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 :( - Joe