sorting - Why is the minimalist, example Haskell quicksort not a "true" quicksort?

Question

Haskell's website introduces a very attractive 5-line quicksort function, as seen below.

quicksort [] = []
quicksort (p:xs) = (quicksort lesser) ++ [p] ++ (quicksort greater)
    where
        lesser = filter (< p) xs
        greater = filter (>= p) xs

They also include a "True quicksort in C".

// To sort array a[] of size n: qsort(a,0,n-1)

void qsort(int a[], int lo, int hi) 
{
  int h, l, p, t;

  if (lo < hi) {
    l = lo;
    h = hi;
    p = a[hi];

    do {
      while ((l < h) && (a[l] <= p)) 
          l = l+1;
      while ((h > l) && (a[h] >= p))
          h = h-1;
      if (l < h) {
          t = a[l];
          a[l] = a[h];
          a[h] = t;
      }
    } while (l < h);

    a[hi] = a[l];
    a[l] = p;

    qsort( a, lo, l-1 );
    qsort( a, l+1, hi );
  }
}

A link below the C version directs to a page that states 'The quicksort quoted in Introduction isn't the "real" quicksort and doesn't scale for longer lists like the c code does.'

Why is the above Haskell function not a true quicksort? How does it fail to scale for longer lists?

Answer 1

The true quicksort has two beautiful aspects:

1. Divide and conquer: break the problem into two smaller problems.
2. Partition the elements in-place.

The short Haskell example demonstrates (1), but not (2). How (2) is done may not be obvious if you don't already know the technique!