Lab 5 Materials

Assets/Lectures/Lab5.md

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+# 20CYS312 - Principles of Programming Languages
+![](https://img.shields.io/badge/Batch-21CYS-lightgreen) ![](https://img.shields.io/badge/UG-blue) ![](https://img.shields.io/badge/Subject-PPL-blue) <br/>
+![](https://img.shields.io/badge/Lecture-2-orange) ![](https://img.shields.io/badge/Practical-3-orange) ![](https://img.shields.io/badge/Credits-3-orange)
+
+## Lab 5 - Higher Order Functions (Continued), Function Composition and Types
+![](https://img.shields.io/badge/-05th_Mar-orange)
+
+### foldr and foldl
+The `foldr` function (right fold) takes a binary function, an initial accumulator value, and a list. It recursively combines the elements of the list from right to left using the binary function and the accumulator.
+
+##### Simple Example
+```
+-- Let's define a function to sum all elements of a list
+sumList :: [Int] -> Int
+sumList xs = foldr (+) 0 xs
+
+-- Summing all elements of a list
+totalSum = sumList [1, 2, 3, 4, 5]
+-- Result: 15
+```
+
+##### Another Example
+```
+-- Function to calculate the factorial of a number
+factorial :: Int -> Int
+factorial n = foldr (*) 1 [1..n]
+
+-- Calculating the factorial of 5
+factorialOfFive = factorial 5
+-- Result: 120 (1 * 2 * 3 * 4 * 5)
+```
+
+##### foldl and foldr
+```
+-- Define a function to subtract two numbers
+subtractNum :: Int -> Int -> Int
+subtractNum x y = x - y
+
+-- List of numbers
+numbersList = [1, 2, 3, 4]
+
+-- Using foldl to perform left fold with subtraction
+foldlResult = foldl subtractNum 0 numbersList
+-- Result: -10 (0 - 1 - 2 - 3 - 4)
+
+-- Using foldr to perform right fold with subtraction
+foldrResult = foldr subtractNum 0 numbersList
+-- Result: 2 (1 - (2 - (3 - (4 - 0))))
+```
+
+### Function Composition
+
+```
+	sanitizeInput :: String -> String
+	sanitizeInput inputString = concatMap replaceAngleBrackets inputString
+	  where
+		replaceAngleBrackets '<' = "<"
+		replaceAngleBrackets '>' = ">"
+		replaceAngleBrackets c   = [c]
+
+	validateInput :: String -> Bool
+	validateInput inputString = length inputString > 5
+
+	saveToDatabase :: String -> IO ()
+	saveToDatabase inputString = putStrLn $ "Saving to database: " ++ inputString
+
+	-- Function composition
+	processUserInput :: String -> IO ()
+	processUserInput x = saveToDatabase (sanitizeInput x)
+
+	-- Example usage:
+	main :: IO ()
+	main = do
+		let userInput = "<script>alert('Hello, 21CYS!')</script>"
+		if validateInput userInput
+			then processUserInput userInput
+			else putStrLn "Input did not pass validation"
+```
+
+Another form
+
+```
+sanitizeInput :: String -> String
+sanitizeInput inputString = concatMap replaceAngleBrackets inputString
+  where
+	replaceAngleBrackets '<' = "<"
+	replaceAngleBrackets '>' = ">"
+	replaceAngleBrackets c   = [c]
+
+validateInput :: String -> Bool
+validateInput inputString = length inputString > 5
+
+saveToDatabase :: String -> IO ()
+saveToDatabase inputString = putStrLn $ "Saving to database: " ++ inputString
+
+-- Function composition
+processUserInput :: String -> IO ()
+processUserInput = saveToDatabase . sanitizeInput
+
+-- Example usage:
+main :: IO ()
+main = do
+	let userInput = "<script>alert('Hello, 21CYS!')</script>"
+	if validateInput userInput
+		then processUserInput userInput
+		else putStrLn "Input did not pass validation"
+
+```
+
+### Types
+- type
+- newType
+- data
+
+#### Example
+```
+-- Define a data type representing shapes
+data Shape = Circle Double | Rectangle Double Double
+
+-- Function to calculate area of a shape
+area :: Shape -> Double
+area (Circle r) = pi * r * r
+area (Rectangle w h) = w * h
+
+-- Example usage
+main :: IO ()
+main = do
+    let circleArea = area (Circle 3.0)
+        rectangleArea = area (Rectangle 2.0 4.0)
+    putStrLn $ "Area of circle: " ++ show circleArea
+    putStrLn $ "Area of rectangle: " ++ show rectangleArea
+```
+
+#### Another Example
+```
+type ProgrammingLanguage = String
+
+-- Function to validate if a string represents a valid programming language
+isValidLanguage :: String -> Bool
+isValidLanguage lang = lang `elem` ["Haskell", "Python", "JavaScript", "Java", "C", "RAM"]
+
+-- Example usage
+main :: IO ()
+main = do
+    putStrLn "Enter a programming language:"
+    language <- getLine
+    if isValidLanguage language
+        then putStrLn $ language ++ " is a valid programming language."
+        else putStrLn $ language ++ " is not a valid programming language."
+```
+
+```
+-- Define a custom data type for programming languages
+data ProgrammingLanguage = Haskell | Python | JavaScript | Java | C | RAM 
+
+-- Function to validate if a string represents a valid programming language
+isValidLanguage :: String -> Bool
+isValidLanguage "Haskell" = True
+isValidLanguage "Python" = True
+isValidLanguage "JavaScript" = True
+isValidLanguage "Java" = True
+isValidLanguage "C" = True
+isValidLanguage "RAM" = True
+isValidLanguage _ = False
+
+-- Example usage
+main :: IO ()
+main = do
+    putStrLn "Enter a programming language:"
+    language <- getLine
+    if isValidLanguage language
+        then putStrLn $ language ++ " is a valid programming language."
+        else putStrLn $ language ++ " is not a valid programming language."
+```
+
+### deriving
+```
+data ProgrammingLanguage = Haskell | Python | JavaScript | Java | C | RAM deriving (Eq)
+
+-- Example usage
+main :: IO ()
+main = do
+    let lang1 = Haskell
+        lang2 = Python
+    if lang1 == lang2
+        then putStrLn "The languages are equal."
+        else putStrLn "The languages are not equal."
+```
+
+```
+-- Define types using newtype
+newtype Meters = Meters Double deriving (Show)
+newtype Feet = Feet Double deriving (Show)
+
+-- Function to convert meters to feet
+metersToFeet :: Meters -> Feet
+metersToFeet (Meters m) = Feet (m * 3.28084)
+
+-- Example usage
+main :: IO ()
+main = do
+    let lengthInMeters = Meters 5.0
+    putStrLn $ "Length in meters: " ++ show lengthInMeters
+    putStrLn $ "Length in feet: " ++ show (metersToFeet lengthInMeters)
+```
+[Click Here](https://wiki.haskell.org/Type) to read more.