[Haskell-cafe] On the purity of Haskell

[Conal Elliott]

On Fri, Dec 30, 2011 at 8:12 AM, Conal Elliott <conal at conal.net> wrote:

> On Fri, Dec 30, 2011 at 12:52 AM, Heinrich Apfelmus <
> apfelmus at quantentunnel.de> wrote:
>
>> Conal Elliott wrote:
>>
>>> I wrote that post to point out the fuzziness that fuels many
>>> discussion threads like this one. See also http://conal.net/blog/posts/*
>>> *notions-of-purity-in-haskell/<http://conal.net/blog/posts/notions-of-purity-in-haskell/>and the
>>> comments.
>>>
>>> I almost never find value in discussion about whether language X is
>>> "functional", "pure", or even "referentially transparent", mainly
>>> because those terms are used so imprecisely. In the notions-of-purity
>>> post, I suggest another framing, as whether or not a language and/or
>>> collection of data types is/are "denotative", to use Peter Landin's
>>> recommended replacement for "functional", "declarative", etc. I
>>> included some quotes and a link in that post. so people can track
>>> down what "denotative" means. In my understanding, Haskell-with-IO is
>>> not denotative, simply because we do not have a
>>> (precise/mathematical) model for IO. And this lack is by design, as
>>> explained in the "toxic avenger" remarks in a comment on that post.
>>>
>>> I often hear explanations of what IO means (world-passing etc), but I
>>> don't hear any consistent with Haskell's actual IO, which includes
>>> nondeterministic concurrency. Perhaps the difficulties could be
>>> addressed, but I doubt it, and I haven't seen claims pursued far
>>> enough to find out.
>>>
>>
>> Personally, the operational semantics given in SPJ's "Tackling the
>> Awkward Squad" always struck me as an accurate model of how GHC performs IO.
>
>
> It might be accurate, but it's not denotational.  - Conal
>

Moreover, afaict, the "Awkward Squad" operational semantics tackles only a
tiny fraction of the IO type.

One of the strengths of Haskell IO is that it can be extended easily via
the FFI. And I guess that strength is also a theoretical weakness in the
sense that only a tiny fraction of the IO interface has even an operational
semantics.

  - Conal
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