"Enhanced While Loop", by W Isaac Carroll

pep-0315.txt

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+PEP: 315
+Title: Enhanced While Loop
+Version: $Revision$
+Last-Modified: $Date$
+Author: W Isaac Carroll <icarroll@pobox.com>
+Status: Draft
+Type: Standards Track
+Content-Type: text/plain
+Created: 25-Apr-2003
+Python-Version: 2.4
+Post-History:
+
+
+Abstract
+
+    This PEP proposes adding an optional "do" clause to the beginning
+    of the while loop to make loop code clearer and reduce errors
+    caused by code duplication.
+
+
+Motivation
+
+    It is often necessary for some code to be executed before each
+    evaluation of the while loop condition.  This code is often
+    duplicated outside the loop, as setup code that executes once
+    before entering the loop:
+
+        <setup code>
+        while <condition>:
+            <loop body>
+            <setup code>
+
+    The problem is that duplicated code can be a source of errors if
+    one instance is changed but the other is not.  Also, the purpose
+    of the second instance of the setup code is not clear because it
+    comes at the end of the loop.
+
+    It is possible to prevent code duplication by moving the loop
+    condition into a helper function, or an if statement in the loop
+    body.  However, separating the loop condition from the while
+    keyword makes the behavior of the loop less clear:
+
+        def helper(args):
+            <setup code>
+            return <condition>
+
+        while helper(args):
+            <loop body>
+
+    This last form has the additional drawback of requiring the loop's
+    else clause to be added to the body of the if statement, further
+    obscuring the loop's behavior:
+
+        while True:
+            <setup code>
+            if not <condition>: break
+            <loop body>
+
+    This PEP proposes to solve these problems by adding an optional
+    clause to the while loop, which allows the setup code to be
+    expressed in a natural way:
+
+        do:
+            <setup code>
+        while <condition>:
+            <loop body>
+
+    This keeps the loop condition with the while keyword where it
+    belongs, and does not require code to be duplicated.
+
+
+Syntax
+
+    The syntax of the while statement
+
+        while_stmt : "while" expression ":" suite
+                     ["else" ":" suite]
+
+    is extended as follows:
+
+        while_stmt : ["do" ":" suite]
+                     "while" expression ":" suite
+                     ["else" ":" suite]
+
+
+Semantics of break and continue
+
+    In the do-while loop the break statement will behave the same as
+    in the standard while loop: It will immediately terminate the loop
+    without evaluating the loop condition or executing the else
+    clause.
+
+    A continue statement in the do-while loop will behave somewhat
+    differently than in the standard while loop.  Instead of jumping
+    back to the loop condition, it will jump to the beginning of the
+    first suite of the loop.  This is to ensure that the setup code
+    has a chance to do its job before the condition is evaluated.
+
+
+Future Statement
+
+    Because of the new keyword "do", the statement
+
+        from __future__ import do_while
+
+    will initially be required to use the do-while form.
+
+
+Implementation
+
+    The first implementation of this PEP can compile the do-while loop
+    as an infinite loop with a test that exits the loop.
+
+
+Copyright
+
+    This document is placed in the public domain.
+
+
+
+Local Variables:
+mode: indented-text
+indent-tabs-mode: nil
+sentence-end-double-space: t
+fill-column: 75
+End: