Henning Thielemann
If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad?
The question is what "right order" means. Let B1..Bn be some arbitrary IO-Actions. Let A1..An be some arbitrary IO Actions passed to unsafeInterleaveIO You're guaranteed that a) Bk+1 is executed after Bk b) Ak+1 is executed after Ak , all by virtue of the IO Monad. However, usage of unsafeInterleaveIO means that you aren't guaranteed that A1..An happens _exactly_ between any pair of Bk and Bk+1: The total ordering of actions is circumvented. Semantically, this is equivalent to sparking a thread on a uniprocessor, and equivalent to sparking a thread on a multiprocessor for all but the rarest combinations of IO actions. There are no lazy monads. Monads imply explicit sequencing... writing kiss girl =><= speakTo girl , denoting "either (fist kiss the girl, then pass the result of that to speakTo girl) or (first speak to the girl, then pass the result of that to kiss girl) would, OTOH, certainly makes sense and certainly (well, depending on girl and setting) is undeterministic in both execution order and result. The question thus becomes: "Does arbitrary ordering of actions result in the same (in the id sense) result, and do I care more about the aggregate result than its ordering, or do I only care about ordering of actions for a certain definition of 'same result'?" Implementation of such a beast is left to the semantically inclined reader ;) In the meantime, I'm happy to just carelessly use unsafe*IO. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.