GROOVY-9946: Add @Pure to mark constant result of method

[daniellansun]

See https://issues.apache.org/jira/browse/GROOVY-9946

[eric-milles]

If this annotation is a marker and not a transform, is there a better package for it? I am not understanding the value proposition here. The ticket does not describe why this would be useful. Is the recreation of GString value a performance problem in some context?

[daniellansun]

If this annotation is a marker and not a transform, is there a better package for it?

Any suggestion is welcome. Currently I put it with @KnownImmutable in the same package. /cc @paulk-asert

The ticket does not describe why this would be useful.

Currently we just cache literal of GString when its all values are immutable, but for more common cases, if the result of value's toString is constant, we could cache too.

[paulk-asert]

I am thinking this could be generalised. If instead of having a very specific class annotation, we had a @Pure or @Constant annotation and place it on the toString method itself, then the annotation could be used in more places. @Memoized could annotate the methods it generates (giving the caching optimization for free on memoized toString methods) and future type checkers could ensure that the return value of "pure" methods only comes from expressions involving constants or other pure methods - kind of what Frege does.

[daniellansun]

OK. Let me try to implement @Pure instead of @ConstantString.

[paulk-asert]

LGTM. The one thing which I think we should do is move the @Pure marker interface into a groovy.annotations package in its own groovy-annotations mandatory module. However, I can do that move later if you like. That would be a good place for other such marker interfaces to also go. Then folks writing Java libraries could make them more Groovy friendly in some places and only need worry about having that one module available as a compile-time only dependency for their builds.

[daniellansun]

@paulk-asert

LGTM. The one thing which I think we should do is move the @pure marker interface into a groovy.annotations package in its own groovy-annotations mandatory module. However, I can do that move later if you like.

Thanks for your reviewing and help ;-)

[paulk-asert]

We can probably swap the order of this clause and the next since this one is probably less common than next line.

[paulk-asert]

Merged, thanks!