I have a couple of files written in Chez Scheme, each about a thousand lines. When I try to load a file into the REPL:
> (load "filexxx.scm")
...
Exception: variable A is not bound
Type (debug) to enter the debugger.
How can I figure out which line from which file triggered this exception?
I mainly use Racket, where DrRacket would have pointed me to the line causing the exception. Is there a similar way to find where the undefined variable A is being used in Chez?
Obtaining source location for exceptions can be frustrating in Chez Scheme. In this particular case, it may work to define identifier syntax for A that reports the location.
filexxx.scm
(define a 1)
(define b (+ A 1))
(define c 3)
(define-syntax A
(make-variable-transformer
(lambda (x)
(errorf #f "variable ~a is not bound at ~s"
(syntax->datum x) (annotation-source (syntax->annotation x))))))
(load "filexxx.scm")
Exception: variable A is not bound at #<source filexxx.scm[26:27]>
Type (debug) to enter the debugger.
This indicates A is referenced at 0-based character offset 26 in filexxx.scm.
Alternatively, you can use the debugger to gain context, though maybe not a line or character number.
If A occurs in a variable definition ...
filexxx.scm
(define my-var (+ A x))
... you'll likely find yourself in the compiler, as seen below. (Enter i to inspect the continuation, then sf to list frames.)
> (load "filexxx.scm")
Exception: variable A is not bound
Type (debug) to enter the debugger.
> (debug)
debug> i
#<system continuation in map> : sf
0: #<system continuation in map>
1: #<system continuation in compile>
2: #<system continuation in map>
3: #<system continuation in compile>
4: #<system continuation in compile>
5: #<system continuation in ksrc>
6: #<system continuation>
7: #<system continuation in dynamic-wind>
8: #<system continuation in dynamic-wind>
9: #<system continuation in $reset-protect>
10: #<system continuation in new-cafe>
#<system continuation in map> :
Use d and s to walk and inspect the stack frames until you see something familiar.
#<system continuation in map> : s
continuation: #<system continuation in compile>
frame and free variables:
0: #<procedure compile>
1: (#[#{Lsrc:Expr:call czsa1fcfzdeh493n-3-44} 44 #[...] #[...] (...)])
2: ()
#<system continuation in map> : d
#<system continuation in compile> : s
continuation: #<system continuation in map>
frame and free variables:
0: #[#{primref a0xltlrcpeygsahopkplcn-2} + 19834 (-1)]
#<system continuation in compile> : d
#<system continuation in map> : s
continuation: #<system continuation in compile>
frame and free variables:
0: #{my-var *top*:my-var}
1: ()
Here, we see my-var, which provides a clue.
On the other hand, if A occurs somewhere other than a variable definition ...
filexxx.scm
(define (my-procedure x)
(+ A x))
(my-procedure 1)
... you'll likely find yourself at the site of the reference.
> (load "filexxx.scm")
Exception: variable A is not bound
Type (debug) to enter the debugger.
> (debug)
debug> i
#<continuation in my-procedure> : sf
0: #<continuation in my-procedure>
1: #<system continuation in ksrc>
2: #<system continuation>
3: #<system continuation in dynamic-wind>
4: #<system continuation in dynamic-wind>
5: #<system continuation in $reset-protect>
6: #<system continuation in new-cafe>
Here, we see my-procedure, another clue. And showing the frame with s reveals the expression within which A occurs.
#<continuation in my-procedure> : s
continuation: #<system continuation in ksrc>
procedure code: (lambda (x) (+ A x))
call code: A
frame and free variables:
0. x: 1
