Update on String Templates (JEP 459)

[Remi Forax]

> From: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> To: "Guy Steele" <guy.steele at oracle.com>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.org>
> Sent: Friday, March 15, 2024 5:31:28 PM
> Subject: Re: Update on String Templates (JEP 459)

> Hi

> On 15/03/2024 16:07, Guy Steele wrote:

>> Then again, now that I ponder the space of use cases, it may be that, despite my
>> initial enthusiasm, having a separate string interpolation syntax may not carry
>> its weight if its uses are relatively rare. We always have the option of using
>> a string template and then applying an interpolation processor (which might be
>> spelled `String.of(<template>)` or `(<template>).interpolate()` or some other
>> way), and about all we lose from that approach is the ability to use string
>> interpolation to specify a constant expression—for which we still have the
>> old-fashioned alternative of using `+` concatenation. If we drop string
>> interpolation, we can then drop the INTERPOLATION prefix, and we are back to a
>> single-prefix model, and the remaining question is whether that prefix is
>> optional, at least in some cases. Okay, I think I now have a better
>> understanding of the relationships among the various proposals in the design
>> space. Thanks for your patience.

> I think the advantage for not having a string interpolation prefix, is that then
> interpolation is “just another processor” e.g. a static method somewhere that
> takes a string template and returns a String. Another String::format, in a way.
> So that leads to a rather uniform design.

>> And now that I have that better understanding, I think I lean toward (a)
>> abandoning string interpolation and (b) having a single, short, _non-optional_
>> prefix for templates (“$” would be a plausible choice), on the grounds that I
>> think it makes code more readable if templates are always distinguished up
>> front from strings—and this is especially helpful when the templates are rather
>> long and any `\{` present might be far from the beginning. It has a minimal
>> number of cases to explain:

>> “…” string literal, must not contain \{…}, type String
>> $”…” template literal, may contain \{…}, type StringTemplate

> Yep, I agreee this a very principled way to look at the problem.

[...] 

This is how i like to explain the design space to myself. 
We have two kind of strings, tainted string and untainted string (this is not new, see [1]). 
An untainted string is a string that can be escaped properly, in our case a StringTemplate. A tainted string is just a String. 

We do not want a String to be a StringTemplate, because it means all untainted strings are tainted strings. 
We do not want a StringTemplate to be a String, because it means that all tainted strings are untainted strings. 
So both are different types, with neither a subtype relationship nor an automatic conversion between them. 

For the literals, we need two different constructs otherwise we will have a conversion between tainted and untainted strings, 
we also need the literal to construct an untainted string to be different and upfront to easily distinguish an untainted string from a tainted string, so 
- "..." constructs a String, a tainted string, 
- TEMPLATE"..." constructs a StringTemplate, an untainted string. 

About string interpolation, this is another way to create a String and this is not directly related to a string being tainted or not, so it's a kind of orthogonal in term of design. 
It can not be a prefix like INTERPOLATE, because this is different in nature from TEMPLATE, TEMPLATE creates another kind of String, interpolation creates just a String. 
Having a static method (a processor) that creates a String from a StringTemplate creates a common conduit to get a tainted string from any untainted strings, which makes the distinction between untainted string and tainted string less relevant. So i would advise to not go in that direction. 

> Maurizio
> ​

Rémi 

[1] [ https://en.wikipedia.org/wiki/Taint_checking | https://en.wikipedia.org/wiki/Taint_checking ] 
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