Re: [Scheme-reports] Seeking review of sets and hash tables proposals

[Alexey Radul <axch@x>]

On Sun, May 26, 2013 at 9:29 PM, Alex Shinn <alexshinn@x> wrote:
> I also think the following utility is nice, as there is no other
> existing utility to use a hash-table as a cache with a single
> operation:
>
>   (hash-table-ref/cache! table key thunk)
>   => (hash-table-search
>       table key
>       (lambda (value update remove)
>         value)
>       (lambda (insert)
>         (let ((res (thunk)))
>           (insert res)
>           res)))

I got in trouble with this one once because the thunk was a call to a
recursive function, and the hash table was that function's memoization
cache.  So calling the thunk caused many entries to be added to the
table, which caused the table to resize itself, which made the
precomputed bucket inside the insert procedure I had invalid, which
made me sad.  That problem can of course be fixed in a careful
implementation (insert can check some serial number or dirtiness bit
and fall back to recomputing the hash), but getting this to be both
clean and efficient is nontrivial.  I wonder what language, if any,
the proposal should include warning either users to avoid or
implementers to solve this problem.

~Alexey

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