使用何處和守衛

鑑於此功能：

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!"

我們可以使用 where 來避免重複並使我們的程式碼更具可讀性。使用 where 檢視下面的替代功能：

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，我們在函式體的末尾使用了 where，消除了重複計算（hourlyRate * (weekHoursOfWork * 52)），我們還使用 where 來組織工資範圍。

使用 let 表示式也可以實現公共子表示式的命名，但只有 where 語法才能使警衛能夠引用那些命名的子表示式。