use assertions for path validation

palps.rkt

@@ -9,10 +9,14 @@
 ;; directory of the current user of the program
 (define call-dir (make-parameter (string->path "/nobody")))
 
+(struct exn:assert exn ()
+  #:extra-constructor-name make-exn:assert
+  #:transparent)
+
 ;; raised by assert-confined-to-call-dir when attemping to traverse
 ;; outside of (call-dir)
-(struct exn:fail:contract:illegal-path exn:fail:contract ()
-  #:extra-constructor-name make-exn:fail:contract:illegal-path
+(struct exn:assert:illegal-path exn:assert ()
+  #:extra-constructor-name make-exn:assert:illegal-path
   #:transparent)
 
 ;; check if path, once resolved, is a child of base
@@ -24,9 +28,50 @@
 ;; asserts that path is within (call-dir), and raises
 ;; exn:fail:contract:illegal-path otherwise.
 (define (assert-confined-to-call-dir path)
+  (when (string? path)
+    (set! path (string->path path)))
+
   (when (not (confined-path? (call-dir) path))
-    (raise (make-exn:fail:contract:illegal-path
-            "assert-confined-to-call-dir: traversed outside of root"
+    (raise (make-exn:assert:illegal-path
+            (format (string-append "assert-confined-to-call-dir: traversed outside of root~n"
+                                   "  path: ~a")
+                    (path->string path))
+            (current-continuation-marks)))))
+
+(struct exn:assert:no-such-directory exn:assert ()
+  #:extra-constructor-name make-exn:assert:no-such-directory
+  #:transparent)
+
+(define (assert-directory-exists path)
+  (when (string? path)
+    (set! path (string->path path)))
+
+  ;; path-only returns #f when path is a single element. this is taken
+  ;; to mean that path is in the root (current-directory), which
+  ;; necessarily exists. this is structured to short circuit such that
+  ;; (directory-exists? #f) is never called (which would raise an
+  ;; exception).
+  (when (not (or (not (path-only path))
+                 (directory-exists? (path-only path))))
+    (raise (make-exn:assert:no-such-directory
+            (format (string-append "assert-directory-exists: directory does not exist~n"
+                                   "  path: ~a")
+                    (path->string (find-relative-path (call-dir) path)))
+            (current-continuation-marks)))))
+
+(struct exn:assert:no-such-file exn:assert ()
+  #:extra-constructor-name make-exn:assert:no-such-file
+  #:transparent)
+
+(define (assert-file-exists path)
+  (when (string? path)
+    (set! path (string->path path)))
+
+  (when (not (file-exists? path))
+    (raise (make-exn:assert:no-such-file
+            (format (string-append "assert-file-exists: file does not exist~n"
+                                   "  path: ~a")
+                    (path->string (find-relative-path (call-dir) path)))
             (current-continuation-marks)))))
 
 (define (call-valid? call)
@@ -122,9 +167,9 @@
 
 (define (session)
   (with-handlers
-    ([exn:fail:contract:illegal-path?
+    ([exn:assert?
       (λ (exn)
-        (displayln "cannot traverse higher than root"))])
+        (displayln (exn-message exn)))])
 
     (display "palps > ")
     (flush-output)
@@ -138,6 +183,7 @@
 
       [(list _ "ed" name)
        (assert-confined-to-call-dir name)
+       (assert-directory-exists name)
 
        (let ([path (if (file-exists? name)
                        (ed (our-make-temporary-file name))
@@ -146,6 +192,7 @@
 
       [(list _ "rm" name)
        (assert-confined-to-call-dir name)
+       (assert-file-exists name)
 
        (if (file-exists? name)
            (delete-file name)