Stack usage with a state monad

[Joe Thornber]

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