RFR: 8367531: Template Framework: use scopes and tokens instead of misbehaving immediate-return-queries [v29]

[Roberto Castañeda Lozano]

On Wed, 12 Nov 2025 15:42:55 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> I got some feedback from users of the Template Framework, especially @galderz . And personally, I already was slightly unsatisfied by some of the issues described below, but did not expect it to be as bad as it is.
>> 
>> So I'm sorry, but I think we need to do a significant re-design. It is now still early enough, and only trivial changes are required for the "real" uses of the framework. Only the framework internal tests require significant changes.
>> 
>> Many thanks to @galderz for trying out the framework, and reporting the issues. And thanks to @chhagedorn for spending a few hours in an offline meeting discussing the issue.
>> 
>> **Major issue with Template Framework: lambda vs token order**
>> 
>> The template rendering involves some state, such as keeping track of hashtag replacements, names and fuel cost.
>> Some methods have side-effects (`addDataName`, `let`, ...) and others are simple queries (`sample`, ...).
>> Sadly, the first version of the template framework was not very consistent, and created tokens (deferred evaluation, during token evaluation) for some, and for others it queried the state and returned the result immediately (during lambda execution). One nasty consequence is that an immediately returning query can "float" above a state affecting token. For example, `addDataName` generated a token (so that we know if it is to be added for the template frame or a hook anchoring frame), but answered sampling queries immediately (because that means we can use the returned value immediately and make decisions based on it immediately, which is nice). Looking at the example below, this had the confusing result that `addDataName` only generates a token at first, then `sample` does not have that name available yet, and only later during token evaluation is the name actually added.
>> 
>> var testTemplate = Template.make(() -> body(
>>     ...
>>     addDataName("name", someType, MUTABLE),
>>     let("name", dataNames(MUTABLE).exactOf(someType).sample().name()),
>>     ...
>> ));
>> 
>> 
>> **Two possible solutions: all-in on lambda execution or all-in on tokens**
>> 
>> First, I thought I want to go all-in on lambda execution, and have everything have immediate effect and return results immediately. This would have the nice effect that the user feels like they are directly in control of the execution order. But I did not find a good way without exposing too many internals to the user, or getting rid of the nice "token lists" we currently have inside Templates (the...
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   inflate abreviations to full names

Hi Emanuel, thanks for improving the design of the template framework, the enforcement of "everything is a token" and the introduction of explicit scope constraints seem like a step in the right direction. Before I go on with the review, I would like to ask two high-level questions (apologies if these are already discussed, it is hard to browse through a PR history):
- The tutorial and the Template documentation remark that we would ideally have used string templates rather than hashtag replacements. Is this still true after the introduction of explicit scoping constraints, i.e. could we still simply use string templates and still enforce the user-provided scoping rules if the feature was available?
- If I got the comments in the tutorial right, it seems that the user has good control over the "transparency level" of scopes, while the transparency rules for templates are hardcoded (hashtag replacements never escape, DataNames always escape, etc.). This felt a bit surprising, would it be feasible to just let the outermost scope in a template determine the template's transparency level?

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PR Review: https://git.openjdk.org/jdk/pull/27255#pullrequestreview-3458766576