RFR: JDK-8285932 Implementation of JEP-430 String Templates (Preview) [v11]

[Jorn Vernee]

On Wed, 2 Nov 2022 19:44:00 GMT, Jim Laskey <jlaskey at openjdk.org> wrote:

>> Enhance the Java programming language with string templates, which are similar to string literals but contain embedded expressions. A string template is interpreted at run time by replacing each expression with the result of evaluating that expression, possibly after further validation and transformation. This is a [preview language feature and API](http://openjdk.java.net/jeps/12).
>
> Jim Laskey has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Internalize TemplateSupport

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 1103:

> 1101:         }
> 1102: 
> 1103:         MethodHandle mh = MethodHandles.dropArguments(newString(), 2, ttypes);

The code here us pretty dense. I suggest adding comments that show the type of `mh` as far as it is known. I think it makes it a much easier to keep track of what all the argument indexes mean. e.g.
Suggestion:

        // (byte[],long,ttypes...) -> String
        MethodHandle mh = MethodHandles.dropArguments(newString(), 2, ttypes);

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 1118:

> 1116:             MethodHandle prepender = prepender(lastFragment.isEmpty() ? null : fragment, ttype);
> 1117:             initialLengthCoder = JLA.stringConcatMix(initialLengthCoder, fragment);
> 1118:             mh = MethodHandles.filterArgumentsWithCombiner(mh, 1, prepender,1, 0, 2 + pos);

Suggestion:

            Class<?> ttype = ttypes[pos];
            // (long,byte[],ttype) -> String
            MethodHandle prepender = prepender(lastFragment.isEmpty() ? null : fragment, ttype);
            initialLengthCoder = JLA.stringConcatMix(initialLengthCoder, fragment);
            // (byte[],long,ttypes...) -> String (unchanged)
            mh = MethodHandles.filterArgumentsWithCombiner(mh, 1, prepender,1, 0, 2 + pos);

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 1125:

> 1123:         MethodHandle newArrayCombinator = lastFragment.isEmpty() ? newArray() :
> 1124:                 newArrayWithSuffix(lastFragment);
> 1125:         mh = MethodHandles.foldArgumentsWithCombiner(mh, 0, newArrayCombinator,

Suggestion:

        // (long,ttypes...) -> String
        mh = MethodHandles.foldArgumentsWithCombiner(mh, 0, newArrayCombinator,

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 1143:

> 1141:                         0, // old-index
> 1142:                         1 + pos // selected argument
> 1143:                 );

Suggestion:

            // (long,ttype) -> long
            MethodHandle mix = mixer(ttypes[pos]);
            boolean lastPType = pos == ttypes.length - 1;

            if (lastPType) {
                // (ttype) -> long
                mix = MethodHandles.insertArguments(mix, 0, initialLengthCoder);
                // (ttypes...) -> String
                mh = MethodHandles.foldArgumentsWithCombiner(mh, 0, mix,
                        1 + pos // selected argument
                );
            } else {
                // (long,ttypes...) -> String
                mh = MethodHandles.filterArgumentsWithCombiner(mh, 0, mix,
                        0, // old-index
                        1 + pos // selected argument
                );

-------------

PR: https://git.openjdk.org/jdk/pull/10889