[asdf-devel] standard io syntax woes

[Robert Goldman]

On 2/20/13 Feb 20 -2:44 PM, Faré wrote:
> Dear Common Lisp hackers,
> 
> Inspecting with Anton Vodonosov the latest batch of cl-test-grid issues
> when running with asdf 2.29.x, we found an interesting case
> that mirrors the previous failure of iolib 0.7.3 with 2.29.
> 
> In the hope of making the semantics of asd files more deterministic,
> with an eye on eventually making .asd files a strict subset of Lisp,
> I had put in 2.27 a with-standard-io-syntax around the loading of a .asd file.
> However, this is specified to bind *readtable* and *print-pprint-dispatch*
> to standard tables that are notionally read-only,
> though this immutability is NOT enforced on most implementations,
> instead there being unspecified bad consequences if you do mutate.
> 
> So, I could conceivably (copy-readtable nil) and (copy-pprint-dispatch nil)
> every time, but that could be expensive on some implementations.
> Or I could say "it's the programmer's responsibility to ensure a proper
> table has been setup before he modifies it",
> but that would be harsh and a notable backward incompatibility
> (and there's no equivalent of named-readtables for pprint-dispatch).
> Or I could preserve the current semantics of a global table
> that everyone modifies causing "interesting" issues, by rebinding
> *print-pprint-dispatch* as well as *readtable* within the w-s-i-s,
> only ensuring that the other syntax variables are standard.
> Or I could remove the with-standard-io-syntax altogether, and say
> "yes, if you're doing any global modification, you suck and you're
> going to break something for someone, but that's none of my business".
> 
> Anton leans for the latter.

I favor the expensive option that keeps safety (copying) and that
provides a predictable environment.

We don't load ASDF system definitions into COMMON-LISP-USER, or
"whatever *PACKAGE* happens to be bound to" and we shouldn't do the
equivalent for *readtable* or *print-pprint-dispatch*.

Doing something nasty to *READTABLE* while loading your ASDF system
*should* break, IMO.

Libraries that want to do this should export functions that modify
readtables and pprint-dispatch tables.

[I'm especially unsympathetic to this because of just this month having
gone through hell fixing code that broke because a library felt like it
could call SET-DISPATCH-MACRO-CHARACTER without specifying the
readtable....]

Cheers,
r