Для чего здесь in?

evenSum :: [Integer] -> Integer
 
evenSum l = accumSum 0 l
 
accumSum n l = if l == []
                  then n
                  else let x = head l 
                           xs = tail l 
                       in if even x
                              then accumSum (n+x) xs
                              else accumSum n xs

let x = head l
xs = tail l

sumEven xs = sumE 0 xs
             where sumE acc [] = acc
                   sumE acc (z:zs) = if even z then sumE (acc+z) zs else sumE acc zs

sumEven xs = sumE 0 xs
where sumE acc [] = acc
sumE acc (z:zs) = if even z then sumE (acc+z) zs else sumE acc zs

sumEven xs = sumE 0 xs
 where sumE acc [] = acc
sumE acc (z:zs) = if even z then sumE (acc+z) zs else sumE acc zs

evenSum = sum . filter even

sumEven :: Integral a => [a] -> a
sumEven = let sumE acc [] = acc
              sumE acc (z:zs) | even z = sumE (acc+z) zs 
                              | otherwise = sumE acc zs
          in sumE 0