2.2.1 True and false

We can compare the magnitude of two numbers:

? 1 = 1;
#t
? 3 < 30;
#t
? 15 > 16;
#f

The functions =, <, and > are predicates. A predicate returns true if the condition it is testing is true; otherwise, it returns false. As you might guess, #t means true and #f means false. False is represented by the unique value #f only, but any object that is not #f is true (thus, 0 is a true value).

Comparison with C and C++: Caution! C and C++ use integers to represent Boolean values — 0 represents false, and any nonzero value is considered true. Dylan has an explicit <boolean> type with two instances: #f represents false, and #t represents the canonical true value. However, any value other than #f is also considered true in a Boolean test. Thus, in Dylan, 0 is considered true.

Comparison with Java: Java has a separate type for Boolean values. Unlike Dylan, C, or C++, the Java Boolean class has only two values, true and false. This design allows the compiler to issue warnings for the common C error if (a=b) ..., because an assignment does not typically yield a Boolean result. An explicit conversion is required to test nonzero in Java: if (a!=0) ....