Q:

Struggling to understand a "Cannot construct the infinite type" error

I'm trying to solve a HackerRank problem and I am getting an error that I cannot figure out. The issue is in the solve1 function. The exact error is: cannot construct the infinite type a ~ t0 a. Expected type ([t0 a], [t0 a], [t0 a]) Actual type ([a], [a], [a]). In the second argument of `tripleMap`, namely '(tripList xs)'.
I keep looking at the types and they continue to appear correct in my eyes. tripList takes a list of numbers and returns a triple of lists of numbers. tripleMap takes a triple of lists of numbers as its second argument.
In testing tripList in my REPL I get the desired results: 
> tripList [1,0,-1,0,1] 
([1,1],[0,0],[-1])

Here is my code:
length' :: (Foldable t, Num b, Fractional b, Ord b) => t a -> b
length' = foldr (\_ acc -> 1 + acc) 0

tripList :: (Num a, Ord a) => [a] -> ([a], [a], [a])
tripList xs =
  ( filter (>0) xs
  , filter (==0) xs
  , filter (<0) xs )

foldSolution :: (Foldable t, Num a, Ord a, Fractional a)
             => a -> t a -> a
foldSolution n = foldr (\x y -> x/n + y/n) 0

tripleMap :: (a -> b) -> ([a], [a], [a]) -> ([b], [b], [b])
tripleMap f (a, b, c) = (map f a, map f b, map f c)

solve1 :: (Num a, Ord a) => [a] -> ([a], [a], [a])
solve1 xs = tripleMap (foldSolution (length' xs)) (tripList xs)

A:

Your foldSolution (length' xs) takes t a and returns a. So, when you tripleMap it on tripList xs (which has type ([a], [a], [a])), you get a tuple of type (a, a, a), not ([a], [a], [a]), which is why you got the error.

