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/ 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.

- Conal

On Thu, Dec 29, 2011 at 4:42 PM, Steve Horne <sh006d3592@blueyonder.co.uk> wrote:

On 30/12/2011 00:16, Sebastien Zany wrote:
Steve Horne wrote:

I haven't seen this view explicitly articulated anywhere before

See Conal Elliott's blog post The C language is purely functional.

Thanks - yes, that's basically the same point. More concise - so clearer, but not going into all the same issues - but still the same theme.

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