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While reading for the Nth time "The interface could be better but we're stuck with it now", I began thinking about ways to provide better backwards and forward compatibility. I have some ideas but nothing ground-breaking and I'd like to know about any other techniques out there.

A few approaches to changes in API or Interfaces are:

  • Don't. Stick with the sub-optimal past
  • Do. Let the new version croak, the old versions still available on CPAN
  • Deprecation cycle, see perlmodstyle:
    Backwards compatibility
    Modules which are "stable" should not break backwards compatibility without at least a long transition phase and a major change in version number.
  • Opt-in back-compat. Require users to ask for a deprecated feature (eg. TT2's V1DOLLAR)
  • Fork. Budd-off into a new namespace, leaving the old API behind
  • Provide a custom VERSION sub, "use Mod 1.1", perldoc -f use
  • use version; the module.

If you plan to evolve an API or want to release a non-binding alpha version, you could provide an alternate name that you document as stable, while the main-line can change. For example:

use Some::Module::alpha; # supported as is use Some::Module::alpha::warn; # as is, opt-in to future warnings use Some::Module; # may evolve without warning

Initially, "alpha" would basically be an proxy for the main module. Later it might be a backward-compatibility wrapper for the new interface. If the backward compatibility becomes impractical or restrictive then the alpha interface could become it's own package. The ::warn name extension (or "compat_warn => 1") could be used to opt-in to future compatibility change warnings.

I'm just brain-storming here and am interested in what others think.

What have other people tried?
What works and, importantly, what doesn't?
Are there already PM nodes addressing this?
How does Perl 6's module versioning system address these issues?
(The naming of v6-alpha inspired me in part)

Brad

In reply to Planning for Backwards Compatibility by bsb

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