"interact" behaves oddly if used interactively

[Olaf Chitil]

Robert Ennals wrote:
> > No, optimistic evaluation does not work well with interact, because it
> > causes the input stream to be evaluated (and therefore demanded) earlier
> > than you would expect.  This is the problem: interact exposes more than
> > just non-strictness, it exposes laziness.
> >
> > In Robert Ennals' implementation of optimistic evaluation he has to fall
> > back to lazy evaluation for lazy I/O, precisely because of this problem.
> 
> Or to put it another way.
> 
> Optimistic Evaluation works fine with "interact". You can write programs that
> use interact, evaluate them optimistically, and they will behave exactly as
> they always did.

Good to hear that ;-)

I still do not quite agree with Simon that 'interact' exposes anything
but non-strictness. Non-strictness means that

  map toUpper _|_ = _|_
  map toUpper ('a':_|_) = ('A':_|_)
  map toUpper ('a':'b':_|_) = ('A':'B':_|_)

and 'interact (map toUpper)' is a great way to experience this property.

However, you can also experience the property without 'interact',
evaluating expressions like

  take 2 (map toUpper ('a':'b':undefined))

I suppose Simon finds it annoying that optimistic evaluation has to deal
specially with 'interact' (or actually the primitive that really
implements it). I do not find that surprising. When you define an
evaluation strategy you have to define it for all language constructs,
including special primitives. If many of the special primitives don't
need special treatment, that is nice, but it cannot be expected in
general.

Olaf

-- 
OLAF CHITIL, 
 Dept. of Computer Science, The University of York, York YO10 5DD, UK. 
 URL: http://www.cs.york.ac.uk/~olaf/
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