On Mon, Nov 3, 2014 at 10:02 PM, Jeffrey Brown <jeffbrown.the@gmail.com> wrote:

Question 1: Are monad stacks "transparent" -- that is, can one use a layer in the middle of a monad stack without thinking about other layers?

In learning about how to stack multiple monads (or more literally, how to stack monad transformers on top of a monad), I was expecting that in order to reach into the middle of the stack, one would have to stay aware of the order in which the stack was created, and unwrap outer transformers to get there.

If you're doing it manually, yes, that would be true. Newtype deriving can be used to hide it, assuming that the levels are distinct enough: you can't easily pick between two MonadReader-s, for example, without lifting past one of them (and therefore having to at least know what order they are in).

Is that true in general? That is, if I build on top of a monad M a stack of monad transformers T1 T2 T3 ... TN M, and each of the Tn have different functions associated with them, call I call those functions without even remembering the order in which the transformers were stacked? Does the order of the stack constrain the programmer in how they chain execution functions like runReader, but not otherwise?

If you use newtype deriving, you can mostly avoid depth or ordering. One remaining sticky case is that of `fail`; if a monad in the middle calls `fail`, state below that monad *may* be lost. (But `fail` is rather badly behaved in general, and is best avoided.)

Stated differently: Monadic code can call pure code. Can pure code ever call monadic code?

Something like runReaderT or runST? Or in the extreme case, unsafePerformIO --- which is unsafe for a reason, and trying to use it to sneak into IO from pure code will usually cause major problems, because pure code does not expect non-deterministic results. Otherwise, the type system does its best to avoid you unexpectedly dipping into e.g. IO from inside pure code.

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net