IO and ST RealWorld are indeed represented the same--in today's GHC. It would be possible to implement the same ST interface differently (e.g., as an operational monad). Adding ioToST to the public interface commits not only GHC, but also future Haskell implementations, to a similar representation. Note: I recently noticed a real difference between IO and ST with regard to strictness analysis. In IO, m >>= undefined is *not* bottom: it may perform an observable action. In ST, it is bottom; anything it mutates is guaranteed to vanish in a puff of smoke. So assuming that these have the same representation actually introduces some complications for optimization. On Jul 20, 2017 1:36 PM, "Andrew Martin" wrote: My understanding is that `ST RealWorld` and `IO` are genuinely the same thing. But I'd want someone with better understanding to corroborate that claim. On Thu, Jul 20, 2017 at 12:02 PM, David Feuer wrote:

This seems to commit to ST and IO really being the same. Does stToIO already make this commitment? I can't tell, but if not, I oppose making that commitment in the "public" interface.

On Jul 20, 2017 11:51 AM, "Andrew Martin" wrote:

There's a function stToIO that is defined in GHC.IO and exported from Control.Monad.ST. There's another function, ioToST, that's also defined in IO, but it isn't exported from Control.Monad.ST. I propose that it be exported from Control.Monad.ST.

-- -Andrew Thaddeus Martin

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -Andrew Thaddeus Martin