Brian Hurt wrote:
This is probably an off-topic question, but I can't think of a better forum to ask it: does the existance of monads imply laziness in a language, at least at the monadic level?
Consider the following: a purely functional, eagerly evaluated programming language, that uses monads to encapsulate the awkward squad. In this programming language, a program generates an IO monad whose encapsulating computation performs side effecting affections- it writes to stdout, say. But this generated monad never has it's value evaluated- the monad is tossed away uninspected. Does the side effect happen? If the answer is at least potientially "no", then monads are lazily evaluated, and thus monads imply laziness (at least at the monadic level). On the other hand, if the answer is "yes", then monads do not imply laziness.
Thanks, Brian
Firstly, this question is not off-topic. Next, strictness and purity have nothing to do with monads. Finally, monadic values -represent- actions, they aren't the actions themselves. If you throw away a string representing some action that you were going to pass to an interpreter, the action it represents never happens, but this has nothing to do with laziness. Monadic IO in Haskell can be understood very much like that. The value of main gets passed to an IO interpreter, and all the work on the Haskell side is just building expressions for that interpreter (and thus it is now easy to see why this preserves purity). Laziness is not involved at all (except, as one person found out on c.l.f a long time ago, (>>) is less useful in a strict language than in a pure one, e.g. repeatM_ m = m >> repeatM_ m doesn't do anything in a strict language).