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Solutions to problems of chapter 3

Scope

  1. These lines of code are from a REPL session. Is y in scope for z?
> x = 5
> y = 7
> z = x * y

Yes

  1. These lines of code are from a REPL session. Is h in scope for g?
> f = 3
> g = 6 \* f + h

No h does not seem to be declared previously.

  1. This code sample is from a source file. Is everything we need to execute area in scope?
area d = pi \* (r \* r)
r = d / 2

No Since d is local to the function area it can be accessed in the expression following r.

  1. This code is also from a source file. Now are r and d in scope for area?
area d = pi \* (r \* r)
  where r = d / 2

Yes Since r is now defined in the local scope of area, it can access d which is defined in its wrapping scope.

Reading syntax

  1. For the following lines of code, read the syntax carefully and decide if they are written correctly. a. concat [[1,2,3], [4,5,6]] correct syntax. concat takes a list of lists and returns one list of type equal to the nested lists after concatenating them together. b. ++ [1, 2, 3] [4, 5, 6] wrong syntax. ++ is a infix operator. In order to use it prefix, it must be wrapped in parenthesis as follows: (++) [1, 2, 3] [4, 5, 6] c. (++) "hello" " world" correct syntax. ++ takes two parameters and wrapped in parenthesis can be used as a prefix operator. d. ["hello" ++ " world] wrong syntax. world is not correctly wrapped in double quotes. e. 4 !! "hello" wrong syntax. (!!) is of type [a] -> Int -> a which means its first parameter is a list and the second parameter is an integer. The correct expression would be: "hello" !! 4 f. (!!) "hello" 4 correct syntax. (!!) takes a list and an Integer and the parenthesis makes it prefix. g. take "4 lovely" wrong syntax. take has the type Int -> [a] -> [a] but we are passing only a string, [Char], to it. The correct syntax is a follows: take 4 "lovely" h. take 3 "awesome" correct syntax.

  2. We have two sets: The first set is lines of code while the other set is results. Read the code and decide which result belongs to which set. a. concat [[1 * 6], [2 * 6], [3 * 6]] => [6, 12, 18] b. "rain" ++ drop 2 "elbow" => "rainbow" c. 10 * head [1, 2, 3] => 10 d. (take 3 "Julie") ++ (tail "yes") => Jules e. concat [tail [1, 2, 3], tail [4, 5, 6], tail [7, 8, 9]] => [2, 3, 5, 6, 8, 9]

Building functions

  1. Given the list-manipulation functions mentioned in this chapter, write functions that take the following inputs and return the expected outputs. a. Given "Curry is awesome", return "Curry is awesome!" concat ["Curry is awesome", "!"] or "Curry is awesome" ++ "!" b. Given "Curry is awesome!", return "y". drop 4 (take 5 "Curry is awesome!") or "Curry is awesome!" !! 4 c. Given "Curry is awesome!", return "awesome!". drop 9 "Curry is awesome!"

  2. Now, take each of the above and rewrite it in a source file as a general function that could take different string inputs as arguments but retain the same behavior. Solution file

  3. Write a function of type String -> Char which returns the third character in a String. Solution file

  4. Now change that function so the string operated on is always the same and the variable represents the number of the letter you want to return. Solution file

  5. Using the take and drop functions write a function called rvrs that takes the string "Curry is awesome" and returns the string "awesome is Curry". Solution file

  6. Add the rvrs function to a module named Reverse Solution file