Are templated string embedded expressions "method parameters" or "lambdas"?

[John Rose]

On Oct 30, 2021, at 2:44 PM, John Rose <john.r.rose at oracle.com<mailto:john.r.rose at oracle.com>> wrote:

its own avowed language feature

And, full disclosure, besides pass-by-name I see two other language
features that could be distinguished from the current design, whether
or not they ever appear separately (as I hope pass-by-name/autoquote
will some day appear, for expression tree processing more than loggers).

2. Reified argument types:  There is a nifty way that javac captures the
static types of the “hole” arguments of the template and makes them
available, in case the template processor would find it useful.

(This is not reified type arguments, but could be a building block.)

This happens also for many uses of method handles and invokedynamic,
but always “under the covers”.  This is the first time static argument types
have been reified as part of an API design.   Perhaps that deserves some
pathfinding discussion at this point.  Perhaps string templates intrinsically
“own the turf” of reifying static argument types, but I don’t see this yet;
I think they have a natural “ask” for reified types, but *are not alone*.
If S.T.s are an early customer of such things, it would be wise to take
a breath and think ahead, to other customers, if possible aligning the
way S.T.s do it from the first with some set of likely futures.

3. Static validation: This is a biggie. We want a hook that allows (though
does not require) a S.T. provider to validate a string template, once for
each distinct context.  What is the static context?  Well, in x."y\{z}"
that would be at least the string containing y plus (see above) the static
type of z, plus something derived from x.  Plus maybe—I hope—an
indy-like context, so that the validation logic has the option (but
not requirement) to look up the stack for permission checks.
Plus, probably, the entire contents of x (which is not static if x is
a proper variable!).

I call it “static” validation because clearly we want to play out the
validation logic the first time some given context is evaluated,
but we don’t want to replay it for later evaluations in the same
context.  The string is clearly constant as are the static types
of the arguments.  The rest is not so clearly constant, so the
prototype uses some tricks with MutableCallSite to make the
evaluation mostly static.  I think this needs scrutiny, and I
would also like to consider other ways (without appealing
to an MCS) to gain the required static evaluation hooks.

Some of the use cases defer static validation until a database
connection is opened and an SQL idiom is determined.
In the more problematic case, the validation is not static
because the database connection is not a constant, and
the idiom may vary from call to call.  The best you can
do in such a circumstance is a cache. A MCS is the wrong
tool in that circumstance; it is best to allow the SQL
template designer to create whatever cache is most
appropriate to the expected set of SQL idioms.  The
ST provider, per se, will in this case do minimal
checking, or none at all, in its “static validation” hook.

In any case, I think SQL idiom validation is very far
from the central use case to design around.

I think format strings and regular expressions are much
more likely as use cases, and those have truly static validation
stories that make sense.  They may require static types to do the
best job, but much validation can be done without argument
types as well.  Clearly, the design will succeed for regular
expressions if and only if Pattern.compile can be executed
reliably once before the first evaluation of the template
as a whole.

To me this suggests that we should be thinking about
what it means to execute a “static hook” for an expression,
which prepares for the subsequent execution of a method
call that finishes the call.

Maybe this is just a design pattern, but there is evidence
that it needs language support.  The language support
come in when the compiler separates arguments meant
only for the static hook from arguments used to finish
the whole expression evaluation.  It would seem that
we need a notion of “static arguments” as opposed to
regular “dynamic arguments”, for expressions that
have a “static hook”.

I don’t want to dilute the S.T. discussion any more
than I already have, so I’ll just say that S.T.’s have
static arguments that are (a) the string itself with
holes marked and (b) maybe the argument types
for the expressions for those holes.

I suggest that, until we roll out more of the machinery
we intend to roll out, such as type classes, that we
restrict the operand x (the receiver LHS of the S.T.)
to be a statically constant expression.  If we do that,
then we can hand three things to a bootstrap method
that can do static validation:

1. the (constant) receiver
2. the string body (with holes marked)
3. maybe the static types of the arguments (this is very natural for indy)

If we allow the receiver to vary dynamically, we have
boxed ourselves into a corner, regarding validation.

By “constant x” I simply mean the name of a static final field.
There are not many other reference-valued expressions which
are “constant” enough to permit static validation.

This would give us a story for validation from day one (using
indy) and would also preserve room to grow to more complex
receivers, though not all at once.

If we roll out a story for constant folding, the set of possible
x naturally grows, without further trouble.  And when we
roll out type classes, then we can (again without much more
trouble) support non-constant “x” receivers, if their type
is able to nominate the appropriate S.T. handler witness.
In this I am assuming that witnesses will always be
constants relative to a given request for a witness.
The Parametric VM design provides a way to do this
even when the witnessed type is a type parameter.
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