add user-defined errors

config.json

@@ -815,6 +815,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "food-chain",
+        "name": "Food Chain",
+        "uuid": "ac1c0246-0fed-46e7-819a-0b013b2b4585",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "circular-buffer",
         "name": "Circular Buffer",

exercises/practice/circular-buffer/.meta/example.el

@@ -5,6 +5,10 @@
 ;;; Code:
 
 
+(define-error 'empty-buffer-error "buffer is empty")
+
+(define-error 'full-buffer-error "buffer is full")
+
 (defclass circular-buffer ()
   ((capacity :initarg :capacity :initform 10)
    (buf-modulus :initform 20)
@@ -13,6 +17,7 @@
    (back :initform 0))
   :documentation "Circular buffer class")
 
+
 (cl-defmethod initialize-instance :after ((buf circular-buffer) &rest _)
   (with-slots (capacity buf-modulus data) buf
     (setf buf-modulus (* 2 capacity)
@@ -45,15 +50,15 @@
 (cl-defmethod read-buff ((buf circular-buffer))
   (with-slots (front data capacity) buf
     (if (buf-empty-p buf)
-        (error "buffer is empty")
+        (signal 'empty-buffer-error nil)
       (let ((v (aref data (mod front capacity))))
         (advance-front buf)
         v))))
 
 (cl-defmethod write ((buf circular-buffer) v)
   (with-slots (back data capacity) buf
     (if (buf-full-p buf)
-        (error "buffer is full")
+        (signal 'full-buffer-error nil)
       (progn
         (aset data (mod back capacity) v)
         (advance-back buf)))))

exercises/practice/circular-buffer/circular-buffer-test.el

@@ -15,8 +15,7 @@
 
 (ert-deftest reading-empty-buffer-should-fail ()
   (let ((buf (create-test-buffer 1)))
-    (should (equal (should-error (read-buff buf))
-                   '(error . ("buffer is empty"))))))
+    (should-error (read-buff buf) :type 'empty-buffer-error)))
 
 
 (ert-deftest can-read-an-item-just-written ()
@@ -29,8 +28,7 @@
   (let ((buf (create-test-buffer 1)))
     (write buf 1)
     (should (equal (read-buff buf) 1))
-    (should (equal (should-error (read-buff buf))
-                   '(error . ("buffer is empty"))))))
+    (should-error (read-buff buf) :type 'empty-buffer-error)))
 
 
 (ert-deftest items-are-read-in-the-order-they-are-written ()
@@ -44,8 +42,7 @@
 (ert-deftest full-buffer-cant-be-written-to ()
   (let ((buf (create-test-buffer 1)))
     (write buf 1)
-    (should (equal (should-error (write buf 2))
-                   '(error . ("buffer is full"))))))
+    (should-error (write buf 2) :type 'full-buffer-error)))
 
 
 (ert-deftest a-read-frees-up-capacity-for-another-write ()
@@ -70,8 +67,7 @@
   (let ((buf (create-test-buffer 1)))
     (write buf 1)
     (clear buf)
-    (should (equal (should-error (read-buff buf))
-                   '(error . ("buffer is empty"))))))
+    (should-error (read-buff buf) :type 'empty-buffer-error)))
 
 
 (ert-deftest clear-frees-up-capacity-for-another-write ()
@@ -128,8 +124,7 @@
     (overwrite buf 4)
     (should (equal (read-buff buf) 3))
     (should (equal (read-buff buf) 4))
-    (should (equal (should-error (read-buff buf))
-                   '(error . ("buffer is empty"))))))
+    (should-error (read-buff buf) :type 'empty-buffer-error)))
 
 
 (provide 'circular-buffer-test)

exercises/practice/circular-buffer/circular-buffer.el

@@ -5,6 +5,12 @@
 ;;; Code:
 
 
+(define-error 'empty-buffer-error
+              (error "Delete this S-Expression and write your own implementation"))
+
+(define-error 'full-buffer-error
+              (error "Delete this S-Expression and write your own implementation"))
+
 (defclass circular-buffer ()
   (error "Delete this S-Expression and write your own implementation"))