[string-templates] Template expression in expression statement?

Alex Buckley alex.buckley at oracle.com
Tue Jul 25 15:51:39 UTC 2023

On 7/25/2023 2:07 AM, Remi Forax wrote:
> ----- Original Message -----
>> From: "Tagir Valeev" <amaembo at gmail.com>
>> To: "amber-spec-experts" <amber-spec-experts at openjdk.org>
>> Sent: Tuesday, July 25, 2023 10:52:04 AM
>> Subject: [string-templates] Template expression in expression statement?
>> Hello!
>> It looks like, the latest javac (build 22-ea+7-489) accepts string
>> templates as expression statements:
>> public class Demo {
>>     public static void main(String[] args) {
>>         STR."hello";
>>     }
>> }
>> The expression statement spec (JLS 14.8, [1]) states that only
>> 'StatementExpression' productions are allowed inside expression
>> statements. This is a closed list of expressions, and it was not
>> updated in the latest spec draft for JEP 430 [2] to include 15.8.6
>> Template Expressions. I'm not sure what the latest consensus is. On
>> one hand, template expression is essentially a method call that may
>> produce a side effect (e.g. logging :D). On the other hand, such kind
>> of usage is probably discouraged. In any case I see the discrepancy
>> here: either spec or javac implementation should be updated.
> The JEP contains these sentences
> "However, it is unwise for a template processor to trigger potentially long-running actions in order to compose a result. It is also unwise to embark upon actions that can have side effects, such as updating a database. The authors of template processors are strongly advised to focus on validating their input and on composing a result that gives maximum flexibility to the client."
> so I believe the spec needs to be updated.

The quoted text focuses on results, not side effects. (In line with 
Brian's "The spirit here is that the template processor is a *function*, 
taking some template-shaped ingredients and turning it into a useful 
*thing*, that you can then further use or manipulate.")

In contrast, expression statements are all about side effects, not 
results (which are discarded).

So, it's odd to read the quoted text and conclude that the _spec_ should 
be updated to make template expressions be expression statements. It's 
javac that should be updated to make template expressions _not_ be 
expression statements. I don't have any reason to think that JLS 14.8 is 


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