def histogram_distance(arr1, arr2, bins=None):
    """ This function returns the sum of the squared error
    Parameters:
        two arrays constrained to 0..1

    Returns:
        sum of the squared error between the histograms
    """
    eps = 1e-6
    assert arr1.min() > 0 - eps
    assert arr1.max() < 1 + eps
    assert arr2.min() > 0 - eps
    assert arr2.max() < 1 + eps

    if not bins:
        bins = [x / 10 for x in range(11)]
    hist1 = np.histogram(arr1, bins=bins)[0] / arr1.size
    hist2 = np.histogram(arr2, bins=bins)[0] / arr2.size

    assert abs(hist1.sum() - 1.0) < eps
    assert abs(hist2.sum() - 1.0) < eps

    sqerr = (hist1 - hist2) ** 2
    return sqerr.sum()