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.