[stringtemplate-interest] indexed access to lists

[Terence Parr]

Hi John, I like your ideas here about indexing.  I think adding more  
stuff like first, trunc, etc... as you suggest (slice) would be  
interesting....no time to think right now!

Ter
On Jun 1, 2007, at 9:13 PM, John Snyders wrote:

> There was a recent discussion on this list about indexing into lists
> from a template
>
> Nate asks:
>
>   Would index access to lists break separation? Eg...
>   $tests[0].name$
>
> Terence answers:
>
>   yep because it is like calling a function with an argument from the
>   template, not that there aren't similar things.
>
> Calling a function with an argument doesn't bother me much as it is
> already done when accessing
> the values of a map. The key is passed as an argument and the key  
> is not
> limited to an identifier it can
> be any string.
>
> I have some different reasons for not liking list indexing
> 1) the index must be a number and a template shouldn't know anything
> about numbers. To me this by itself isn't a very strong argument. I  
> just
> have a feeling that there is no need for numbers as such in a  
> template.
> Still the index could be a string representation of a number.
> 2) Indexing lists (or arrays) is really begging for the for loop and
> math. This is the main reason why I don't like the list index  
> syntax. I
> think the  StringTemplate list iteration syntax is much nicer than  a
> for loop.  Allowing math would clearly break separation by allowing
> business logic in the template. The indexing syntax could be used  
> to get
> a single specific values from the list but most often it is used in
> conjunction with for loop iteration.
>
> If you just want one item out of the list have the program put it in a
> scalar attribute. If there is truly something special about a single
> element then it probably isn't in a list anyway. I think the  
> request for
> indexing is mostly about wanting to iterate over a list differently.
> Just the first five, all but the last 2, every third one, etc.
>
> One solution for iterating a subset of a list is to have the program
> either copy the desired subset to a new list or wrap  the list in a  
> new
> list that gives out an iterator that returns the subset and  
> provides the
> length of the subset. Some reusable wrapper classes could be created.
> Example
>   List sublist = (List) new SliceListWrapper(0, 5, mylist);
> SliceListWrapper implements List and in this case would return 5 for
> length and the iterator would return elements 0 to 4 inclusive from  
> mylist.
>
> If you really really need to provide indexed access to a list then you
> could wrap it in a map. Then you can access item 5 like so $mylist. 
> ("5")$.
>
> Another possibility would be to add a slice function to  
> StringTemplate.
> It would return a new list with the given range.
> Example
> $slice(mylist, "0", "5"): { [$it$] }$
> would output the first 5 items in mylist enclosed in [].
> I'm thinking it would work like Python list slicing.
>
> I think this is safer than list indexing and more in the spirit of
> trying to iterate over a subset of a list. It is safe because you are
> not calling a method on a user object but a specific string template
> function.
>
> Functions first and last should still be kept as convenient shortcuts
> and for backward compatibility. The trunc function need not be
> implemented now because slice can do it and more.
>
> -John
>
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