When a $placeholder lookup fails, Cheetah throws a
NameMapper.NotFound_ error.  But sometimes it's desirable to provide a
default value.  For instance, the BatchHandling_ proposal that is
currently being implemented.  It needs a CGI parameter 'start', which
it sets on subsequent links to itself (= to the current page).  But
the first time around, the user may have come by typing the URL rather
than following a hyperlink, and she may have forgotten (or didn't know
about) the required 'start' parameter, which would be 'start=0' the
first time around.

Current plan is to use::

   $getVar('request.field("start")', 0)

if it works.  This is verbose and requires too much quoting.  But if
=$getVar`` is not usable in this manner, the user will have to resort
to::

    #try
    #set $start = $request.field("start")
    #except NameMapper_.NotFound_
    #set $start = 0
    #end try

which is even less desirable.  Compounding the problem is that the
exception will not be thrown if an errorCatcher is active; instead it
will substitute some HTML which is not numeric and will make the
$batch() method barf.

The following solution would be the cleanest::

    ${request.field("start"), default=0}

The syntax follows precedent.  Currently, 'default' would be would be
considered a filter argument, to be passed to the #filter.  I propose
making 'default' a special name that Cheetah would substitute if
NameMapper.NotFound_ is raised, before calling the filter, and then
removing 'default' from the filter's argument set.

-- MikeOrr_ - 13 Nov 2001

$getVar and $varExists are what I hope to stick with.  The
implementation of the proposal above would probably have a speed
impact across all Cheetah $placeholders.  If someone wants to go to
the effort of making the parser compile different code for this
special case, go for it. Not something I'm interested in doing though
-- it would probably be a full days work.

-- TavisRudd_ - 13 Nov 2001

I don't see why it should have any (significant) speed impact.  If you
change NameMapper.valueFromSearchList_ to take an optional
default=NoDefault argument, then at the last line where it raises
NotFound(name), instead test if default is NoDefault, and either
return default or raise the error.  _namemapper.valueFromSearchList
already needs some work to give correct error messages anyway.  I
expect the parser isn't the hard part of implementing this? --
IanBicking_ - 13 Nov 2001