2025-11-16 Backpack Review

More review exercises covering the fundamentals. Originally from enrichment.

Hand can hold a Left integer or a Right string. Write a data definition for Hand.
Backpack is a data type that holds an Int numPencils, a Double money and a String owner. Write the data definition.
Update the contents of the backpack. If you get a left hand, add the int to the number of pencils. If you get a right hand, set the owner to be the string.
```
 update :: Backpack -> Hand -> Backpack
```
Use different methods:
1. Pattern matching
2. Updater/accessor functions.
Write a pencil counting function: how many pencils are there total, in all of the backpacks.
```
 pc :: [Backpack] -> Int
```
1. List comprehension
2. Fold
3. Recursion

Simpler Review Problems

Use map to change a list of words into a list of their lengths.
```
 q5 ["cat","be"] == [3,2]
```
Use filter to remove all words longer than 6 letters from a list of words.
```
 q6 ["Workaholic","student","study","now"] == ["study","now"]
```
Use foldl to concatenate all of the strings in a list, except skip “Bob”.
```
 q7 ["Bob"," ran ","fast"] == " ran fast"
```