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

Eliot Miranda eliot.miranda at gmail.com
Wed Mar 11 21:10:33 UTC 2020

Hi All,

    I'v committed the core support for read-only objects, adapted from
Clément'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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