我正在尝试使用 haskell 中的警卫进行分类

Question

classify n: returns a letter grade based upon a numeric grade, according to the following schema:

• if n ≥ 90, return 'A'
• if 89 ≤ n ≤ 70, return 'B'
• if 69 ≤ n ≤ 50, return 'C'
• otherwise, return 'D'.

this is my haskell code:

classify n
       | n => 90 = 'A'
       | 89 <= n <= 70 = 'B'
       | 69 <= n <= 50 = 'C'
       | otherwise = 'D' 

but it keeps giving me errors

Answer 1

For greater than or equal, >= is used, not => , furthermore you can not chain conditions like in Python, you thus should work with 89 >= n && n >= 70 :

classify n
    |  = 'A'
    | 89 >= n  >= 70 = 'B'
    | 69 >= n  >= 50 = 'C'
    | otherwise = 'D' 

You can simplify the expression to:

classify n
    |  = 'A'
    | n >= 70 = 'B'
    | n >= 50 = 'C'
    | otherwise = 'D' 

this works for integers because the guard n >= 90 will fire for all values greater than or equal to 90. This thus means that no such values will be evaluated for the next guards and only integers less than or equal to 89 will be evaluated by the other guards.