Re: `unreadablep'

[Ihor Radchenko]

Po Lu <luangruo@yahoo.com> writes:

>> Do you imply that `print-unreadable-function' should return the new
>> value to be printed? If so, it is indeed more flexible. Passing the
>> unreadable value to the function would also be useful (e.g. we can print
>> buffer file name instead of buffer object).
>
> It would accept a function that is called to print characters, like the
> `printcharfun' argument to prin1.

I was confused by the name. `printcharfun' in prin1 can be a buffer, a
marker, etc. Doing the same for the `print-unreadable-function' makes it
tricky to implement. How does `print-unreadable-function' print into a
buffer or marker? It would be cumbersome to consider all such cases if
the user simply wants to print nil in place of unreadable objects.

If the `printcharfun' passed to `print-unreadable-function' is strictly
an ordinary function that does actual printing, I can see a problem with
circular objects. Consider something like
(#1=a #<unreadable> #2=b #1 #1 #2).
If the main print loop is not aware of what was printed by
`print-unreadable-function' and the function tries to print reference to
b, we may get incorrect object:
(#1=a b #2=b #1 #1 #2)
The object printed instead of unreadable value will break circular
structure.

Of course, the described problem will depend on the details of
implementation.

Best,
Ihor