Re: ``Update functions'' in Scheme.

[Oliver Laumann <net%TUB.BITNET@MITVMA.MIT.EDU>]

> [example where a function (setter accessor) obtains the mutator for
> a given accessor function by table lookup]

> The key point here is that Scheme already has enough power to express
> the abstract relationship between accessors and mutators; there is no
> need to "extend" the language to provide this feature.

If this is true, then please tell me how you would do the following
using the method you described.

Suppose I want to write a new print function that involves a ``print length''.
The function (print-length) should serve both as an accessor and as a
mutator for the actual print length.  That is,

   (print-length)

simply returns the current print length, while

   (set! (print-length) 20)

should be used to change the print length (changing it would include some
kind of sanity check).

Using the extension to Scheme I outlined in a previous message, the actual
variable holding the print length (say, *print-length*) could be a local
variable to the combined accessor/mutator function:

   (define print-length
      (let ((*print-length* 100))
        (lambda ...


How would you do something like this using the method you proposed
without employing a global variable?

--
Regards,
    Oliver Laumann, Technical University of Berlin, Germany.
    ...!pyramid!tub!net   or   net@TUB.BITNET