Haskell Language Syntax in Functions Using where and guards
Example
Given this function:
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!"
We can use where to avoid the repetition and make our code more readable. See the alternative function below, using where:
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)
As observed, we used the where in the end of the function body eliminating the repetition of the calculation (hourlyRate * (weekHoursOfWork * 52)) and we also used where to organize the salary range.
The naming of common sub-expressions can also be achieved with let expressions, but only the where syntax makes it possible for guards to refer to those named sub-expressions.
