Update on String Templates (JEP 459)

[Brian Goetz]

OK, so let's summarize the EG discussion so far.  (As a reminder, 
syntax-heavy features like this are even more subject to "armchair 
theorization" than most, so please, take that into account when 
commenting.  As a further reminder, the best thing we could do right now 
is write more API code that manipulates string templates.)

Overall, I think everyone agrees that the "make string templates the 
star of the show" approach is a winning direction.  No one seems too 
busted up at the loss of processors.

I'm going to try and focus for now on "potential problems that might 
prompt further adjustment", rather than specific solutions.

There is some ambient discomfort that the "sublanguage" of a template 
becomes a dynamic property of a template, introducing new opportunities 
for users to make mistakes with unprocessed templates.  (This was 
present before as well using the RAW processor, but much less 
prominent.)  But, I don't think this is a significant issue, its just 
something new to get used to.

Most of the concerns have to do with the visual similarity between 
string literals and template literals.  While this is of course 
intended, there are some concerns that they may be "too similar". 
Concerns raised include:

  - In a code-generation scenario that leans on templates, sometimes we 
want to use a string literal as a degenerate form of template.  It may 
be surprising that this doesn't "just work", and alternatives (e.g., 
conversion functions, casting, etc) may have varying degrees of 
discoverability and yuck-factor.

  - Given (a) the visual similarity of string and template literals and 
(b) the lenient treatment of concatenation between strings and 
everything else, users may well be tempted to concatenate string 
literals with template literals, and may be surprised at the outcome.

  - Because template literals may be broad and wide, and their 
evaluation may involve side effects, we may want to give a lexical 
heads-up of "weird thing coming", rather than having template literals 
be framed more like "strings with benefits."

Have I covered the concerns raised so far?

Before we get too caught up in solutions, let's try to get on the same 
page about which of these are problems that need to be solved right now.


(As a small matter of housekeeping, given that the preview train is 
already rolling, we will soon have to make a decision to (a) withdraw 
the current preview entirely, (b) re-preview the current design even 
though we know it will change, or (c) gain the requisite confidence in a 
new design in time to preview that. From my vantage point, (c) is 
starting to look increasingly unlikely, and I suspect (a) is a better 
choice than (b).  But I bring this up not to start a project management 
discussions, as much as to raise awareness that there are project 
management constraints.)




On 3/8/2024 1:35 PM, Brian Goetz wrote:
>
> Time to check in with where were are with String Templates.  We’ve 
> gone through two rounds of preview, and have received some feedback.
>
> As a reminder, the primary goal of gathering feedback is to learn 
> things about the design or implementation that we don’t already know. 
>  This could be bug reports, experience reports, code review, careful 
> analysis, novel alternatives, etc.    And the best feedback usually 
> comes from using the feature “in anger” — trying to actually write 
> code with it. (“Some people would prefer a different syntax” or “some 
> people would prefer we focused on string interpolation only” fall 
> squarely in the “things we already knew” camp.)
>
> In the course of using this feature in the `jextract` project, we did 
> learn quite a few things we didn’t already know, and this was 
> conclusive enough that it has motivated us to adjust our approach in 
> this feature.  Specifically, the role of processors is “outsized” to 
> the value they offer, and, after further exploration, we now believe 
> it is possible to achieve the goals of the feature without an explicit 
> “processor” abstraction at all!  This is a very positive development.
>
> First, I want to affirm that that the goals of the project have not 
> changed.  From JEP 459:
>
> Goals
>
> • Simplify the writing of Java programs by making it easy to express 
> strings that include values computed at run time.
> • Enhance the readability of expressions that mix text and 
> expressions, whether the text fits on a single source line (as with 
> string literals) or spans several source lines (as with text blocks).
> • Improve the security of Java programs that compose strings from 
> user-provided values and pass them to other systems (e.g., building 
> queries for databases) by supporting validation and transformation of 
> both the template and the values of its embedded expressions.
> • Retain flexibility by allowing Java libraries to define the 
> formatting syntax used in string templates.
> • Simplify the use of APIs that accept strings written in non-Java 
> languages (e.g., SQL, XML, and JSON).
> • Enable the creation of non-string values computed from literal text 
> and embedded expressions without having to transit through an 
> intermediate string representation.
>
> Non-Goals
> • It is not a goal to introduce syntactic sugar for Java's string 
> concatenation operator (+), since that would circumvent the goal of 
> validation.
> • It is not a goal to deprecate or remove the StringBuilder and 
> StringBuffer classes, which have traditionally been used for complex 
> or programmatic string composition.
>
> Another thing that has not changed is our view on the syntax for 
> embedding expressions.  While many people did express the opinion of 
> “why not ‘just' do what Kotlin/Scala does”, this issue was more than 
> fully explored during the initial design round.  (In fact, while 
> syntax disagreements are often purely subjective, this one was far 
> more clear — the $-syntax is objectively worse, and would be doubly so 
> if injected into an existing language where there were already string 
> literals in the wild.  This has all been more than adequately covered 
> elsewhere, so I won’t rehash it here.)
>
>
> Now, let’s talk about what we do think should change: the role of 
> processors and the StringTemplate type.
>
> Processors were envisioned as a means to abstract the transformation 
> of templates to their final form (whether string, or something else.) 
>  However, Java already has a well established means of abstracting 
> behavior: methods.   (In fact, a processor application can be viewed 
> as merely a new syntax for a method call.)  Our experience using the 
> feature highlighted the question: When converting a SQL query 
> expressed as a template to the form required by the database (such as 
> PreparedStatement), why do we need to say:
>
>   DB.”… template …”
>
> When we could use an ordinary Java library:
>
>   Query q = Query.of(“…template…”)
>
> Indeed, one of the worst things about having processors in the 
> language is that API designers are put in the difficult situation of 
> not knowing whether to write a processor or an ordinary API, and often 
> have to make that choice before the consequences are fully understood. 
>  (To add to this, processors raise similar questions at the use site.) 
> But the real criticism here is that template capture and processing 
> are complected, when they should be separate, composable features.
>
> This motivated us to revisit some of the reasons why processors were 
> so central to the initial design in the first place.  And it turned 
> out, this choice had been influenced — perhaps overly so — by early 
> implementation experiments.  (One of the background design goals was 
> to enable expensive operations like `String::format` to be (much) 
> cheaper.  Without digressing too deeply on performance, String::format 
> can be more than an order of magnitude worse than the equivalent 
> concatenation operation, and this in turn sometimes motivates 
> developers to use worse idioms for formatting.  The FMT processor 
> brough that cost back in line with the equivalent concatenation.) 
>  These early experiments biased the design towards needing to know the 
> processor at the point of template capture, but upon reexamination we 
> realized that there are other ways to achieve the desired performance 
> goals without requiring processors to be known at capture time.  This, 
> in turn, enabled us to revisit a point in the design space we had 
> transited through earlier, where string templates were “just a new 
> kind of literal” and the job performed by processors could instead be 
> performed by ordinary APIs.
>
> At this point, a simpler design and implementation emerged that met 
> the semantic, correctness, and performance goals: template literals 
> (“Hello \{name}”) are simply the literal form of StringTemplate:
>
>   StringTemplate st = “Hello \{name}”;
>
> String and StringTemplate remain unrelated types.  (We explored a 
> number of ways to interconvert them, but they caused more trouble than 
> they solved.)  Processing of string templates, including 
> interpolation, is done by ordinary APIs that deal in StringTemplate, 
> aided by some clever implementation tricks to ensure good performance.
>
> For APIs where interpolation is known to be safe in the domain, such 
> as PrintWriter, APIs can make that choice on behalf of the domain, by 
> providing overloads to embody this design choice:
>
>    void println(String) { … }
>    void println(StringTemplate) { … interpolate and delegate to 
> println(String) …. }
>
> The upshot is that for interpolation-safe APIs like println, we can 
> use a template directly without giving up any safety:
>
>    System.out.println(“Hello \{name}”);
>
> In this example, the string template evaluates to StringTemplate, not 
> String (no implicit interpolation), and chooses the StringTemplate 
> overload of println, which in turn chooses how to process the 
> template. This stays true to the design principle that interpolation 
> is dangerous enough that it should be an explicit choice in the code — 
> but it allows that choice to be made by libraries when the library is 
> comfortable doing so.
>
> Similarly, the FMT processor is replaced by an overload of 
> String::format that interprets templates with embedded format 
> specifiers (e.g., “%d”):
>
>   String format(String formatString, Object… parameters) { … same as 
> today … }
>   String format(StringTemplate template) {... equivalent of FMT ...}
>
> And users can call this as:
>
>   String s = String.format(“Hello %12s\{name}”);
>
> Here, the String::format API has chosen to interpret string templates 
> according to the rules previously specified in the FMT processor (not 
> ordinary interpolation), but that choice is embedded in the library 
> semantics so no further explicit choice at the use site is required. 
>  The user already chose to pass it to String::format; that’s all the 
> processing selection that is needed.
>
> Where APIs do not express a choice of what template expansion means, 
> users continue to be free to process them explicitly before passing 
> them, using APIs that do (such as String::format or ordinary 
> interpolation.).
>
> The result is:
>
> - The need for use-site "goop" (previously, the processor name; now, 
> static or instance methods to process a template) goes away entirely 
> when dealing with libraries that are already template-friendly.
> - Even with libraries that require use-site goop, it is no more 
> intrusive than before, and can be reduced over time as APIs get with 
> the program.
> - StringTemplate is just another type that APIs can support if they 
> want.  The "DB" processor becomes an ordinary factory method that 
> accepts a string template or an ordinary builder API.
> - APIs now can have _more_ control over the timing and meaning of 
> template processing, because we are not biasing so strongly towards 
> early processing.
> - It becomes easier to abstract over template processing (i.e., 
> combine or manipulate templates as templates before processing)
> - Interpolation remains an explicit choice, but ST-aware libraries can 
> make this choice on behalf of the user.
> - The language feature and API surface get considerably smaller, which 
> is good.  Core JDK APIs (e.g., println, format, exception 
> constructors) get upgraded to work with string templates.
>
> The remaining question that everyone is probably asking is: “so how do 
> we do interpolation.”  The answer there is “ordinary library methods”. 
>  This might be a static method (String.join(StringTemplate)) or an 
> instance method (template.join()), shed to be painted (but please, not 
> right now.).
>
> This is a sketch of direction, so feel free to pose questions/comments 
> on the direction.  We’ll discuss the details as we go.
>
>
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