Re: [Chicken-users] with-*-*-port

[Joerg F. Wittenberger]

true, but there is a problem though

Felix Winkelmann <address@hidden> writes:

> Tony Garnock-Jones wrote:
> >>(define with-input-from-port
> >>  (let ((values values))
> >>    (lambda (port thunk)
> >>      (let ((old ##sys#standard-input))
> >>        (##sys#dynamic-wind
> >>         (lambda () (set! ##sys#standard-input port))
> >>         thunk
> >>         (lambda () (set! ##sys#standard-input old))) ) ) ) )
> > Perhaps this would be better capturing standard-input on each
> 
> > transition, rather than just the first?
> > (Untested code follows)
> 
> > (define with-input-from-port
> 
> >   (let ((values values))
> >     (lambda (inner thunk)
> >       (let ((outer '*))
> >         (##sys#dynamic-wind
> >          (lambda ()
> >        (set! outer ##sys#standard-input)
> >        (set! ##sys#standard-input inner))
> >          thunk
> >          (lambda ()
> >        (set! inner ##sys#standard-input)
> >        (set! ##sys#standard-input outer)))))))
> > Does this make sense, or is it too paranoid?
> 
> It *is* paranoid, but it's the safest method.

True, it is paranoid, it is the safest method and it SHOULD be overly
paranoid, i.e., not needed:

1. The ##sys#standard-input is never changed at application level (it
   must be hidden from that level), only chicken internal code can
   change it.

2. Clean coding would never assign to ##sys#standard-input except by
   means of with-input-from-port and friends.

3. As long as this holds true, there will always be a propper nesting
   of BEFORE/AFTER hooks from ##sys#dynamic-wind.

Hence this safety should not be needed.  The code would have no actual
effect, would it?

If I'm right (skip conclusion otherwise), it would even harm for two
reasons:

a) It wastes some CPU cycles and memory objecst.  Not so much of a
   problem sometimes.

b) It would encourage to rely on the code and assign to
   ##sys#standard-* . I.e., it could lead to really bad coding
   practice.  And *this* would be bad.

Cheers

/Jörg

-- 
The worst of harm may often result from the best of intentions.