Programs, module declarations, and expressions

A Dylan program is composed of expressions. Each expression is associated with a module. Within an expression, variables are referenced by variable names. The module associated with the expression provides the mapping from variable name to variable used within the expression. It is an error to reference a variable name for the purpose of getting or setting its value if the variable name does not designate either a variable locally bound with a scope that includes the reference or a variable accessible in the associated module.

Module declarations are expressions. Like other "defining forms," module declarations are only allowed at top level or inside of begin. The variable names in module declarations are relative either to the module being declared or to a module that it uses, as specified in part b, and thus are not affected by the module declaration's associated module. A module declaration can be associated with any module where define module has its normal meaning.

Before an expression can be compiled, the module declaration for the module associated with the expression must be compiled and then made available to the development environment in an implementation-defined way.