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Re: argument against using '() for null values? ([Chicken-users] DBI)


From: Graham Fawcett
Subject: Re: argument against using '() for null values? ([Chicken-users] DBI)
Date: Wed, 27 Feb 2008 21:58:42 -0500

On Wed, Feb 27, 2008 at 8:29 PM, Ozzi <address@hidden> wrote:
> > Three that I can think of:
>  >
>  > 1) It would make alist representations ugly:
>  > (query "select foo, NULL as bar, baz from stuff") =>
>  > ((foo . 1) (bar) (baz . "a string"))
>
>  This doesn't bother me.
>
>  > 2) It is not a disjoint type. (list? '()) => #t. That's bad.
>  >
>  > 3) As a special case of (2), some databases have array-type columns.
>  > An empty list may be a valid column value.
>
>  All right, there's the big problem.
>
>  How about keeping '() for sql NULL, and using vectors to represent array-type
>  columns?

Vectors are currently used in the Postgres egg to represent date-time
values. Not saying it's good, but there it is. (BTW, a dbapi needs
consistent date/time support too...)

In Common Lisp, NIL is the same as '(), and both mean "false". In
Scheme we have an explicit #f. For the same reasons we should have an
explicit #<sql-null>. It is important that (sql-null? (sql-null)) is
#t, and (sql-null? anything-else) is #f.

>  Since SQL NULL values are common, I really like the simple null? test. It's 
> intuitive.

How is (sql-null?) harder or less intuitive?

>  On the other hand, I didn't even know array-type columns existed until 
> tonight.
>  I'd prefer to go with something slightly less intuitive for representing 
> them if
>  it will let us use '() for NULL.

...and perhaps we should use '() for #f as well? ;-)

Graham




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