Does String extend StringTemplate? (Was: Update on String Templates (JEP 459))

[Brian Goetz]

Splitting off into a separate thread.

I would like to redirect this discussion from the mechanical challenges 
and consequences to the goals and semantics.

If we are considering "String extends StringTemplate", we are making a 
semantic statement that a String *is-a* StringTemplate. While I can 
imagine convincing oneself that this is true "if you look at it right", 
this sets off all my "self-justification" detectors.

So, I recommend we step back and examine why we think this is a good 
idea before we descend into the mechanics.  My suspicion is that this is 
motivated by "I want to be able to automatically use String where a 
StringTemplate is desired", and that this seems a clever-enough hack to 
get there.  (I think we probably also need to drill further, into "why 
do we think it is important to be able to use String where 
StringTemplate is desired", and I suspect further that part of it will 
be "but the APIs are not yet fully equilibrated" (which would be a truly 
bad reason to give String a new supertype.))




On 3/12/2024 1:24 PM, Tagir Valeev wrote:
> Hello, Maurizio!
>
> Thank you for the detailed explanation!
>
> On Mon, Mar 11, 2024 at 1:16 PM Maurizio Cimadamore 
> <maurizio.cimadamore at oracle.com> wrote:
>
>     Hi all,
>     we tried mainly three approaches to allow smoother interop between
>     strings and string templates: (a) make String a subclass of
>     StringTemplate. Or (b) make constant strings bs /convertible/ to
>     string templates. Or, (c) use target-typing. All these approaches
>     have some issues, discussed below.
>
>     The first approach is slightly simpler, because it can be achieved
>     entirely outside of the Java language. Unfortunately, adding
>     “String implements StringTemplate” adds overload ambiguities in
>     cases such as this:
>
>     |format(StringTemplate) // 1 format(String, Object...) // 2 |
>
>     This is actually a very important case, as we predice that
>     StringTemplate will serve as a great replacement for methods out
>     there accepting a string/Object… pack.
>
>     Unfortunatly, if String <: StringTemplate, this means that calling
>     format with a string literal will resolve to (1), not (2) as
>     before. The problem here is that (2) is not even applicable during
>     the two overload resolution phases (which is only allowed to use
>     subtyping and conversions, respectively), as it is a varargs
>     method. Because of this, (1) will now take the precedence, as
>     that’s not varargs. While for String::format this is probably
>     harmless, changing results of overload selection is something that
>     should be done with care (esp. if different overloads have
>     different return types), as it could lead to source compatibility
>     issues.
>
> I would still like to advocate for String <: StringTemplate solution. 
> I think that the overloading is not a big problem. Simply making 
> String implements StringTemplate will not break any of existing code 
> because there are no APIs yet that accept the StringTemplate instance. 
> The problem may appear only when an API author actually adds such an 
> overload and does this in an incompatible way with an existing String 
> overload. This would be an extremely bad design choice, and the blame 
> goes to the API author. You've correctly mentioned that for 
> String::format this is harmless because the API is well-designed. We 
> may suggest in StringTemplate documentation that the API designers 
> should provide the same behavior for foo(String) and 
> foo(StringTemplate) when they add an overload.
>
> I must say that we already had an experience of introducing new 
> interfaces in the hierarchy of widely-used library classes. Closable 
> got AutoClosable parent, StringBuilder became comparable, and so on. 
> So far, the compatibility issues introduced were tolerable. Well, 
> probably I'm missing something but we have preview rounds just for 
> this purpose: to find out the disadvantages of the approach.
>
>     On top of these issues, making all strings be string templates has
>     the disadvantage of also considering “messy” strings obtained via
>     concatenation of non-constant values string templates too, which
>     seems bad.
>
> I think that most of the APIs will still provide String overload. 
> E.g., for preparing an SQL statement, it's a perfectly reasonable 
> scenario to have a constant string as the input. So 
> prepareStatement(String) will stay along with 
> prepareStatement(StringTemplate). And people will still be able to use 
> concatenation. I don't think that the absence of String <: 
> StringTemplate relation will protect anybody from using the 
> concatenation. On the other hand, if String actually implements 
> StringTemplate, it will be a very simple static analysis rule to warn 
> if the concatenation occurs in this context. If the expected type for 
> concatenation is StringTemplate, then something is definitely wrong. 
> Without 'String implements StringTemplate', one will not be able to 
> write a concatenation directly in StringTemplate context. Instead, 
> String-accepting overload will be used, and the expected type will be 
> String, so static analyzer will have to guess whether it's dangerous 
> to use the concatenation here. In short, I think that it's actually an 
> advantage: we have an additional hint here that concatenation is 
> undesired. Even compilation warning could be possible to implement.
>
> So, I don't see these points as real disadvantages. I definitely like 
> this approach much more than adding any kind of implicit conversion or 
> another literal syntax, which would complicate the specification much 
> more.
>
> With best regards,
> Tagir Valeev.
>
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