That's very attainable in Haskell and is often well-modeled off the ST monad. You also have a number of "Data types a la carte"-like methods which allows you to

That's very attainable in Haskell and is often well-modeled off the ST monad. You also have a number of "Data types a la carte"-like methods which allows you to build very specific effect contexts in order to precisely guarantee things like the fact that mutable array code never produces events in the UI. These can be achieved by products and compositions of applicative types, free monads, monad transformers, or the "sum of effects" "a la carte" method. Another big deal that comes out of these more specific effects is that you can have the compiler produce stream fusion to automatically optimize some inner loops, though I don't know how often that actually happens as of today. (Oh: if you want to play with "Data types a la carte" they're in the IOSpec package, http://hackage.haskell.org/package/IOSpec http://hackage.haskell.org/package/IOSpec and are described---somewhat densely---in http://www.staff.science.uu.nl/~swier004/Publications/DataTypesALaCarte.pdf http://www.staff.science.uu.nl/~swier004/Publications/DataTy...)