20-CS-4003-001 |
Organization of Programming Languages |
Fall 2017 |
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Functors |

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**Basics**

{- deriving other classes - all named types (String and Int here) must already derive from Eq - ops available: == /=, not available <, > -} data Person = Person { firstName :: String , lastName :: String , age :: Int } deriving (Eq) p4 = Person { firstName = "John", lastName = "Franco", age = 22 } p5 = Person { firstName = "Jim", lastName = "Ravella", age = 25 } p6 = Person { firstName = "John", lastName = "Franco", age = 22 } p7 = Person { firstName = "John", lastName = "Franco", age = 21 } x1 = (firstName p4) == (firstName p7) lst = [p4,p5,p6,p7] {- can be done because Person derives from Eq which is needed by elem -} x2 = (Person "John" "Franco" 22) `elem` lst {- Bool derives from Ord so compare can be used -} x3 = compare True False x4 = True < False {- Note: (* 3) is a function so does not derive from Ord and (*3) < (*5) is not allowed : even Just (*3) < Just (*4) is not allowed -} {- Can derive from Enum because there are no type parameters, since there are seven values, can derive from Bounded, deriving from Eq and Ord give the values an order, increasing from left to right -} data Day = Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday deriving (Eq, Ord, Show, Read, Bounded, Enum) x5 = Monday < Tuesday x6 = read "Saturday"::Day x7 = compare Tuesday Monday {- data Either a b = Left a | Right b deriving (Eq, Ord, Read, Show) If Left is used, contents are of type a, if Right is used contents are of type b Note: now one can have a function return more than one type: -} x8 = Right 20 x9 = Left "w00t" {- func :: (Integral a, Num a1) => a -> Either a1 [Char] -} func x = if (even x) then Right "OK"; else Left 34 {- Another example fm :: Ord b => [b] -> Either [a] b -} fm lst = if (lst == []) then Left [] else Right (last y) where y = (head lst):[ max a l | a <- y | l <- lst] {- Without Left and Right fm1 :: Ord a => [[a]] -> [a] because output must be a list of some type and y is a single element from the input list -} fm1 lst = if (lst == []) then [] else last y where y = (head lst):[ max a l | a <- y | l <- lst] {- Another possibility - fm2 :: Ord a => [a] -> [a] -} fm2 lst = if (lst == []) then [] else [last y] where y = (head lst):[ max a l | a <- y | l <- lst] |
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