<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; charset=iso-8859-1"> <META content="MSHTML 6.00.6000.16939" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV><FONT face="Courier New" size=2>Consider the following code extracted from a larger piece of code:</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>#lang scheme</FONT></DIV><FONT face="Courier New" size=2> <DIV><BR>(define-syntax (make-box stx)<BR> (syntax-case stx ()<BR>  ((_) #`#,(box 1))))</DIV> <DIV><BR>(define my-box (make-box))</DIV> <DIV><BR>(immutable? my-box) ; --> #t</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>The box delivered by the expander appears to be immutable, which seems appropriate. However, I do not understand how in this case an immutable box is produced. I checked with check-syntax. Indeed:</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>(define my-box (make-box)) ==> (define-values (my-box) '#&1)<BR><BR>But inspecting the expansion of (define-syntax (make-box stx) ...) I don't see how transformer make-box produces an immutable box.</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>NB. This is not a question how to produce a mutable box, for I could easily write</FONT></DIV> <DIV><FONT face="Courier New" size=2>#'(box 1)) in stead of</FONT></DIV> <DIV><FONT face="Courier New" size=2>#`#,(box 1) or even simpler:</FONT></DIV> <DIV><FONT face="Courier New" size=2>(define (make-box) (box 1))</FONT></DIV> <DIV><FONT face="Courier New" size=2>I am simply interested in the background mechanism.</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>Thanks, Jos</FONT></DIV></BODY></HTML>