fix string

string.rkt

@@ -3,12 +3,12 @@
 
 
 (define+provide/contract (starts-with? str starter)
-  (string? string? . -> . coerce/boolean?)
+  (coerce/string? coerce/string? . -> . coerce/boolean?)
-  (define starter-pattern (regexp (format "^~a" starter)))
+  (and (<= (string-length starter) (string-length str)) 
-  (regexp-match starter-pattern str))
+       (equal? (substring str 0 (string-length starter)) starter)))
 
 
 (define+provide/contract (ends-with? str ender)
-  (string? string? . -> . coerce/boolean?)
+  (coerce/string? coerce/string? . -> . coerce/boolean?)
-  (define ender-pattern (regexp (format "~a$" ender)))
+  (and (<= (string-length ender) (string-length str)) 
-  (regexp-match ender-pattern str))
+       (equal? (substring str (- (string-length str) (string-length ender)) (string-length str)) ender)))

tests.rkt

@@ -83,6 +83,7 @@
 (check-true ("foobar" . starts-with? . "f"))
 (check-true ("foobar" . starts-with? . "foobar"))
 (check-false ("foobar" . starts-with? . "bar"))
+(check-false ("foobar" . starts-with? . "."))
 (check-true ("foobar" . ends-with? . "bar"))
 (check-true ("foobar" . ends-with? . "r"))
 (check-true ("foobar" . ends-with? . "foobar"))