Add a char-in flat contract

pkgs/racket-doc/scribblings/reference/contracts.scrbl

@@ -253,6 +253,12 @@ An alias for @racket[between/c].}
 Returns a flat contract that requires the input to be an exact integer
 between @racket[j] and @racket[k], inclusive.}
 
+@defproc[(char-in [a char?] [b char?]) flat-contract?]{
+
+Returns a flat contract that requires the input to be a character whose
+code point number is between the code point numbers of @racket[a] and
+@racket[b], inclusive.}
+
 
 @defthing[natural-number/c flat-contract?]{
 

pkgs/racket-test/tests/racket/contract/flat-contracts.rkt

@@ -41,6 +41,10 @@
   (test-flat-contract '(integer-in 0 10) 10 3/2)
   (test-flat-contract '(integer-in 0 10) 1 1.0)
   (test-flat-contract '(integer-in 1 1) 1 1.0)
+  (test-flat-contract '(char-in #\a #\z) #\a #\Z)
+  (test-flat-contract '(char-in #\a #\z) #\z #\A)
+  (test-flat-contract '(char-in #\a #\z) #\b "b")
+  (test-flat-contract '(char-in #\a #\a) #\a #\b)
   (test-flat-contract '(real-in 1 10) 3/2 20)
   (test-flat-contract '(string-len/c 3) "ab" "abc")
   (test-flat-contract 'natural-number/c 5 -1)

racket/collects/racket/contract/private/misc.rkt

@@ -17,6 +17,7 @@
          not/c
          =/c >=/c <=/c </c >/c between/c
          integer-in
+         char-in
          real-in
          natural-number/c
          string-len/c
@@ -550,6 +551,16 @@
       (and/c start exact?)
       (integer-in-ctc start end)))
 
+(define (char-in a b)
+  (check-two-args 'char-in a b char? char?)
+  (let* ([x (char->integer a)]
+         [y (char->integer b)]
+         [ctc (integer-in x y)])
+    (flat-named-contract
+     `(char-in ,a ,b)
+     (λ (c) (and (char? c)
+                 (ctc (char->integer c)))))))
+
 (define/final-prop (real-in start end)
   (check-two-args 'real-in start end real? real?)
   (make-real-in-s start end))