[stringtemplate-interest] Understanding template recursion

[John Snyders]

One more note on this.

I thought I could get around the problem by using the literal list 
construction to force a new list to be created
as in

recurse2(x) ::= "$x$ $if(x)$$recurse2([rest(x)])$$endif$"

or

$useItTwice([rest(items)])$

But this does not help because the list construct returns an iterator as 
well
In ActionEvaluator.java: list method
        value = new CatIterator(elements);
       ...
        return value;

This too will need to be fixed.

-John


John Snyders wrote:
> Hi Trevor,
>
> recurse(x) ::= "<x><rest(x):recurse()>"
>
> In this case on the first call x is the whole list so "abcd" is output. 
> then it loops over the rest of x calling recurse with the x being the 
> current item it. It is the same as recurse(x=it).
> When I try this I get "abcdbcd"
> For me this is behaving as you and I would expect. I don't know why it 
> doesn't work for you. We may have different versions of ST.
>
> recurse2(x) ::= "<x><recurse2(rest(x))>"
>
> I think what might be happening here is infinite recursion. The typical 
> pattern for recursion is to have a test for the case that is going to 
> stop the recursion. You might intend something like this (I just happen 
> to use $$ rather than <>)
>
> recurse2(x) ::= "$x$ $if(x)$$recurse2(rest(x))$$endif$"
>
> Sadly this does not do what I expect. The first time it should output 
> "abcd" because x is items. Now it calls recurse2 with the rest of x 
> which is [ b, c, d ] and that is what should be output next "bcd" Now it 
> recurses again with [ c, d ] and should output "cd" and again resulting 
> in "d" at which point rest is null and the if would be false. So the 
> total output I expect is "abcd bcd cd d "
>
> What I get is "abcd bcd "
>
> The reason I believe has to do with how ST uses iterators. On the first 
> recursive call to recurse2(rest(x)) rest(x) returns an iterator and not 
> a new list that is a slice of the old list. Now on the recursive call x 
> is an iterator and as soon as you use it once it is used up. The next 
> recursive call rest(x) returns null because x was used up in $x$.
>
> This is a very serious bug and I thought I had reported it before in 
> some way but now I can't find it in the archives.
> A very simple way to see this bug is with the following
> useItTwice(x) ::= <<
> once : $x;separator=", "$
> twice: $x;separator=", "$
>  >>
> test() ::= <<
> $useItTwice(items)$
> $useItTwice(rest(items))$
>  >>
> With your definition of items calling
> $useItTwice(items)$
> $useItTwice(rest(items))$
> it should print
> once : a, b, c, d
> twice: a, b, c, d
> once : b, c, d
> twice: b, c, d
>
> but it prints
> once : a, b, c, d
> twice: a, b, c, d
> once : b, c, d
> twice:
>
> Iterators should never be passed to templates. This bug will prevent you 
> from doing anything interesting with recursive templates.
>
> P.S. I went to umass Amherst back in the 80's.
>
> -John
>
> Trevor Strohman wrote:
>   
>> I'm having trouble understanding recursion in templates.
>>
>> Suppose I have a list called "items" that contains Strings: "a", "b",  
>> "c", "d", and these two templates:
>> recurse(x) ::= "<x><rest(x):recurse()>"
>> recurse2(x) ::= "<x><recurse2(rest(x))>"
>>
>> I expect <recurse(items)> to become:
>> 	abcdcdd
>> and <recurse2(items)> to become:
>> 	abcd
>>
>> However, <recurse(items)> becomes "abcd" and <recurse2(items)> throws  
>> an Exception.  What am I misunderstanding?
>>
>> Also, I notice that functions first, last and rest don't seem to be  
>> first-class citizens in the language.  These expressions throw  
>> exceptions:
>> 	<if(first(rest(items))>
>> 	<first(items).isEnabled>
>> This has been a problem for me this afternoon.  I feel like I must be  
>> misunderstanding the right way to approach recursive template  
>> invocation.
>>
>> Trevor
>>
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