[squeak-dev] enabling read-only literals...

David T. Lewis lewis at mail.msen.com
Wed Mar 11 22:42:53 UTC 2020

+1 from me, please do it.

And don't worry about your VisualWorks experience. They were dealing
with commercial customers, and if you ever want to see a lot of really
bad code, take a look at the in-house code base of just about any
commercial organization.

In Squeak we have our share of bad code, but it's not *that* bad ;-)


On Wed, Mar 11, 2020 at 02:10:33PM -0700, Eliot Miranda wrote:
> Hi All,
>     I'v committed the core support for read-only objects, adapted from
> Cl??ment's Pharo implementation.  Enabling read-only literals is a very
> small step from that (three methods modified in the parser, and one method
> modified in Symbol class).  I would like to do this as soon as possible,
> but it may break code.  Hence my asking for permission/sending out a
> warning before hand.  Note that the only code it will break is bad code;
> code that writes to literals.
> With the changes installed doing this:
> #(1 2 3) at: 2 put: 'nevermore'
> raises a ModificationForbidden exception with the message text
> "ModificationForbidden:  #(1 2 3) is read-only, hence its field 2 cannot be
> modified with 'nevermore'"
> When we added read-only object support to VisualWorks some of the
> engineering staff were of the opinion that insulating customers from the
> change was a necessary thing, and so we implemented a preference to allow
> automatic mutating of read-only literals so that customers whose code did
> modify literals could set the preference rather than fix their code.  I
> *really* don't ant to do this.  It is a lot of complication for little
> gain; the right fix is just to rewrite the code not to write to literals.
> Note that that's as easy as:
> #(1 2 3) copy at: 2 put: 'nevermore'; yourself
> =>
> #(1 'nevermore' 3)
> since copies of read-only objects are mutable.
> So do I have everyone's consent in changing trunk over to read-only
> literals?  If there are no strenuous complaints by tomorrow noon, PST, I
> shall effect the change.
> To avoid recompiling (which can produce unbound methods, etc) I shall
> change literals to be read-only via a pass over all literals and a pass
> over the Symbol table.  Hence the change should be non-invasive.  If your
> code (like probably 99.9% of all code in the system) doesn't write to
> literals you won't notice.
> _,,,^..^,,,_
> best, Eliot


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