Re: Making while interruptable

[Miles Bader <miles@gnu.org>]

Lennart Borgman <lennart.borgman@gmail.com> writes:
>>> The idea is that the user with some let bound variable can control how
>>> every call to `while' handles pending input. If pending input is
>>> detected then `while' will throw to a catch symbol that is let bound
>>> by the user.
>>
>> This is very, very, bad.  It _will_ screw up lots of code in hard to
>> debug and confusing ways.
>
> Could you tell me exactly why you think so?  Are you aware of that I
> placed the code for interruption at the same place as QUIT is placed
> in the c code. So I assumed that it was ok to jump out there

Most code is not prepared to deal with random non-local control
transfers.  In such cases, a quit _also_ has the potential to screw
something up, but a quit is much much rarer, is explicitly invoked by
the user, and the user knows that after quitting, things may be in a
funny state (it's an emergency action of sorts).  This limits the
problem enough that it's something we can live with.

The mere presence of _input_ on the other hand is _normal_, and your
flag is not connected with any explicit user action.  So with your
system, the number of unexpected non-local control transfers would go
_way_ up, and would not be obviously connected with any user action.

-Miles

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