On Fri, Dec 30, 2011 at 12:52 AM, Heinrich Apfelmus < apfelmus@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.
Best regards, Heinrich Apfelmus
It might be accurate, but it's not denotational. - Conal