bugKawa - Bugs: bug #27042, Bad interaction between...


bug #27042: Bad interaction between syntax-rules and call-with-values

Submitted by:  Helmut Eller <ellerh>
Submitted on:  Thu 16 Jul 2009 05:53:09 PM UTC  
Category:  None Severity:  3 - Normal
Item Group:  None Status:  Fixed
Privacy:  Public Assigned to:  Per Bothner <bothner>
Open/Closed:  Closed

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Sun 06 Dec 2009 01:13:46 AM UTC, comment #1: 

Thanks for the bug report, and sorry for it taking so long!

If turned out to have nothing to do specifically with call-with-values - it actually was a bad interaction between syntax-rules and lambda.  The following example showed the same problem:

(let-syntax ((dlet
              (syntax-rules ()
;; var==x val==(+x 10) body==x
                ((dlet (var val) body)
                 (let ((saved var)) ; saved=0
                   (set! var val) ; x = 10
   ((lambda (result) ; result=10
                       (set! var result) ; x=10
                       (set! var saved) ; x=0
                       result) ; 10
  (let ((x 0))
    (display x)
     (dlet (x (+ x 10))
    (display x)

I tracked it down, and fixed it.  Fix and test-case checked in.

Per Bothner <bothner>
Project AdministratorIn charge of this item.
Thu 16 Jul 2009 05:53:09 PM UTC, original submission:  

This example

(let-syntax ((dlet
              (syntax-rules ()
                ((dlet (var val) body)
                 (let ((saved var))
                   (set! var val)
                   (call-with-values (lambda () body)
                     (lambda (result)
                       (set! var saved)
  (let ((x 0))
    (list x
          (dlet (x (+ x 10))

should return (0 10 0) but returns (0 10 10). 
The intention of dlet is to temporarily rebind the variable.
Sorry, couldn't come up with a simpler expression.

If it helps: referencing var inside the second lambda, e.g. with
something like (set! var var), works around the problem.

Helmut Eller <ellerh>


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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2009-12-06 bothner Assigned toNone => bothner
        Open/ClosedOpen => Closed
        StatusNone => Fixed

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