On 12/14/11 10:58 PM, Gregory Crosswhite wrote:
Of course, this is not a simple change at all because it would have to be done in such a way as to respect the ordering of actions --- that is, we can't have each action executed only when the corresponding element of the list demanded is forced, or else actions would undesirably interleave.
Therein lies the issue. To put this in a monadic context, this is the same reason why we can't just say: evalState (repeatM getNext) init e.g., to generate an infinite list of pseudorandom numbers and then discard the final seed because we have all the numbers we'll ever need. Hidden lurking in this expression is the fact that the state being passed around eventually becomes bottom. We don't especially care, since evalState is discarding the state, but the fact remains that we have to compute it. Indeed, we can't even define 'repeatM' sensibly, for the same reason. We can only compute a list of responses lazily if we happen to be in a monad/applicative where we can guarantee that pulling all the effects up to the top is equivalent to performing them lazily/interleaved. However, even in the cases where that can be guaranteed, we don't have any especially good mechanism for informing GHC about that fact. The reason why 'some' and 'many' can escape this ---in the case of parsers at least--- is that they will run for a (deterministic) fixed length of time and then return. If you're parsing any finite-length text, then there's an upper bound on how many times the action can run before it fails. This is the same reason why we can define 'replicateM' even though we can't define 'repeatM'. The only difference is that with 'replicateM' the termination criterion is extrinsic to the monad (it's induction on Int), whereas with 'some' and 'many' the termination criterion is intrinsic to whatever the side-effects of the action are. The problem is that for Maybe and lists, there is no intrinsic state which would allow an action to succeed sometimes and fail other times (thereby providing an intrinsic means for termination). -- Live well, ~wren