4.1 Notation

This chapter (and the rest of the documentation) uses a slightly different notation than the character-based grammars of the Scheme Essentials chapter. The grammar for a use of a syntactic form something is shown like this:

(something [id ...+] an-expr ...)

The italicized meta-variables in this specification, such as id and an-expr, use the syntax of Scheme identifiers, so an-expr is one meta-variable. A naming convention implicitly defines the meaning of many meta-variables:

• A meta-variable that ends in id stands for an identifier, such as x or my-favorite-martian.

• A meta-identifier that ends in keyword stands for a keyword, such as #:tag.

• A meta-identifier that ends with expr stands for any sub-form, and it will be parsed as an expression.

• A meta-identifier that ends with body stands for any sub-form; it will be parsed as either a local definition or an expression. A body can parse as a definition only if it is not preceded by any expression, and the last body must be an expression; see also Internal Definitions.

Square brackets in the grammar indicate a parenthesized sequence of forms, where square brackets are normally used (by convention). That is, square brackets do not mean optional parts of the syntactic form.

A ... indicates zero or more repetitions of the preceding form, and ...+ indicates one or more repetitions of the preceding datum. Otherwise, non-italicized identifiers stand for themselves.

Based on the above grammar, then, here are a few conforming uses of something:

(something [x])

(something [x] (+ 1 2))

(something [x my-favorite-martian x] (+ 1 2) #f)

Some syntactic-form specifications refer to meta-variables that are not implicitly defined and not previously defined. Such meta-variables are defined after the main form, using a BNF-like format for alternatives:

(something-else [thing ...+] an-expr ...)

thing   =   thing-id     |   thing-keyword

The above example says that, within a something-else form, a thing is either an identifier or a keyword.