Re: ‘set-cdr!’ and weak-cdr pairs

[Andy Wingo]

On Sun 13 Mar 2011 16:25, address@hidden (Ludovic Courtès) writes:

> The problem is that ‘hash-create-handle!’ above created a weak-cdr
> pair—i.e., a pair whose cdr is /not/ scanned for pointers—but ‘set-cdr!’
> did not register a disappearing link from O to K+V.  Consequently, O
> eventually gets collected, but K+V remains; the storage of O then gets
> reused, and the cdr of K+V ends up containing either an unrelated or >an
> invalid Scheme object.

Given that we don't expose weak-pair constructors or accessors to
Scheme, we should not expose weak pairs to Scheme.  What do you think
about making it an error to hash-create-handle! on a weak table?  That
way you never expose a weak pair to Scheme.  It does appear possible to
discriminate in C between calls to create-handle! that occur due to ref
/ set! and those that are called explicitly.

Also that would free up our implementation to use something other than
weak pairs for weak hash tables.  In particular, I think first-class
weak references are not a bad idea, as Chez Scheme does.

WDYT?

Andy
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