CS 5010: Guided Practice 4.1 — Solution

The question was:

Consider the following data definition:

    ;; A StringList is one of
    ;; -- empty
    ;; -- (cons String StringList)
  

Which of the following expressions has a value that is a StringList ?

Here are the answers:

1. (cons "abc" empty)
   Answer: Yes.
   empty is a StringList and "abc" is a string, so (cons "abc" empty) is a StringList.
2. (cons "bcd")
   Answer: No.
   This expression has no value, because cons is given a wrong number of arguments.
3. (cons "cde" (cons "ef" empty))
   Answer: Yes.
   This is a StringList.
4. (cons "3" (cons "4" empty))
   Answer: Yes.
   This too is a StringList.
5. (cons 3 (cons 4 empty))
   Answer: No.
   3 and 4 are numbers, not strings.