[Haskell-cafe] Are bottoms ever natural?

[Brandon Allbery]

I'm tempted to refer you to the "Floop and Bloop and Gloop" chapter in
_Gödel, Escher, Bach: an Eternal Golden Braid_.

How exactly do you prove to the compiler that you have made sufficient
progress? It's not enough to assume that doing any particular operation
constitutes progress. And even in cases where you can provide such a proof,
it usually requires dependent types to express because "progress" depends
on some value. Consider that any given action may appear to have no side
effects even in C unless the entire program is considered as a single unit.
And also consider that Haskell expressions usually do not have "side
effects" in this sense at all, unless you are in IO... and now you have to
formalize what an IO "side effect" is in order to prove that you have
actually accomplished something.

On Tue, Dec 19, 2017 at 2:20 AM, (IIIT) Siddharth Bhat <
siddharth.bhat at research.iiit.ac.in> 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.bhat 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
>>> --
>>> Sending this from my phone, please excuse any typos!
>>>
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>>
>>
>>
>> --
>> brandon s allbery kf8nh                               sine nomine
>> associates
>> allbery.b at gmail.com
>> ballbery at sinenomine.net
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>> _______________________________________________
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>
> --
> Sending this from my phone, please excuse any typos!
>



-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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