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Booleans and Conditionals In Scheme, falsity is represented by the value false, written #f. Conceptually, #f is a pointer to a special object, the false object. Predicates are procedures that return either #t or #f, and don't have side effects. Calling a predicate is like asking a true/false question--all you care about is a yes or no answer. Scheme>(> 1 2) #f Here we told Scheme to apply the predicate procedure to 1 and 2; The important thing about #f is its use in conditionals. If the first subexpression (the condition) of an if expression returns the value #f, the second subexpression is not evaluated, and the third one is; Scheme>(if #f 1 2) 2 Here the second subexpression was just the literal 2, so 2 was returned. Now try it using the predicate > Scheme>(if (> 1 2) 1 2) 2 This is clearer if we indent it like this, lining up the "then" part (the consequent) and the "else" part (the alternative) under the condition. Scheme>(if (> 1 2) 1 2) 2 This is the right way to indent code when writing a Scheme program in an editor, and most Scheme systems will let you indent code this way when using the system interactively--the you can hit <RETURN>, and type in extra spaces. Scheme won't try to evaluate the expression until you write the last closing parenthesis and hit <RETURN>. This helps you format your code readably even when typing interactively, so that you can see what you're doing. The false value makes a conditional expression (like an if) go one way, and a true value will make it go another. In Scheme, any value except #f counts as true in conditionals. Try this: Scheme> (if 0 1 0) What result value does Scheme print? One special value is provided, called the true object, written #t. There's nothing very special about it, though--it's just a handy value to use when you want to return a true value, making it clear that all you're doing is returning a true value.
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