Add support for Java 8 syntax in equations and method bodies

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Issue #266 resolved
Jesper Öqvist created an issue

JastAdd currently supports Java 6 syntax, and parts of Java 7 syntax, in attribute equations and aspect methods. It would be great if full Java 8 support could be added.

Java 8's lambda expressions and method/constructor references are very useful for making Java code concise. These features could benefit JastAdd code too, by making attribute equations shorter. For example, here is a reachability attribute declared using lambdas:

  syn Set<FuncDecl> FuncDecl.reachable() circular [Collections.emptySet()] =
      calls().stream().flatMap(func -> Stream.concat(func.reachable().stream(),
          Stream.of(func))).collect(Collectors.toSet());

  coll Set<FuncDecl> FuncDecl.calls() [new HashSet<>()] root FuncDecl;

  Call contributes decl() to FuncDecl.calls();

If the same code were written in Java 6, it would require a loop to iterate over the calls set and build a new reachable estimate, requiring many more lines of code.

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