5.13.3 Interpret/Compile states

A standard program is not permitted to change state explicitly. However, it can change state implicitly, using the words [ and ]. When [ is executed it switches state to interpret state, and therefore the text interpreter starts interpreting. When ] is executed it switches state to compile state and therefore the text interpreter starts compiling. The most common usage for these words is for switching into interpret state and back from within a colon definition; this technique can be used to compile a literal (for an example, see Literals) or for conditional compilation (for an example, see Interpreter Directives).