C3 superclass linearization

DEP-Number: 3
Type: Standards Track
Affects-DRM: Yes
Author: Hannes Mehnert
Status: Accepted
Created: 09-Jan-2012
Last-Modified: 09-Jan-2012
Post-History: 09-Jan-2012
Target-Version: 2012.1

Abstract

The Dylan superclass linearization is sometimes counter-intuitive. The C3 superclass linearization algorithm is more intuitive and allows for greater optimization.

Specification

The C3 superclass linearization has been proposed in 1996 by Kim Barrett, Bob Cassels, Paul Haahr, David A. Moon, Keith Playford, and P. Tucker Withington in a paper (html version).

A superclass linearization (also known as a class precedence list) is used for resolving conflicts among multiply-inherited superclasses which provide differing definitions of the same method.

Unfortunately the algorithm presented in the Dylan Reference Manual (computing the class precedence list) is not consistent with the extended precedence graph, and may lead to counter-intuitive linearizations. To fix that, the C3 linearization was developed.

Motivation

In order to allow for more optimizations, especially compression of dispatch tables, which requires monotonicity of method orderings, a consistent superclass linearization algorithm is needed.

After the C3 linearization was proposed in 1996, it was subsequently adapted in Python 2.3 (article) and Perl 6 (article).

Rationale

The C3 linearization has been around for a long time, does not break any existing code, and is commonly agreed upon as being the right thing to do (in mailing list discussions).

Examples

The example from the paper is given here, consider the following Dylan classes:

define class <pane> (<object>) end;
define class <scrolling-mixin> (<object>) end;
define class <editing-mixin> (<object>) end;
define class <scrollable-pane> (<pane>, <scrolling-mixin>) end;
define class <editable-pane> (<pane>, <editing-mixin>) end;
define class <editable-scrollable-pane> (<scrollable-pane>, <editable-pane>) end;

The Dylan linearization is:

<editable-scrollable-pane>, <scrollable-pane>, <editable-pane>, <pane>, <editing-mixin>, <scrolling-mixin>, <object>

The C3 linearization is:

<editable-scrollable-pane>, <scrollable-pane>, <editable-pane>, <pane>, <scrolling-mixin>, <editing-mixin>, <object>

The difference in the ordering of <scrolling-mixin> and <editing-mixin> and having that match the ordering of <scrollable-pane> and <editable-pane> is clearly more intuitive and consistent.

Backwards Compatibility

In the first release a serious warning will be issued for superclass linearization which changed, in subsequent releases a warning should be issued (depending on a strict compatibility to DRM switch).

Experiments show that there are some differences in existing code; we found so far three, two of them in duim-gadgets, one in win32-duim. According to Scott McKay, the author of DUIM, the first two look better when C3 is used (mailing list post). The latter does not alter behaviour (post).

The class precedence list of <row-splitter-pane> differ, Dylan:
#(<row-splitter-pane>, <row-splitter>, <splitter>, <single-child-wrapping-pane>, <cached-space-requirement-mixin>, <client-overridability-mixin>, <wrapping-layout-mixin>, <composite-layout-mixin>, <layout-mixin>, <layout>, <single-child-mixin>, <basic-sheet>, <sheet>, <basic-gadget>, <gadget>, <abstract-gadget>, <abstract-sheet>, <event-handler>, <object>);
C3:
#(<row-splitter-pane>, <row-splitter>, <splitter>, <single-child-wrapping-pane>, <cached-space-requirement-mixin>, <client-overridability-mixin>, <wrapping-layout-mixin>, <composite-layout-mixin>, <layout-mixin>, <layout>, <basic-gadget>, <gadget>, <abstract-gadget>, <single-child-mixin>, <basic-sheet>, <sheet>, <abstract-sheet>, <event-handler>, <object>)
The class precedence list of <column-splitter-pane> differ, Dylan:
#(<column-splitter-pane>, <column-splitter>, <splitter>, <single-child-wrapping-pane>, <cached-space-requirement-mixin>, <client-overridability-mixin>, <wrapping-layout-mixin>, <composite-layout-mixin>, <layout-mixin>, <layout>, <single-child-mixin>, <basic-sheet>, <sheet>, <basic-gadget>, <gadget>, <abstract-gadget>, <abstract-sheet>, <event-handler>, <object>);
C3:
#(<column-splitter-pane>, <column-splitter>, <splitter>, <single-child-wrapping-pane>, <cached-space-requirement-mixin>, <client-overridability-mixin>, <wrapping-layout-mixin>, <composite-layout-mixin>, <layout-mixin>, <layout>, <basic-gadget>, <gadget>, <abstract-gadget>, <single-child-mixin>, <basic-sheet>, <sheet>, <abstract-sheet>, <event-handler>, <object>)
The class precedence list of <win32-viewport> differ, Dylan:
#(<win32-viewport>, <viewport>, <basic-gadget>, <gadget>, <abstract-gadget>, <win32-pane-mixin>, <standard-input-mixin>, <sheet-with-event-queue-mixin>, <client-overridability-mixin>, <scrolling-sheet-mixin>, <permanent-medium-mixin>, <sheet-with-medium-mixin>, <mirrored-sheet-mixin>, <sheet-with-resource-mixin>, <single-child-composite-pane>, <single-child-mixin>, <basic-composite-pane>, <cached-space-requirement-mixin>, <composite-layout-mixin>, <layout-mixin>, <layout>, <basic-sheet>, <sheet>, <abstract-sheet>, <event-handler>, <object>);
C3:
#(<win32-viewport>, <viewport>, <basic-gadget>, <gadget>, <abstract-gadget>, <win32-pane-mixin>, <standard-input-mixin>, <sheet-with-event-queue-mixin>, <client-overridability-mixin>, <scrolling-sheet-mixin>, <permanent-medium-mixin>, <mirrored-sheet-mixin>, <sheet-with-resource-mixin>, <sheet-with-medium-mixin>, <single-child-composite-pane>, <single-child-mixin>, <basic-composite-pane>, <cached-space-requirement-mixin>, <composite-layout-mixin>, <layout-mixin>, <layout>, <basic-sheet>, <sheet>, <abstract-sheet>, <event-handler>, <object>)

Reference Implementation

The pull request #168 was finally merged into master.