Dear list, I decided to extract some common code I use in my data analysis code and end up with some interesting pattern.
-- | Confined array operation. The type Arr here is actually used as -- a C-like array with index runs from 0 to n-1. type Arr = Array Int
-- | Element fetching function, no bounds checking. Works with array -- with index [0..n-1] (!>) :: Arr e -> Int -> e (!>) = unsafeAt {-# INLINE (!>) #-}
-- | Length of the array. arrLength :: Arr e -> Int arrLength = numElements {-# INLINE arrLength #-}
-- | Sequentially loop over all elements in an array. loopArrM :: (Monad m) => (e -> m a) -- ^ function to apply to elements -> Arr e -- ^ the array -> m () loopArrM = loopArrMp (\ _ -> True) {-# INLINE loopArrM #-}
-- | Same as loopArrM but only to those with index satisfies predicate. loopArrMp :: (Monad m) => (Int -> Bool) -- ^ predicate -> (e -> m a) -- ^ function to apply to elements -> Arr e -- ^ the array -> m () loopArrMp p f arr = loopArrMG 0 end p f arr where end = arrLength arr {-# INLINE loopArrMp #-}
-- | Same as loopArrM but only to indices within a range. loopArrMr :: (Monad m) => Int -- ^ start index -> Int -- ^ end index -> (e -> m a) -- ^ function to apply to elements -> Arr e -- ^ the array -> m () loopArrMr start end = loopArrMG start end (\ _ -> True) {-# INLINE loopArrMr #-}
-- | Generic loopArrM for indices with in a range [start, end-1] and -- satisfy predicate. loopArrMG :: (Monad m) => Int -- ^ start index -> Int -- ^ end index -> (Int -> Bool) -- ^ predicate -> (e -> m a) -- ^ function to apply to elements -> Arr e -- ^ the array -> m () loopArrMG start end p f arr = go start where go !j | j == end = return () | p j = f (arr !> j) >> go (j + 1) | otherwise = go (j + 1) {-# INLINE loopArrMG #-}
It is quite frequently to loop over Arr with various conditions, so I wrote loopArrMG to do it. I know a list comprehension would just do the trick, but the extra time and heap allocation is not what I want. My question here is that whether there is a better data structure available to accomplish such task? If not, is there a better way to write the above code? Thanks, Xiao-Yong -- c/* __o/* <\ * (__ */\ <