Entry format (Revised(7) Report on the Algorithmic Language Scheme)

1.3.3 Entry format

Chapters Expressions and Standard procedures are organized into entries. Each entry describes one language feature or a group of related features, where a feature is either a syntactic construct or a procedure. An entry begins with one or more header lines of the form

category:

template

for identifiers in the base library, or

name library category:

template

where name is the short name of a library as defined in Standard Libraries.

If category is “syntax,” the entry describes an expression type, and the template gives the syntax of the expression type. Components of expressions are designated by syntactic variables, which are written using angle brackets, for example ⟨expression⟩ and ⟨variable⟩. Syntactic variables are intended to denote segments of program text; for example, ⟨expression⟩ stands for any string of characters which is a syntactically valid expression. The notation

⟨thing₁⟩ …

indicates zero or more occurrences of a ⟨thing⟩, and

⟨thing₁⟩ ⟨thing₂⟩ …

indicates one or more occurrences of a ⟨thing⟩.

If category is “auxiliary syntax,” then the entry describes a syntax binding that occurs only as part of specific surrounding expressions. Any use as an independent syntactic construct or variable is an error.

If category is “procedure,” then the entry describes a procedure, and the header line gives a template for a call to the procedure. Thus the header line

procedure:

vector-ref vector k

indicates that the procedure bound to the vector-ref variable takes two arguments, a vector vector and an exact non-negative integer k (see below). The header lines

procedure:	make-vector `k`
procedure:	make-vector `k` `fill`

indicate that the make-vector procedure must be defined to take either one or two arguments.

It is an error for a procedure to be presented with an argument that it is not specified to handle. For succinctness, we follow the convention that if an argument name is also the name of a type listed in Disjointness of types, then it is an error if that argument is not of the named type. For example, the header line for vector-ref given above dictates that the first argument to vector-ref is a vector. The following naming conventions also imply type restrictions:

alist: association list (list of pairs)
boolean: boolean value (#t or #f)
byte: exact integer 0 ≤ byte < 256
bytevector: bytevector
char: character
end: exact non-negative integer
k, k₁, … k_j, …: exact non-negative integer
letter: alphabetic character
list, list₁, … list_j, …: list (see Pairs and lists)
n, n₁, … n_j, …: integer
obj: any object
pair: pair
port: port
proc: procedure
q, q₁, … q_j, …: rational number
start: exact non-negative integer
string: string
symbol: symbol
thunk: zero-argument procedure
vector: vector
x, x₁, … x_j, …: real number
y, y₁, … y_j, …: real number
z, z₁, … z_j, …: complex number

The names start and end are used as indexes into strings, vectors, and bytevectors. Their use implies the following:

It is an error if start is greater than end.
It is an error if end is greater than the length of the string, vector, or bytevector.
If start is omitted, it is assumed to be zero.
If end is omitted, it assumed to be the length of the string, vector, or bytevector.
The index start is always inclusive and the index end is always exclusive. As an example, consider a string. If start and end are the same, an empty substring is referred to, and if start is zero and end is the length of string, then the entire string is referred to.