DevTut

# Syntax in Functions

# Pattern Matching

Haskell supports pattern matching expressions in both function definition and through case statements.

A case statement is much like a switch in other languages, except it supports all of Haskell's types.

Let's start simple:

longName :: String -> String
longName name = case name of
                   "Alex"  -> "Alexander"
                   "Jenny" -> "Jennifer"
                   _       -> "Unknown"  -- the "default" case, if you like

Or, we could define our function like an equation which would be pattern matching, just without using a case statement:

longName "Alex"  = "Alexander"
longName "Jenny" = "Jennifer"
longName _       = "Unknown"

A more common example is with the Maybe type:

data Person = Person { name :: String, petName :: (Maybe String) }

hasPet :: Person -> Bool
hasPet (Person _ Nothing) = False
hasPet _ = True  -- Maybe can only take `Just a` or `Nothing`, so this wildcard suffices

Pattern matching can also be used on lists:

isEmptyList :: [a] -> Bool
isEmptyList [] = True
isEmptyList _  = False

addFirstTwoItems :: [Int] -> [Int]
addFirstTwoItems []        = []
addFirstTwoItems (x:[])    = [x]
addFirstTwoItems (x:y:ys)  = (x + y) : ys

Actually, Pattern Matching can be used on any constructor for any type class. E.g. the constructor for lists is : and for tuples ,

# Using where and guards

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.

# Guards

A function can be defined using guards, which can be thought of classifying behaviour according to input.

Take the following function definition:

absolute :: Int -> Int  -- definition restricted to Ints for simplicity
absolute n = if (n < 0) then (-n) else n

We can rearrange it using guards:

absolute :: Int -> Int
absolute n 
  | n < 0 = -n
  | otherwise = n

In this context otherwise is a meaningful alias for True, so it should always be the last guard.

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