Haskell Language Syntax in Functions Pattern Matching
Example
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 ,
