[racket] arrows in macro stepper

Eric Tanter wrote:
> Hi,
> 
> A simple question: what exactly are those arrows with question mark
> in the macro stepper? (see picture below) I noticed that they seem to
> appear only if there is an occurrence of the same identifier with a
> different color in between (ie. in the example below, if (display it)
> is not there, then the second arrow is blue, and without question
> mark).
> 
> I couldn't find a precise explanation in the guide.

The blue arrows indicate "definite" references and the purple arrows 
indicate "apparent" references. A reference is definite when it lies in 
the exapnded part of the program. In the unexpanded part of the program 
(approximately, the part after the redex), things that look like 
references might be changed by intervening macros. They might become 
binding occurrences themselves, or expansion might uncover a binding 
closer than the one apparent before expansion starts.

To illustrate, run the following program in the macro stepper:

   #lang racket
   (define (x)
     (cons x
           (let ([x 1])
             (cons x
                   (let ([x 2])
                     x)))))

Set the macro hiding policy to "Custom"; click any occurrence of 'let' 
in the program and click "Show macro". At first, all of x's seem to 
refer to the module-level binding, but as macro expansion progresses it 
discovers the binding structure. Some purple arrows turn blue and others 
disappear as new bindings are uncovered.

Check Syntax also has purple arrows that point into syntax constants 
(eg, macro templates). They mean roughly the same thing: who knows how 
apparent references in the syntax will eventually turn out?

Ryan