使用何处和守卫

鉴于此功能：

annualSalaryCalc :: (RealFloat a) => a -> a -> String
annualSalaryCalc hourlyRate weekHoursOfWork
  | hourlyRate * (weekHoursOfWork * 52) <= 40000  = "Poor child, try to get another job"
  | hourlyRate * (weekHoursOfWork * 52) <= 120000 = "Money, Money, Money!"
  | hourlyRate * (weekHoursOfWork * 52) <= 200000 = "Ri¢hie Ri¢h"
  | otherwise = "Hello Elon Musk!"

我们可以使用 where 来避免重复并使我们的代码更具可读性。使用 where 查看下面的替代功能：

annualSalaryCalc' :: (RealFloat a) => a -> a -> String
annualSalaryCalc' hourlyRate weekHoursOfWork
  | annualSalary <= smallSalary  = "Poor child, try to get another job"
  | annualSalary <= mediumSalary = "Money, Money, Money!"
  | annualSalary <= highSalary   = "Ri¢hie Ri¢h"
  | otherwise = "Hello Elon Musk!"
  where 
      annualSalary = hourlyRate * (weekHoursOfWork * 52)
      (smallSalary, mediumSalary, highSalary)  = (40000, 120000, 200000)

如上所述，我们在函数体的末尾使用了 where，消除了重复计算（hourlyRate * (weekHoursOfWork * 52)），我们还使用 where 来组织工资范围。

使用 let 表达式也可以实现公共子表达式的命名，但只有 where 语法才能使警卫能够引用那些命名的子表达式。