f, it is not checked.  For example, (hash-pred
+;;; dict) is equivalent to (hash dict).
+(define (hash-pred dict #:key-pred [key-pred #f] #:val-pred [val-pred #f])
   (and (hash? dict)
        (if (not (hash-empty? dict))
-           (symbol? (car (hash-keys dict)))
-           #t)))
+           (if key-pred
+               (key-pred (car (hash-keys dict)))
+               #t)
+           (if val-pred
+               (val-pred (car (hash-values dict)))
+               #t))))
+
+;;; A variable mapping is a hash table mapping symbols to values.
+(define (variable-mapping? dict) (hash-pred dict #:key-pred symbol?))
 
 ;;; Given a (HashTable Symbol a) and a sequence of symbols, binds
 ;;; these symbols to the values they are associated to in the hash