Date: 2013-04-20 05:29 am (UTC)
winterkoninkje: shadowcrane (clean) (Default)
The problem is that monads are everywhere. Everything is an effect. I think it's a bit myopic to focus only on things like state, IO, and exceptions which model the blatantly effectful structure of imperative languages. (That's how DDC started out, btw.) Continuations are a perfectly good effect, and is part of the blatantly effectful structure of Scheme et al. Nondeterminism is a perfectly good effect, and is part of the blatantly effectful structure of logic programming. Things like unification and futures are also perfectly good effects; building futures into the language was essentially the whole selling point for Oz and AliceML! And of course laziness is a perfectly good effect. Not to mention Lindsey and Ryan's work on deterministic parallelism. Nor all the recent work on memory regions. ...

The problem, of course, is that monads do not compose. That is, in general we do not have that m (n a) is isomorphic to n (m a). This is a major problem that has been known for quite a while. Things like monad transformers give us a bit of compositionality, but they provably can't go far enough. There are a few other categorical goodies people have tried using to improve the compositionality/modularity aspect of things, but none of them appears to have been especially promising in the long run.

I don't have a reference for MacQueen's talk, alas. Oleg's rants include this one about Lazy IO and unsafeInterleaveIO, which was discussed to death on reddit recently, and his followup which follows my suggestion on reddit that really the problem already exists for unsafeInterleaveST. At this point, I'm not sure how worthwhile the reddit comments will be to sift through; there's a fair bit of nuhuh-ing and handwringing which doesn't make any intellectual progress on the actual issues that Oleg brings up. I do think, though, that the thread me and Lennart Augustsson had touches on some important issues at stake in this whole line of argumentation about referential transparency (and what we mean by RT, or what we want to mean by RT, and why RT isn't the be-all-and-end-all of programming).
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