<div class="im" style><blockquote class="gmail_quote" style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div style="word-wrap:break-word"><div>In the example that doesn't work, the first mult is needed for another use of ==>. In the simpler examples, it is not. Notice that the orphaned mult occurs on the second step when another use is expanded, while in the simpler examples another use of ==> does not occur. In the simple cases, mult is known to the expanded code; in the example that doesn't work, mult is not known to repeated use of the macro. Another clue is: if you run the test in the interactions window, it works because the definition of mult (in definitions) is automatically provided for interactions.</div>
<div><br></div><div>I haven't even read your macro; the amazing and amusing macro stepper knows all and tells all.</div><div><br></div></div></blockquote><div><br></div></div><div style>Your comment caused me to think about why the repeated calls to ==> would 'mult' to become "unknown". Eventually, I realized that the syntax->datum call was probably doing too much; i.e. it was stripping away too much information so when I built back up the new expression, the binding of 'mult' was lost. </div>
<div style><br></div><div style>At least, that's my guess. I think the Racket documentation is fantastic but I find a bit of experimentation is necessary to figure some things out and that's what I've come up with. (Perhaps an experience macrologist can comment?). </div>
<div style><br></div><div style>Anyhow, here's a version that works (er...at least it seems to work so far...):</div><div style><br></div><div style><div class="im" style="color:rgb(80,0,80)"><div>(define-syntax ==></div>
<div> (lambda (stx)</div><div> (define (replace-in-first exp-list new-val)</div><div> (let* ([new-exp </div><div> (map (ë (x) </div></div><div> (if (eq? (syntax-e x) '_)</div>
<div> new-val</div><div> x))</div><div> (syntax->list (car exp-list))) ]</div></div>