问题
I am trying to learn how to use the DrRacket debugger's annotate function. My ultimate aim is to build a REPL that you can execute from within a closure and have access to everything that's in scope. (see my previous question on that topic and Greg Hendershott's well-researched answer) For the time being, I'm just trying to explore how the annotate function works. You can see my first exploratory attempt at using it, and the results, here.
The error, which is happing inside of the annotator, seems to arise when it tries to match he application of string-append. The matcher is looking for a #%plain-app and the expanded syntax I'm presenting to it contains an #%app. I'm unsure if I should be expanding the syntax differently so it comes out as a #%plain-app or if there's something else I'm doing wrong to produce the syntax I'm feeding into the annotator. Does anybody see where my error is?
回答1:
This revision to my previous pasterack post is swallowed without complaint. It seems that the syntax match must take place on a top-level syntax object (ruling out anything that could happen in an expansion phase, like a macro), and expansion must take place with a current namespace attached. There are some more subtleties in the syntax match, particularly around the fact that the syntax object needs to be free-identifier=? to #%plain-app. For all the gory details, refer to the mailing list thread I started.
来源：https://stackoverflow.com/questions/21151678/how-ought-i-run-the-annotate-function-in-gui-debugger-annotator-on-a-datum