Although one semicolon is enough to begin a comment, most Scheme programmers have the habit of using the number of repetitions of the semicolon character to indicate the scope or importance of a comment. A three-semicolon comment line is more likely to apply to the program as a whole, or to be extremely important. A similar line beginning with just one semicolon is likely to apply only to the line that immediately precedes or follows it and is typically just an off-hand remark. Exercise 7 Add a comment to sample.ss in which you give your name, the date, and a description of why you wrote the program stored in that file. Save the file and use the load procedure to load it into the Scheme interactive interpreter again.
Scheme treats a sequence of characters enclosed in double quotation marks as a string .
Exercise 8 Submit the definition of greeting to the Scheme interactive interpreter. Symbols and quotation String literals should not be confused with symbols , which are never enclosed in double quotation marks, are not thought of as being divisible into separate characters, and constitute a completely separate type of data. Section 1.3 in the textbook contains several examples in which symbols are used as data. With the quotation mark, Scheme makes no attempt to evaluate the symbol, but instead treats it as a simple datum. Exercise 9 Give Scheme a definition that will cause it to recognize gross as a name for the number 144.