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Can SKILL++ symbols be unbound?

tweeks

In SKILL mode, you can unbind a symbol's value slot by assigning the special symbol unbound to it:

> f = 3
3
> boundp 'f
t
> f = 'unbound
unbound
> f
*Error* toplevel: undefined variable - f
<<< Stack Trace >>>
> boundp 'f
nil

You can also unbind the function slot by putting nil there:

> putd 'f (getd 'car)
primop:car
> fboundp 'f
primop:car
> (f '(1 2 3))
1
> putd 'f nil
nil
> fboundp 'f
nil
> (f '(1 2 3))
*Error* eval: undefined function - f

However, in SKILL++ mode, symbols use the function slot exclusively, so putting nil there is the same as assigning the symbol to nil.  Continuing from the previous example:

> toplevel 'ils
ILS-> f
nil

Is there a way to actually undefine a SKILL++ symbol?  It seems you would need to unintern it from the SKILL oblist and/or remove it from the toplevel environment frame.

  • Tom,

    It's slightly tricky - you have to define a SKILL (not SKILL++ function) with the same name, and then putd() the symbol to nil. Since this is an internal implementation detail and not exactly obvious, I've been talking with R&D about providing a macro or syntax form to do this which works in both SKILL and SKILL++ (ILS) modes.

    Here's a macro that I wrote that you can use in the meantime which does the job:

    (defmacro CCFunbind (varName)
  `(if (theEnvironment)
     (inSkill 
       ;-----------------------------------------------------------------
       ; Bit messy as can't swallow warning from the defun.
       ; The idea is that it defines a function for the variable name
       ; and then sets it to nil afterwards.
       ;-----------------------------------------------------------------
       (let ((nullport (outfile "/dev/null")))
         (unwindProtect
           (let ((woport nullport))
             (defun ,varName () nil) 
             (putd (quote ,varName) nil))
           (close nullport)
           )))
     (setq ,varName 'unbound)))
    With this, you can just do:

     

    a=23
    CCFunbind(a)

    and it will work in either mode - it will unbind a top level variable.

    Regards,

    Andrew.

  • The CCR number for my request for such a function in SKILL is 1320516.

    Andrew.

  • This is cool.  :)  Thanks Andrew!

    I learned another trick from you: temporarily rebinding woport for situations where getWarn isn't enough.

    Also, using (if (theEnvironment) ...) to check for SKILL++ mode.

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