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
_______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
