谓词在列表长度上的表现

Question

I have read (and also reasoned) that calculating the length of a list is not good for performance in Haskell. However, i have long lists in my program and my requirement is to find that if length is greater than or less than some number X.

Is their already something in build in Haskell for these kind of predicates ? or i have to resort to manual looping.

Answer 1

On vanilla lists, you can check this using drop :

cmpLen :: Int -> [a] -> Ordering
cmpLen n xs
    | n < 0     = GT
    | otherwise = case drop (n-1) xs of
        []  -> GT
        [_] -> EQ
        _   -> LT

However, this still takes as long as the value (not size, as is typical of asymptotic analysis!) of n . If you intend to do this often, you can take a cue from Okasaki and build a new structure that caches the operation you want to be efficient. I have wanted this a few times before, and found the following sort of interface convenient in those cases:

type LenList a = (Sum Word, [a])
singleton x = (1, [x])
cons x = (singleton x<>)
length = getSum . fst
elems  = snd

Note that, since LenList a is already a Monoid , you get some of the usual operations for free, eg there is an empty LenList a named mempty , and concatenation is given by (<>) . Some operations (notably the ones that produce infinite lists) will not be implementable for this type. However, you pay an O(1) price on each construction operation to make asking for the length of one of these O(1), which can be a nice tradeoff in many situations.