Allow else after a while loop

NEWS

@@ -18,6 +18,7 @@ Changes from 0.13.0
    * new `comment` macro
    * support EDN `#_` syntax to discard the next term
    * `return` has been implemented as a special form
+   * `while` loops may now contain an `else` clause, like `for` loops
 
    [ Bug Fixes ]
    * Numeric literals are no longer parsed as symbols when followed by a dot

hy/compiler.py

@@ -2146,12 +2146,20 @@ class HyASTCompiler(object):
         expr.pop(0)  # "while"
         ret = self.compile(expr.pop(0))
 
+        orel = Result()
+        # (while cond body (else …))
+        if expr and expr[-1][0] == HySymbol("else"):
+            else_expr = expr.pop()
+            for else_body in else_expr[1:]:
+                orel += self.compile(else_body)
+                orel += orel.expr_as_stmt()
+
         body = self._compile_branch(expr)
         body += body.expr_as_stmt()
 
         ret += ast.While(test=ret.force_expr,
                          body=body.stmts,
-                         orelse=[],
+                         orelse=orel.stmts,
                          lineno=expr.start_line,
                          col_offset=expr.start_column)
 

tests/compilers/test_ast.py

@@ -324,6 +324,7 @@ def test_ast_bad_with():
 def test_ast_valid_while():
     "Make sure AST can't compile invalid while"
     can_compile("(while foo bar)")
+    can_compile("(while foo bar (else baz))")
 
 
 def test_ast_valid_for():

tests/native_tests/language.hy

@@ -266,6 +266,39 @@
   (assert (= count 0))
   (assert (= fact 120)))
 
+(defn test-while-loop-else []
+  (setv count 5)
+  (setv fact 1)
+  (setv myvariable 18)
+  (while (> count 0)
+    (setv fact (* fact count))
+    (setv count (- count 1))
+    (else (setv myvariable 26)))
+  (assert (= count 0))
+  (assert (= fact 120))
+  (assert (= myvariable 26))
+
+  ; multiple statements in a while loop should work
+  (setv count 5)
+  (setv fact 1)
+  (setv myvariable 18)
+  (setv myothervariable 15)
+  (while (> count 0)
+    (setv fact (* fact count))
+    (setv count (- count 1))
+    (else (setv myvariable 26)
+          (setv myothervariable 24)))
+  (assert (= count 0))
+  (assert (= fact 120))
+  (assert (= myvariable 26))
+  (assert (= myothervariable 24))
+
+  ; else clause shouldn't get run after a break
+  (while True
+    (break)
+    (else (setv myvariable 53)))
+  (assert (= myvariable 26)))
+
 
 (defn test-branching []
   "NATIVE: test if branching"