Stricter defexamples checking

* dev/dash-defs.el (dash--example-to-test): Expand docstring.
(defexamples): Ensure each triple is well-formed.
* dev/examples.el (-intersection): Fix badly parenthesized cases.

dev/dash-defs.el

@@ -53,7 +53,8 @@ differences in implementation between systems.  Used in place of
          dash--epsilon)))
 
 (defun dash--example-to-test (example)
-  "Return an ERT assertion form based on EXAMPLE."
+  "Return an ERT assertion form based on EXAMPLE.
+Signal an error if EXAMPLE is malformed."
   (pcase example
     (`(,actual => ,expected) `(should (equal ,actual ,expected)))
     (`(,actual ~> ,expected) `(should (approx= ,actual ,expected)))
@@ -75,13 +76,14 @@ See `dash--groups'."
   "Define a set of EXAMPLES and corresponding ERT tests for FN.
 See `dash--groups'."
   (declare (indent defun))
-  (setq examples (-partition 3 examples))
-  `(progn
-     (push (cons ',fn ',examples) dash--groups)
-     (ert-deftest ,fn ()
-       ;; Emacs 28.1 complains about an empty `let' body if the test
-       ;; body is empty.
-       ,@(or (mapcar #'dash--example-to-test examples) '(nil)))))
+  (let (triples tests)
+    (while (let ((triple (-take 3 examples)))
+             (push (dash--example-to-test triple) tests)
+             (push triple triples)
+             (setq examples (nthcdr 3 examples))))
+    `(progn
+       (push (cons ',fn ',(nreverse triples)) dash--groups)
+       (ert-deftest ,fn () ,@(nreverse tests)))))
 
 ;; Added in Emacs 25.1.
 (defvar text-quoting-style)

dev/examples.el

@@ -1356,8 +1356,8 @@ related predicates."
     => '(a b)
     (let ((dash--short-list-length 0)) (-intersection '(a b) '(b a a)))
     => '(a b)
-    (let ((-compare-fn #'string=)) (-intersection '(a) '("a")) => '(a))
-    (let ((-compare-fn #'string=)) (-intersection '("a") '(a)) => '("a")))
+    (let ((-compare-fn #'string=)) (-intersection '(a) '("a"))) => '(a)
+    (let ((-compare-fn #'string=)) (-intersection '("a") '(a))) => '("a"))
 
   (defexamples -powerset
     (-powerset '()) => '(())