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Re: error in hash table reference implementation



On Tue, Aug 09, 2005 at 09:13:41PM -0400, Jonathan Kraut wrote:
> But before you do, I found an error in the reference implementation:
> (alist->hash-table '((a . b) (c . d)))
> calls make-hash-table with incorrect arguments:
> (make-hash-table #f #f 64),
> which succeeds but is wrong.

This is similar to Per's notice about (make-hash-table) not using
hash-by-identity; it's a sign of (my) bad coding style that
implementation details of some functions -- like hash-by-identity,
make-hash-table -- are exposed in other functions -- like
make-hash-table and alist->hash-table, respectively.

Again, it is only a problem for implementors that make partial use of
the SRFI implementation.  I could fix them, but I also think that hash
tables are of such a central nature that implementors should provide
native support for them.  The SRFI code is not the greatest,
most-optimised, neatest code ever; it just shows one way to implement
the requirements of the SRFI.

> Here is a fixed version:
> (define (alist->hash-table alist . args)
>   (let ((hash-table
>           (make-hash-table
>             (if (not (null? args))
>                 (car args)
>                 equal?)
>             (if (and (not (null? args)) (not (null? (cdr args))))
>                 (cadr args)
>                 hash)
>             (if (and (not (null? args)) (not (null? (cdr args)))
>                      (not (null? (cddr args))))
>                 (caddr args)
>                 (max *default-table-size* (* 2 (length alist)))))))
>     (for-each (lambda (elem)
>                 (hash-table-set! hash-table (car elem) (cdr elem)))
>               alist)
>     hash-table))

This does not account for the special case of string-ci=?.  To really
fix this, we should extract the hash function deduction logic into a
separate function.  I can do that if there are some seconds to declare
this important enough.

Panu

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