Haskell: Is there a safe/total version of (!!)?

Question

I tried looking up Int -> [a] -> Maybe a on hoogle, but no luck.

I feel like this should be in a standard library somewhere, but I don't know where.

frosch03 · Accepted Answer

There is a library called Safe on Hackage which holds a function:

atMay :: [a] -> Int -> Maybe a

shang · Answer

There isn't one in the standard library (would be nice, though), but here's one way to implement it (also works for infinite lists):

(!!!) :: [a] -> Int -> Maybe a
[]     !!! _ = Nothing
(x:xs) !!! n
    | n < 0     = Nothing
    | n == 0    = Just x
    | otherwise = xs !!! (n-1)

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