"""Get the quality grade corresponding to a given quality score.

The function maps a quality score to a letter grade based on the following thresholds:
- A: 0.9 <= score <= 1.0
- B: 0.8 <= score < 0.9
- C: 0.7 <= score < 0.8
- D: 0.6 <= score < 0.7
- F: score < 0.6

Args:
    quality_score (float): A numeric score representing the quality, typically in the range [0.0, 1.0].

Returns:
    str: The corresponding letter grade as a single character ('A', 'B', 'C', 'D', or 'F').

Note:
    The function assumes that the input `quality_score` is a float within the range [0.0, 1.0]. 
    Scores outside this range may produce unexpected results.
"""