Don't treat [else…] or ("else"…) as else clauses

NEWS

@@ -44,6 +44,7 @@ Changes from 0.13.0
      instead of silently ignoring them
    * Multiple expressions are now allowed in the else clause of
      a for loop
+   * `else` clauses in `for` and `while` are recognized more reliably
    * Argument destructuring no longer interferes with function docstrings.
 
    [ Misc. Improvements ]

hy/compiler.py

@@ -376,6 +376,14 @@ def is_unpack(kind, x):
             and x[0] == "unpack_" + kind)
 
 
+def ends_with_else(expr):
+    return (expr and
+            isinstance(expr[-1], HyExpression) and
+            expr[-1] and
+            isinstance(expr[-1][0], HySymbol) and
+            expr[-1][0] == HySymbol("else"))
+
+
 class HyASTCompiler(object):
 
     def __init__(self, module_name):
@@ -1825,7 +1833,7 @@ class HyASTCompiler(object):
 
         orel = Result()
         # (for* [] body (else …))
-        if expression and expression[-1][0] == HySymbol("else"):
+        if ends_with_else(expression):
             else_expr = expression.pop()
             for else_body in else_expr[1:]:
                 orel += self.compile(else_body)
@@ -1854,7 +1862,7 @@ class HyASTCompiler(object):
 
         orel = Result()
         # (while cond body (else …))
-        if expr and expr[-1][0] == HySymbol("else"):
+        if ends_with_else(expr):
             else_expr = expr.pop()
             for else_body in else_expr[1:]:
                 orel += self.compile(else_body)

tests/native_tests/language.hy

@@ -221,6 +221,22 @@
       (+= count 10)))
   (assert (= count 161))
 
+  ; don't be fooled by constructs that look like else
+  (setv s "")
+  (setv (get (globals) "else") True)
+  (for [x "abcde"]
+    (+= s x)
+    [else (+= s "_")])
+  (assert (= s "a_b_c_d_e_"))
+
+  (setv s "")
+  (setv (get (globals) "else") True)
+  (with [(pytest.raises TypeError)]
+    (for [x "abcde"]
+      (+= s x)
+      ("else" (+= s "z"))))
+  (assert (= s "az"))
+
   (assert (= (list ((fn [] (for [x [[1] [2 3]] y x] (yield y)))))
              (list-comp y [x [[1] [2 3]] y x])))
   (assert (= (list ((fn [] (for [x [[1] [2 3]] y x z (range 5)] (yield z)))))
@@ -308,7 +324,26 @@
   (while True
     (break)
     (else (setv myvariable 53)))
-  (assert (= myvariable 26)))
+  (assert (= myvariable 26))
+
+  ; don't be fooled by constructs that look like else clauses
+  (setv x 2)
+  (setv a [])
+  (setv (get (globals) "else") True)
+  (while x
+    (.append a x)
+    (-= x 1)
+    [else (.append a "e")])
+  (assert (= a [2 "e" 1 "e"]))
+
+  (setv x 2)
+  (setv a [])
+  (with [(pytest.raises TypeError)]
+    (while x
+      (.append a x)
+      (-= x 1)
+      ("else" (.append a "e"))))
+  (assert (= a [2 "e"])))
 
 
 (defn test-branching []