On 5/19/05, Michele Simionato <[EMAIL PROTECTED]> wrote:
>
> You misunderstood Felix. The question is
>
> """
> what's the rationale for
>
> (hash-table-ref h 'non-existing-key)
>
> returning #f instead of an error?
> """
>
> Returning #f is ambiguous, it could mean that the key exists with value #f!
>
It doesn't matter what is returned: any value will be ambiguous. That's
the reason why one should use (for example):
(hash-table-ref h 'non-existing-key
<some-unique-value-known-to-the-application-that-created-and-uses-the-hashtable>)
to disambiguate the result:
(let ((unique (list 1)))
(if (eq? unique (hash-table-ref ht 'non-existing-key unique))
...not found...
...found...))
Why `#f' in particular? Seems to be used most. (void) would be ok as well,
I guess, but that should normally be reserved for the implementation, since
it's not really well defined.
cheers,
felix
cheers,
felix
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