Cat is a higher-order stack-oriented language. A Stack-oriented language (also known as stack-based) store and share data on one or more memory structures called the stack. Some assembly languages and intermediate languages (CIL and Java bytecode) work are examples of stack-oriented languages. Cat has three different levels of specification: level-0, level-1, and level-2. This is so that tools can identify their degree of compliance with the specification. Each level of the specification specifies a number of primitive function. These functions can be built-in, or can be defined in an implicitly linked standard library.
"Hello, World!" writeln
In the Cat specification, instructions are referred to as "functions," regardless of whether they have side effects. New functionsare defined using the define keyword, and have global scope. Functions cannot be redefined, and are only visible after they aredefined.