fix: for loop must access expression via value stack

The compilation of for loop statements was accessing the expression
that evaluates what collection it will iterate over via a recursive
call to the compiler, and that only worked for other cases by luck.

In order to test this feature, I've added a new filter: `sort`, it
doesn't take any arguments for now but could be a bit more flexible.

Refs: #11

templatel-tests.el

@@ -762,12 +762,29 @@ base-end")))))
      '())
     "b")))
 
-(ert-deftest render-template-forloop ()
+(ert-deftest render-template-for-loop ()
   (should (equal
+           "One Two Three "
            (templatel-render-string
             "{% for name in names %}{{ name }} {% endfor %}"
-            '(("names" . ("One" "Two" "Three"))))
-           "One Two Three ")))
+            '(("names" . ("One" "Two" "Three")))))))
+
+(ert-deftest render-template-for-loop-with-filter ()
+  (should (equal
+           "1 2 3 4 5 6 7 8 9 "
+           (templatel-render-string
+            "{% for i in a|sort %}{{ i }} {% endfor %}"
+            '(("a" . (3 6 1 4 9 5 8 2 7)))))))
+
+(ert-deftest render-if-with-filter ()
+  (should
+   (equal
+    "test"
+    (templatel-render-string "{% if list | first %}test{% endif %}" '(("list" . (1))))))
+  (should
+   (equal
+    "test"
+    (templatel-render-string "{% if 1.6 | round > 1.4 %}test{% endif %}" '()))))
 
 (ert-deftest render-if-else-elif-no-else ()
   (should

templatel.el

@@ -1238,6 +1238,7 @@ finished."
                           ("min"         . templatel-filters-min)
                           ("round"       . templatel-filters-round)
                           ("safe"        . templatel-filters-safe)
+                          ("sort"        . templatel-filters-sort)
                           ("sum"         . templatel-filters-sum)
                           ("title"       . templatel-filters-title)
                           ("upper"       . templatel-filters-upper)
@@ -1598,7 +1599,9 @@ Otherwise its HTML entities are escaped."
   "Compile for statement off TREE."
   (let ((id (cdar tree)))
     `(let ((subenv '((,id . nil)))
-           (iterable ,(templatel--compiler-run (cadr tree))))
+           (iterable (progn
+                       ,(templatel--compiler-run (cadr tree))
+                       (pop rt/valstk))))
        (push subenv rt/varstk)
        (mapc
         (lambda(id)
@@ -1808,6 +1811,9 @@ Hi <b>you</b>!
 #+END_SRC"
   (templatel-mark-safe s))
 
+(defun templatel-filters-sort (s)
+  "Return S sorted."
+  (sort s #'<))