Re: TIP #185: Null Handling

From: Andreas Leitgeb (avl_at_gamma.logic.tuwien.ac.at)
Date: 04/13/04 

Date: 13 Apr 2004 09:40:19 GMT

John H Harris <JHHarris@valley.net> wrote:
> Tcl lacks the ability to handle /nulls/, data with missing or unknown
> values. In this TIP I suggest a means for representing and propagating
> nulls, and command modifications for manipulating them.

I disagree with the target of this TIP.
Tcl itself doesn't need a NULL value, but most likely your
database-extension needs some way to deal with NULLs.

I haven't yet done Database-stuff with tcl (only in C), but with
an ideal API, you should be able to specify a "one-or-zero-llength
list of strings" where your current db-API only accepts strings
directly, and have an empty list interpreted as null, whereas a list
containing an empty string distinguishable a non-NULL empty string.

Perhaps there even already exist db-extensions that do it this way.

> set result [ db eval {select id, name from person}
> foreach -null <unknown> { id name } $result {
> puts "$id: $name" }
proc value {li {def {}}} { if {[llength $li]} {lindex $li 0} {return $def} }
set result [ idealdb -list eval {select id, name from person} ]
foreach {_id _name} $result {
  set id [value $_id <unknown>]; set name [value $_name <unknown>]
  puts "$id: $name"
}

idealdb would use option -list to embed values in lists, and thus
 make NULL-values recognizeable. That's not a proposal, just a
 sketch on how it could be done. Similar list-embedding functionality
 would be required for the API that writes values to the DB.

This list-embedding is principially equivalent to your quote-prepending,
but the main point is, that the db-extension has to translate the list
(or your quote)-trick to possible NULLs, forward and backward.

The database-extension would have to be modified anyway, (unless it
already has such an API).