A top-level program specifies an entry point for defining and running a Scheme program. A top-level program specifies a set of libraries to import and code to run. Through the imported libraries, whether directly or through the transitive closure of importing, a top-level program defines a complete Scheme program.
A top-level program is a delimited piece of text, typically a file, that has the following form:<import form> <top-level body>
An <import form> has the following form:(import <import spec> ...)
A <top-level body> has the following form:<top-level body form> ...
A <top-level body form> is either a <definition> or an <expression>.
The <import form> is identical to the import clause in libraries (see section 7.1), and specifies a set of libraries to import. A <top-level body> is like a <library body> (see section 7.1), except that definitions and expressions may occur in any order. Thus, the syntax specified by <top-level body form> refers to the result of macro expansion.
When uses of begin, let-syntax, or letrec-syntax from the (rnrs base (6)) library occur in a top-level body prior to the first expression, they are spliced into the body; see section 11.4.7. Some or all of the body, including portions wrapped in begin, let-syntax, or letrec-syntax forms, may be specified by a syntactic abstraction (see section 9.2).
A top-level program is executed by treating the program similarly to a library, and evaluating its definitions and expressions. The semantics of a top-level body may be roughly explained by a simple translation into a library body: Each <expression> that appears before a definition in the top-level body is converted into a dummy definition(define <variable> (begin <expression> <unspecified>))
where <variable> is a fresh identifier and <unspecified> is a side-effect-free expression returning an unspecified value. (It is generally impossible to determine which forms are definitions and expressions without concurrently expanding the body, so the actual translation is somewhat more complicated; see chapter 10.)
On platforms that support it, a top-level program may access its command line by calling the command-line procedure (see library section on “Command-line access and exit values”).