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The toolset zenspider and I built into ParseTree for rewriting Ruby’s AST allows us to make features work that otherwise would be very difficult by rewriting them into easier to implement features.

One feature that makes Ruby so clean is string interpolation but currently ruby2c doesn’t support it. In Ruby there are two types of String nodes, a plain :str node ([:str, "mystring"]) for regular strings and a :dstr node that contains all the parts of the dynamic string which the interpreter concatenates ([:dstr, "val: ", [:vcall, :val]]).

Implementing :dstr in a runtime would be too hard to be worthwhile so I used SexpProcessor to rewrite a :dstr into a series of calls to #<< and #to_s that append to the leading :str node.

Now a dynamic string like:

[:dstr,
 "a",
 [:vcall, :x],
 [:str, "b"],
 [:vcall, :y],
 [:str, "c"]]

Gets transformed into this ugly sexp:

s(:call,
  s(:call,
    s(:call,
      s(:call,
        s(:str, "a"),
          :<<,
          s(:arglist,
            s(:call, s(:call, nil, :x, nil), :to_s, nil))),
        :<<,
        s(:arglist,
          s(:call, s(:str, "b"), :to_s, nil))),
      :<<,
      s(:arglist,
        s(:call, s(:call, nil, :y, nil), :to_s, nil))),
    :<<,
    s(:arglist,
      s(:call, s(:str, "c"), :to_s, nil)))

I don’t know about the inside-outness of it though, I may have to fix that.