Replacing placeholders in a tree

[Simon Ochsenreither]

Hi Tony,

I have a language which looks a bit like this:

"test" match {

  if _ == "bar" then ...

  if _.size < 5 then ...

  else ...

}

Given

MatchExpr(subject: Expr, ifBranches: List[Expr], /*let's ignore the then expr for now */ elseBranch: Expr)

, the AST looks like this:

MatchExpr(
  Literal("test"),

  List(

    BinaryExpr(Placeholder, EQ, Literal("bar")),

    BinaryExpr(Call(Placeholder, Method("size")), LT, Literal(5))

  ),
  ...
)

What would be the most idiomatic way in Kiama to replace the Placeholder with the subject?

Basically I want to say something like "given a match expression, I want to traverse all nodes under it, and replace some of them (placeholders) with a value from the match expression (subject)", so that I get a tree like this:

MatchExpr(
  Literal("test"),

  List(

    BinaryExpr(Literal("test"), EQ, Literal("bar")),

    BinaryExpr(Call(Literal("test"), Method("size")), LT, Literal(5))

  ),
  ...
)

I'm a bit unsure how to capture the subject or pass the context (the subject) around with the the right macro invocation. Do you have any suggestions?

Thanks,

Simon

[Tony Sloane]

Hi Simon,

This sounds like a pretty standard application of a recursive traversal operator. E.g., if you really want to do the replacement everywhere then try the everywhere combinator which does a top-down traversal.

Here’s a version from the Kiama JSON example

https://bitbucket.org/inkytonik/kiama/src/default/library/src/test/scala/org/bitbucket/inkytonik/kiama/example/json/Rewriter.scala

rewrite(everywhere(rule[(JName, JNumber)] {

      case (n @ JName("salary"), JNumber(d)) =>

             (n, JNumber(d / 2))

}))(c)

This finds all pairs of JName and JNumber values where the JName refers to “salary” and halves the salary. c is the root of the structure that you want to rewrite in.

cheers,

Tony

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