String Templates Question - are expressions within a TemplateString literal evaluated immediately, or when a template is applied?

[Nathan Walker]

Hi folks,

Question regarding TemplateStrings that I could not seem to find the answer
to in the JEP write up or the Javadoc for TemplateString:  Is the
expression evaluation lazy, or immediate?

In other words, if I do something like this:

     List<Integer> list=new ArrayList<>();
     TemplateString ts = "size = \{list.size()}"
     list.add(1);
     System.out.println(ts.apply(STR));

Will it print out size = 0, or size = 1?

My main reason for asking is that I would love for string templates to
result in logging interfaces with overloaded methods that can take
TemplateStrings instead of strings, and which are only evaluated if the log
level is enabled.  So that instead of something like this:

     if (logger.isEnabled(Level.DEBUG)) {
          logger.log(Level.DEBUG, "Expensive value is " + lookup() );
     }

we can just do:

     log.log(Level.DEBUG, "Expensive value is \{lookup()}");

And be confident that the cost of invoking lookup() will only be paid if
DEBUG is enabled.

Probably over 80% of the string building I have seen over my career has
been related to building log messages.  And the mistakes I have most often
seen are:

     1. Expensive message construction not guarded by checking if logging
is enabled at that level
     2. Expensive message construction guarded, but by checking the *wrong*
logging level (e.g. check INFO, log at ERROR)
     3. Using some sort of parameterized message/string format but getting
the parameters wrong (out of order, missing one, having too many, etc.)

If TemplateStrings evaluate their expression fragments only when processed
then every one of these problems gets addressed, and logging code becomes
*a lot* simpler to read, write, and code review.

(Random side note: I *love* the choice of \ instead of $.  It took some
time for me to get use to the idea, but keeping the same escape character
is fantastic and I wish more languages had done that instead of using $)

Thanks for your time,
Nathan
-- 
Nathan H. Walker
nathan.h.walker at gmail.com
(703) 987-8937
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