2025-10-31 Daily Worksheet

1. Write a function cutter :: String -> Maybe String. It changes str to “xstrx”, except that if str has fewer than 5 letters it becomes Nothing

    cutter s = s
   
    check_cutter = [cutter "Box" == Nothing
                   ,cutter "BigBox" == Just "xBigBoxx" ]
   

2. Write a recursive function cutmap that begins by applying the cutter to each item in a list. When the result is Nothing, skip that item in the output. When the result is Just str, put str in the output.

    check2 = [ cutmap ["Box", "BigBox"] == ["xBigBoxx"] ]
   

3. A shopping list [(Int, (String, Double))] has a list of how many of each item to buy, the item name, and the cost.

    shop0 = [(5, ("Apple", 0.75)), (2, ("Cabbage", 0.99))]
   

   Use foldl with a named update function to find the total cost of all items except skip any food with a “g” in the name.

    check_fc = (foldcost shop0 == 3.75)
   

4. What is the benefit of using (Num a) and not Double in the signature below?

    subtract_posn :: (Num a) => (a,a) -> (a,a) -> (a,a)
    subtract_posn (x0,y0) (x1,y1) = (x0-x1, y0-y1)
   

Data

5. Use record syntax to create a data type called Welcome that holds a String name, a String gift, and an int level.

    exw = Persona {name="Pomerance", gift="telekinesis", level=4}
   

6. Write a function enhance :: Persona -> Persona that prepends “strong " to the gift and increases the level by one.

7. A function partner :: Persona -> (Maybe String) -> Persona might append a new second name.

    partner_tests = [
      partner exw (Just "Smith") ==
          Persona{name="Pomerance Smith",
                  gift="telekinesis",
                  level=4}
     ,partner exw Nothing == exw
    ]