[stringtemplate-interest] dealing with empty list values

[Joshua Royalty]

On Mon, Feb 8, 2010 at 1:51 PM, Terence Parr <parrt at cs.usfca.edu> wrote:

> On Feb 7, 2010, at 5:02 PM, Joshua Royalty wrote:
> Hi Terence,
> > If $foo$ is 'missing', then
> '<tr>$if(foo)<b>$foo$</b>$else$MISSING$endif$</tr>' will still print
> '<tr>MISSING</tr>', but $values:{ v |
> <tr>$if(v)<b>$v$</b>$else$MISSING$endif$</tr>}$ will not print
> '<tr>MISSING</tr>' if one of the values is 'missing'.
>
> unless you use the null option, right?
>

is null=false the intended way to do this?  If so it.. works, but it doesn't
seem like a clean solution.  First you end up with lists with multiple types
of values..  which is just generally not a good idea.  Second.. it is nice
that it preserves backwards compatibility, and its relatively easy to
implement.. but it really seems like a solution tacked on at the last minute
and its not intuitive at all to a programmer.


>
> >  Obviously in the first case ST does not know what text it should skip
> and the user obviously intended it to print <tr>MISSING</tr>, and I contend
> that ST also cannot know if it should skip in the second case and that the
> user obviously intended it to print <tr>MISSING</tr> for values that were
> 'missing'.  When I use a list iterator I intuitively expect it to iterate
> over every value in the list, it should be the same as repeating the code
> inside the template for every value in the list, even if that value is
> 'missing'.
>
> Well, if foo is missing then $foo:blort()$ should not evaluate blort() and
> it doesn't. Therefore when foo his multi-valued, it should skip any empty
> values to be consistent, right?
>

Depends on how you think about it..  See below


>
> > Also, length(values) return the length of the list with the nulls
> included, and in order to get the length without them you need to call
> length(strip(values)).
>
> You're right.  that is inconsistent. on the other hand, given that you
> might be using the null option, there has to be a way to figure out how many
> total elements you're going to have.
>

Again, the inconsistencies make it seem like this was tacked on instead of
integrated into the framework..  And that makes it harder to remember and
use.


>
> > To be consistent with the current ST list iterator length(values) should
> automatically strip nulls...  However, another fix would be to not
> automatically skip nulls in the list iterator.  This would fix both of the
> inconsistencies and be much more intuitive.  If people want to skip the
> nulls entirely when iterating they can use strip as a shortcut,
> $strip(values):{ v | <tr><b>$v$</b></tr>}$.  For backwards compatibility /
> convienience you can add a global 'AutoStripLists' option if you think its
> necessary.
>
> I understand what you're saying, but what about the case when foo a single
> value that I have above? should it invoke blort()?
>

I think I may have found the communications disconnect.  I've been treating
':' as the 'list iterator' operator... you seem to be treating it as the
'apply template'/'expand list and apply template' operator..

If you think of ':' as the 'list iterator' operator (which is how the cheat
sheet describes it..), then actually, no, blort should not be invoked
because by using the 'list iterator' operator (instead of the 'invoke
template' ($<template>(<attr>)$) operator) you are implying that it is a
multi-valued attribute.  (so $<attr>:<template>$ is a shortcut for
$if(<attr>)$$<template>(<attr>)$$endif$ when <attr> is single valued...
(though personally I would spell it out the second way..))



I guess the real question it comes down to is what is more important..
convienience in the case that you want to not want to apply the template to
null items in a list or having a consistent (and thereby intuitive)
framework.  Needing to skip null elements in a list seems like a pretty
uncommon case to me.. (I could be wrong), and its the only piece that would
not be backwards compatible.  If people really need to skip nulls, they
could simply use 'strip'.. or there could be a global 'UseStripIterator'
setting..
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