[Haskell-cafe] Are bottoms ever natural?

[Alexey Muranov]

Siddharth,

how would you deal with functions that terminate for some 
arguments/inputs but do not terminate for the others ?

Alexey.

On Tue, 2017-12-19 at 07:20 +0000, (IIIT) Siddharth Bhat wrote:
> > Is that really true, though?
> > Usually when you have an infinite loop, you have progress of some
> > sort. Infinite loops with no side effects can be removed from the
> > program according to the C standard, for example. So, in general, we
> > should allow programmers to express termination / progress, right? 
> At
> > that point, no computation ever "bottoms out"?
> > Shouldn't a hypothetical purely functional programming language
> > better control this (by eg. Forcing totality?) It seems like we lose
> > much of the benefits of purity by muddying the waters with
> > divergence.
> > From an optimising compiler perspective, Haskell is on some weird
> > lose-lose space, where you lose out on traditional compiler
> > techniques that work on strict code, but it also does not allow the
> > awesome stuff you could do with "pure" computation because bottom
> > lurks everywhere.
> > What neat optimisations can be done on Haskell that can't be done in
> > a traditional imperative language? I genuinely want to know.
> > What are your thoughts on this?
> > Cheers
> > Siddharth
> >
> > On Tue 19 Dec, 2017, 08:09 Brandon Allbery, <allbery.b at gmail.com>
> > wrote:
>> > > Define "natural".
>> > >
>> > > You might want to look into the concept of Turing completeness. 
>> One
>> > > could define a subset of Haskell in which bottoms cannot occur...
>> > > but it turns out there's a lot of useful things you can't do in
>> > > such a language. (In strict languages, these often are expressed 
>> as
>> > > infinite loops of one kind or another. Note also that any
>> > > dependency on external input is an infinite loop from the
>> > > perspective of the language, since it can only be broken by the
>> > > external entity providing the input.)
>> > >
>> > > On Tue, Dec 19, 2017 at 1:47 AM, (IIIT) Siddharth Bhat 
>> <siddharth.b
>> > > hat at research.iiit.ac.in> wrote:
>>> > > > I've been thinking about the issue of purity and speculation
>>> > > > lately, and from what little I have read, it looks like the
>>> > > > presence of bottom hiding inside a lazy value is a huge issue.
>>> > > >
>>> > > > How "natural" is it for bottoms to exist? If one were to 
>>> change
>>> > > > Haskell and declare that any haskell value can be speculated
>>> > > > upon, what ramifications does this have?
>>> > > >
>>> > > > Is it totally broken? Is it "correct" but makes programming
>>> > > > unpleasant? What's the catch?
>>> > > >
>>> > > > Thanks,
>>> > > > Siddharth