[Asdf-devel] syntax-control branch

[Faré]

On Sun, Jun 8, 2014 at 12:09 PM, Robert P. Goldman <rpgoldman at sift.info> wrote:
> Faré wrote:
>> On Sat, Jun 7, 2014 at 8:03 PM, Robert P. Goldman <rpgoldman at sift.info> wrote:
>>> I'm still groping after a clear statement of exactly what class of bugs
>>> it is that we are proposing to fix.  The existing design document is
>>> still too vague, proposing to fix
>>>
>>> "uncontrolled, unintentional leaking of readtable side effects to
>>> systems that depend on such effects not happening,"
>>>
>>> I think we need a clear example of a bug (ideally from a real system,
>>> although it's fine if that bug has since been fixed) that would be fixed
>>> by the change.
>>>
>> I probably should create test cases.
>>
>> Scenario A: dealing with *readtable* being bound to a new value
>> 1- setup the test environment
>> 2- save current packages
>> 3- load fare-quasiquote (or anything that has its own readtable)
>> 4- while in-readtable fare-quasiquote, load-system :force t
>>    something that uses normal quote,
>>    and see it corrupt its fasl with fare-quasiquote annotations.
>> 5- delete all new packages since 2
>> 6- repeat 3, 4 — and see whether it breaks [used to]
>
> OK, so the bug is not that systems leak the readtable *out*, it's that
> readtables leak *into* systems. I had not understood that.
>
> So unlike *PACKAGE*, the current (REPL) value of *READTABLE* leaks into
> every system that you load.  It seems to me that this is a deficiency in
> CL: if CL had an IN-READTABLE equivalent to IN-PACKAGE, we would not be
> having this discussion.
>
Oh, the outer *package* also leaks in, but people have the discipline
to use defpackage and in-package, and to not shadow them away in
incompatible ways.

The syntax-control branch ought to *also* set *package* to
*shared-package* which should be cl-user for maximum backward
compatibility, and asdf-user or something like that for maximum
portability. Or maybe, like Stelian once suggested, some package that
*only* has defpackage and in-package defined (maybe also
uiop:define-package?), for maximum strictness.

>> Scenario B: dealing with the *readtable* object being modified
>> 1- setup the test environment
>> 2- load and compile x1, that define #^
>> 3- load and compile x2, that uses #^ as per x1
>> 4- load and compile x3, that defines a different #^
>> 3- load and compile x4, that uses #^ as per x3
>> 4- force recompilation of x2 — it will use the definition from x4 — oops
>> 5- force recompilation of x1 and x4 — it will use the definition from
>> x2 — oops again
>
> This is the one I had in mind, and this is the one I don't think is
> ASDF's problem.  If x1 exported *X1-READTABLE* and ADD-X1-SYNTAX (this
> function adds X1 reader macros to an existing readtable passed as
> argument), then the Right Thing would simply be for X1 not to alter the
> global readtable, and for X2 and X4 to explicitly employ the readtables
> they want.
>
> This problem does not seem worthy of fixing in ASDF.
>
Indeed, and I'm not proposing that ASDF should do anything about it.
I'm just explaining what the restriction *already is* for sane
programs to never define or use conflicting readtable modifications.
This restriction had to be documented (which I did with ASDF 3.1).

> Scenario A seems like a stronger argument, but I'm inclined to say
> "instead of limping along with a complex fix in ASDF, let's just push
> NAMED-READTABLES use and leave ASDF alone."
>
> NAMED-READTABLES seems like a much better fix than complicating ASDF
> further, because it gets at the root of the problem, instead of
> band-aiding around it.
>
> The counter-argument, I suppose, is that Scenario A can only be fixed if
> *EVERYONE* fixes their code to put IN-READTABLE at the top of their
> source files, just the way we have to put IN-PACKAGE at the head of
> every source file.
>
Exactly. Unless you're demanding that every library should depend on
named-readtables and use it, that doesn't work. And even then, you'd
forbid any readtable that is incompatible enough with the standard one
to interfere with the evaluation of the in-readtable form, e.g. a
readtable for Python syntax, etc. — or even something as innocuous as
fare-quasiquote, if a file uses ` before in-readtable.

This can't fly. And named-readtables itself can't depend on
named-readtables. In the end, you'd have to move it into ASDF, which
is more complex than the simple use of *shared-readtable*.

> My question here came from thinking only about your Scenario B, and
> overlooking Scenario A.
>
> I'm starting to reluctantly come around, but I still need to figure out
> how my system went up the spout when using syntax-control ASDF. If I
> can't answer on my own behalf, I can hardly claim to understand the
> patch well enough to apply it....
>
If I can help with this understanding, I'm at your service to inspect
code of discuss behavior.

—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org
Guns & bullets don't kill people — blood loss and organ damage kills people.