Hi Josef, You wrote:
...Experience tells us that most programmers program as if >>= were strict and the lazy version usually gives problems. That is why, for instance, the ST monad is strict by default.
That is only true for IO. And for ST, which is really just IO restricted to IORefs. In these unusual monads that interact with the Real World, one expects strictness. The monads of the mtl library - and most others - are meant for pure programs that lend themselves to monadic paradigms. Here one expects laziness, as usual in Haskell. Many monad beginners do seem to make the mistake you are referring to. The reason is that they start by using only the IO monad before learning much else about what monads are for. I think that will improve once we begin to see more introductions to Haskell geared to practical programmers.
As an example, consider a server of some sort with an inner loop using a state monad. Making
= lazy will mean that the server will inevitably run out of memory.
Servers interact with the Real World, so you would use IO and ST. But you are right, it would be great to have special strict versions of various monads (and containers like Data.Map) for these situations. Regards, Yitz