[stringtemplate-interest] How best to do recursive hetero template calls?

[Terence Parr]

On Feb 16, 2010, at 2:49 PM, Gerald Rosenberg wrote:
> Hi Ter,
> 
> Yes, slippery slope and all that.  Quite agree with the argument against 
> model view entanglement -- actually the argument is mostly against 
> putting combinatorial logic in the view to evaluate model aspects.

Yup.

> To explain a bit, the attribute being provided to the view is not some 
> fancy data structure, but a simple, literal representation of the view 
> to be generated.  Nested templates are a direct map to the view - each 
> node's type field is the literal name of the template to be applied to 
> the node.

ah. ok, if there is no logic, then I think it is okay to use recursion to walk the tree to spit out XML or whatever.

>  A two-dimensional walk in ST (no exceptions, no conditionals) 
> should be functionally little different from a one-dimensional ST list 
> iteration. So, this narrow use of recursion should not be too much of an 
> entanglement violation, right?

 should be okay; after all, that's how ST v3 he evaluates trees of templates ;)

> Given the view representation is heavily nested, if I instead use 
> external walker I wind up having to not just choose node templates, but 
> also have to provide the decision logic to stitch together parent entry 
> and exit templates for each node.  And, manage the correspondence 
> between the walker logic and the model and all of the view templates.  
> Doable, but seems clumsy.
> 
> Anyway, I have started to experiment using ST4 and, so far, looks 
> promising.
> 
> Purely functional hetero template calls work nicely! 
> <(first(element.children).type)(...)>

sweet...

> Only issue found is that STGroup#getInstanceOf(String name) needs a null 
> check on name - the rest operator can return a null list and that can 
> result in an NPE when rest is used to provide an attribute to another 
> template.

Fixed.

Ter