Mid-Term Solutions

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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)
+
+## Mid-Term Question Paper with Solutions
+
+#### 1. Write the \textit{type signature} of the below functions.
+(a) **zip :: [a] → [b] → [(a, b)]** <br/>
+(b) **filter :: (a → Bool) → [a] → [a]**
+
+#### 2. Identify the type of the following values.
+ (a) "C" -  **String or [Char]** <br/>
+ (b) (85, 'Y', [True]) - **(Int, Char, [Bool])** <br/>
+ (c) ["S":[]] - **[[String]] or [[[Char]]]**
+
+#### 3. Write a function '_pairswap_' that swaps the elements of a list on a pair-wise basis.
+Assumption: the list has an even number of elements. 
+Usage Example:
+ 1.  \> _pairswap_ [1..6]
+       [2,1,4,3,6,5]
+ 2.  \> _pairswap_ "ammu"
+       "maum"
+ 3.  \> _pairswap_ []
+       []
+
+**Solution:**
+```
+pairswap:: [a] -> [a]
+pairswap [ ] = [ ]
+pairswap (x:y:xs) = y:x:pairswap(xs)
+```
+
+#### 5. Write the expression equivalent to the following list comprehension using map and/or filter.
+(a) [ reverse s | s ← strs, even (length s) ] <br/>
+(b) [ length word | word ← words, length word > 5 ] <br/>
+(c) [ (x, y) | x ← [1..5], y ← [1..5], x + y == 6 ] <br/>
+(d) [ x^2 | x ← [1..10], even x ] <br/>
+
+**Solution:** <br/>
+(a) <br/>
+(b) <br/>
+(c) <br/>
+(d) <br/>
+
+#### 6. Write a Haskell code to generate the below HTML file with custom _body_ and _footer_ as mentioned. 
+ - body = "Mid-Term Examination"
+ - footer = "19-March-2024"
+```
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <title>Haskell Generated HTML</title>
+</head>
+<body>
+
+    <main>
+        <p>Content generated by Haskell:</p>
+        <div id="haskell-generated-content">
+            /* body content as mentioned above */
+        </div>
+    </main>
+
+    <footer>
+        <p>Generated on: /* footer as mentioned above */
+    </footer>
+</body>
+</html>
+```
+
+**Solution:**
+```
+
+```
+
+#### 7. Implement a Queue with the isEmpty, enqueue, dequeue and front function using Haskell data type with Maybe.
+
+**Solution:**
+```
+data Queue a = Queue { elements :: [a] }
+
+-- Check if the queue is empty
+isEmpty :: Queue a -> Bool
+isEmpty (Queue []) = True
+isEmpty _          = False
+
+-- Enqueue an element into the queue
+enqueue :: a -> Queue a -> Queue a
+enqueue x (Queue xs) = Queue (xs ++ [x])
+
+-- Dequeue an element from the queue
+dequeue :: Queue a -> Maybe (a, Queue a)
+dequeue (Queue [])     = Nothing
+dequeue (Queue (x:xs)) = Just (x, Queue xs)
+
+-- Dequeue an element from the queue
+-- dequeue :: Queue a -> Maybe (Queue a)
+-- dequeue (Queue [])     = Nothing
+-- dequeue (Queue (_:xs)) = Just (Queue xs)
+
+
+-- Get the front element of the queue
+front :: Queue a -> Maybe a
+front (Queue [])    = Nothing
+front (Queue (x:_)) = Just x
+
+main :: IO ()
+main = do
+    let emptyQueue = Queue ([] :: [Int])
+    let queue1 = enqueue 1 emptyQueue
+    let queue2 = enqueue 2 queue1
+
+    putStrLn "Testing Queue Implementation:"
+    putStrLn "Initial queue:"
+    putStrLn $ "Is empty? " ++ show (isEmpty emptyQueue)
+    putStrLn $ "Front element: " ++ show (front emptyQueue)
+    putStrLn ""
+
+    putStrLn "After enqueueing 1:"
+    putStrLn $ "Is empty? " ++ show (isEmpty queue1)
+    putStrLn $ "Front element: " ++ show (front queue1)
+    putStrLn ""
+
+    putStrLn "After enqueueing 2:"
+    putStrLn $ "Is empty? " ++ show (isEmpty queue2)
+    putStrLn $ "Front element: " ++ show (front queue2)
+    putStrLn ""
+
+    putStrLn "Dequeueing:"
+    case dequeue queue2 of
+        Just (element, newQueue) -> do
+            putStrLn $ "Dequeued element: " ++ show element
+            putStrLn $ "New front element: " ++ show (front newQueue)
+            putStrLn $ "Is empty? " ++ show (isEmpty newQueue)
+        Nothing -> putStrLn "Queue is empty"
+```
+
+#### 8. Which of the following is NOT a key characteristic of Haskell?
+**Solution:** Mutable variables
+
+#### 9. Given the following data type declaration, What does the deriving (Show) part indicate?
+``` data Color = Red | Green | Blue deriving (Show). ```<br/> <br/>
+**Solution:** It automatically generates a Show instance for the Color type
+
+#### 10. Complete the following Haskell function findMax. 
+```
+-- Consider the following incomplete Haskell function definition for
+finding the maximum element in a list
+findMax :: [Int] -> Int
+findMax [] = ???
+findMax (x:xs) = ???
+-- Your task is to complete the definition of the ’findMax’ function to correctly find and
+   return the maximum element in the given list ’xs’.
+-- Assumption: the list ’xs’ is not empty
+```
+**Solution:**
+```
+findMax :: [Int] \rightarrow Int
+findMax [x] = x
+findMax (x:xs) = max$ x (findMax xs)
+```