At 15:37 31/12/03 +0000, Joe Thornber wrote:
On Wed, Dec 31, 2003 at 02:38:06PM +0000, Graham Klyne wrote:
getOrCachePositionValue pos = do { mcache <- gets (findPos pos) -- Query cache for position ; case mcache of Just cached -> return (cachedVal cached) -- Return cached value Nothing -> -- Not in cache: do { let val = calculatePosVal pos -- Calculate new value ; modify (addToCache pos val) -- Cache new value ; return val -- Return new value } }
(This code of off-the-cuff, and may contain errors)
My point is that the function 'calculatePosVal' used here to evaluate a position not in the cache simply returns the calculated value, not a monad. This function is wrapped in "high level" code that queries and/or updates the cache which is kept in a state monad. Thus, the return type of 'getOrCachePositionValue' would be a monad of the appropriate type.
But I want calculatePosVal to use the cache too :(
Well, maybe you really do need to run your calculation in the state monad, but I'll ask one more question: to you need your 'calculatePosVal' to *use* the cache, or to *update* it? If you simply need access to the cache, then the function could accept cache data extracted from the state monad as an additional argument; e.g. getOrCachePositionValue pos = do { cacheState <- get -- Get current cache state ; let mcache = findPos pos cacheState -- Query cache for position ; case mcache of Just cached -> return (cachedVal cached) -- Return cached value Nothing -> -- Not in cache: do { let val = calculatePosVal pos cacheState -- Calculate new value ; modify (addToCache pos val) -- Cache new value ; return val -- Return new value } } If your calculation really needs to update the cache state as it goes along, then I agree that it needs to be run in the state monad. But even then, I'd be inclined to look for sub-calculations that can be evaluated as ordinary functions. Anyway, I think I've probably added enough noise to this debate. Whatever approach you may use, have fun! #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact