Update on String Templates (JEP 459)

[Guy Steele]

> On Mar 13, 2024, at 4:34 PM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> ----- Original Message -----
>> From: "Guy Steele" <guy.steele at oracle.com>
>> To: "John Rose" <john.r.rose at oracle.com>
>> Cc: "Tagir Valeev" <amaembo at gmail.com>, "Brian Goetz" <brian.goetz at oracle.com>, "amber-spec-experts"
>> <amber-spec-experts at openjdk.org>
>> Sent: Wednesday, March 13, 2024 9:13:30 PM
>> Subject: Re: Update on String Templates (JEP 459)
> 
>>> On Mar 13, 2024, at 3:33 PM, John Rose <john.r.rose at oracle.com> wrote:
>>> 
>>> On 9 Mar 2024, at 3:48, Tagir Valeev wrote:
>>> 
>>>> The idea is interesting. There's a thing that disturbs me though.
>>>> Currently, proc."string" and proc."string \{template}" are uniformly
>>>> processed, and the processor may not care much about whether it's a string
>>>> or a template: both can be processed uniformly. After this change, removing
>>>> the last embedded expression from the template (e.g., after inlining a
>>>> constant) will implicitly change the type of the literal from
>>>> StringTemplate to String. This may either cause a compilation error, or
>>>> silently bind to another overload which may or may not behave like a
>>>> template overload with a single-fragment-template. For API authors, this
>>>> means that every method accepting StringTemplate should have a counterpart
>>>> accepting String. The logic inside both methods would likely be very
>>>> similar, so probably both will eventually call a third private method. For
>>>> API user, it could be unclear how to call a method accepting StringTemplate
>>>> if I have simple string in hands but there's no String method (or it does
>>>> slightly different thing due to poor API design). Should I use some ugly
>>>> construct like "This is a string but the API wants a template, so I append
>>>> an empty embedded expression\{""}"?
>>> 
>>> This is a huge thread that I hesitate to dive into, but here’s me putting in one
>>> toe:  Why do we care so much about no-arg string templates?  It’s a small
>>> corner case!  The workarounds (for the no-arg case) are totally straightforward
>>> even if the string template literals (as a syntax) are required to have at
>>> least one argument.
>>> 
>>> Can we have a plausible use case, please, for why a ST with no arguments would
>>> be important, so important that we are motived to invent a sigil syntax or
>>> special type system rules, to avoid requiring the user to invoke a static
>>> factory?
>>> 
>>> Also, Tagir’s workaround of adding a fake argument looks like it would work just
>>> fine, of course depending on which processor was eventually used.
>>> 
>>> And in that vein let me add one new (very bike-sheddy) suggestion before I beat
>>> a hasty retreat:  Instead of in (1) a sigil before the quote like Guy’s
>>> $"hello", put it (1b) after the quote, and in the ST case only.  The ST syntax
>>> could explicitly allow that a no-arg string template would be spelled with a
>>> leading sequence "\{}... which looks like the coder started writing a ST
>>> argument, but in fact dropped it.  So "hello" is a 5-char string, in any
>>> context.  And "\{}hello" is a 5-char no-arg string template, in any context.
>>> That’s Tagir’s workaround, elevated a bit into a new corner case of (existing)
>>> syntax.
>>> 
>>> But even that teeny bit of syntax strikes me as overkill, because I don’t see
>>> the importance of the use cases (no-arg STs) it helps.  Just call
>>> ST.of("hello") and call it a day.
>>> 
>>> In any case, it seems fine to let the IDE take the lead with no-arg STs, helping
>>> the user decide when and how to disambiguate strings from no-arg STs.  Putting
>>> in syntax or type system help for this is surely more expensive than punting to
>>> the IDE, unless there is going to be heavy use of no-arg STs for some use cases
>>> I am not seeing.
>> 
>> Well, just off the top of my head as a thought experiment, if I had a series of
>> SQL commands to process, some with arguments and some not, I would rather write
>> 
>> SQL.process($”CREATE TABLE foo;”);
>> SQL.process($”ALTER TABLE foo ADD name varchar(40);”);
>> SQL.process($”ALTER TABLE foo ADD title varchar(30);”);
>> SQL.process($”INSERT INTO foo (name, title) VALUES (‘Guy’, ‘Hacker’);”);
>> SQL.process($”INSERT INTO foo (name, title) VALUES (\{other name}, \{other
>> job});”);
>> 
>> than
>> 
>> SQL.process(ST.of(”CREATE TABLE foo;”));
>> SQL.process(ST.of(”ALTER TABLE foo ADD name varchar(40);”));
>> SQL.process(ST.of(”ALTER TABLE foo ADD title varchar(30);”));
>> SQL.process(ST.of(”INSERT INTO foo (name, title) VALUES (‘Guy’, ‘Hacker’);”));
>> SQL.process(”INSERT INTO foo (name, title) VALUES (\{other name}, \{other
>> job});”);
>> 
>> especially if I thought that maybe down the road I might want to change the
>> constants 30 and 40 and ‘Hacker' to variables. I don't want to have to keep
>> adding and deleting calls to ST.of as I edit the template strings during
>> program development to have different numbers of interpolated expressions.
> 
> Given what Maurizio said and this, i think the only missing piece in the puzzle is what about existing methods taking a String as parameter.
> 
> We know that for SQL.process(), we do not want process() to take a String but only a StringTemplate.
> But what about the existing methods that takes a String.
> 
> Given a method Logger.warning(String), should
>  LOG.warning($”CREATE TABLE foo;”);
>  LOG.warning($”INSERT INTO foo (name, title) VALUES (\{other name}, \{other job});”);
> 
> be legal ? Is there an auto-conversion (a kind of boxing conversion) from StringTemplate to String ?

In my proposal, the answer would be “no”. Instead you would have two choices:

(1) Instead of string template expressions as in the example just given, you could use string literals or string interpolation expressions (omit the “$” characters):

 LOG.warning(”CREATE TABLE foo;”);
 LOG.warning(”INSERT INTO foo (name, title) VALUES (\{other name}, \{other job});”);

(2) If instead you have some other sort of expression (such as a variable) whose type is StringTempate, you can write

 LOG.warning(String.of(myStringTemplate));

This makes quite explicit that a conversion is happening from StringTemplate to String.