Revised^6 Report on the Algorithmic Language Scheme

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Chapter 6
Entry format

The chapters that describe bindings in the base library and the standard libraries 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 built-in procedure. An entry begins with one or more header lines of the form

`template` category

The category defines the kind of binding described by the entry, typically either “syntax” or “procedure”. An entry may specify various restrictions on subforms or arguments. For background on this, see section 5.4.

## 6.1 Syntax entries

If category is “syntax”, the entry describes a special syntactic construct, and the template gives the syntax of the forms of the construct. The template is written in a notation similar to a right-hand side of the BNF rules in chapter 4, and describes the set of forms equivalent to the forms matching the template as syntactic data. Some “syntax” entries carry a suffix (`expand`), specifying that the syntactic keyword of the construct is exported with level 1. Otherwise, the syntactic keyword is exported with level 0; see section 7.2.

Components of the form described by a template are designated by syntactic variables, which are written using angle brackets, for example, <expression>, <variable>. Case is insignificant in syntactic variables. Syntactic variables stand for other forms, or sequences of them. A syntactic variable may refer to a non-terminal in the grammar for syntactic data (see section 4.3.1), in which case only forms matching that non-terminal are permissible in that position. For example, <identifier> stands for a form which must be an identifier. Also, <expression> stands for any form which is a syntactically valid expression. Other non-terminals that are used in templates are defined as part of the specification.

The notation

<thing1> `...`

indicates zero or more occurrences of a <thing>, and

<thing1> <thing2> `...`

indicates one or more occurrences of a <thing>.

It is the programmer’s responsibility to ensure that each component of a form has the shape specified by a template. Descriptions of syntax may express other restrictions on the components of a form. Typically, such a restriction is formulated as a phrase of the form “<x> must bea `...`”. Again, these specify the programmer’s responsibility. It is the implementation’s responsibility to check that these restrictions are satisfied, as long as the macro transformers involved in expanding the form terminate. If the implementation detects that a component does not meet the restriction, an exception with condition type `&syntax` is raised.

## 6.2 Procedure entries

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

`(vector-ref vector k)` procedure

indicates that the built-in procedure `vector-ref` takes two arguments, a vector vector and an exact non-negative integer object k (see below). The header lines

`(make-vector k)` procedure
`(make-vector k fill)` procedure

indicate that the `make-vector` procedure takes either one or two arguments. The parameter names are case-insensitive: Vector is the same as vector.

As with syntax templates, an ellipsis `...` at the end of a header line, as in

`(= z1 z2 z3 `...`)` procedure

indicates that the procedure takes arbitrarily many arguments of the same type as specified for the last parameter name. In this case, `=` accepts two or more arguments that must all be complex number objects.

A procedure that detects an argument that it is not specified to handle must raise an exception with condition type `&assertion`. Also, the argument specifications are exhaustive: if the number of arguments provided in a procedure call does not match any number of arguments accepted by the procedure, an exception with condition type `&assertion` must be raised.

For succinctness, the report follows the convention that if a parameter name is also the name of a type, then the corresponding argument must be of the named type. For example, the header line for `vector-ref` given above dictates that the first argument to `vector-ref` must be a vector. The following naming conventions imply type restrictions:

obj | any object
z | complex number object
x | real number object
y | real number object
q | rational number object
n | integer object
k | exact non-negative integer object
bool | boolean (`#f` or `#t`)
octet | exact integer object in {0, `...`, 255}
byte | exact integer object in { `-` 128, `...`, 127}
char | character (see section 11.11)
pair | pair (see section 11.9)
vector | vector (see section 11.13)
string | string (see section 11.12)
condition | condition (see library section on “Conditions”)
bytevector | bytevector (see library chapter on “Bytevectors”)
proc | procedure (see section 1.6)

Other type restrictions are expressed through parameter-naming conventions that are described in specific chapters. For example, library chapter on “Arithmetic” uses a number of special parameter variables for the various subsets of the numbers.

With the listed type restrictions, it is the programmer’s responsibility to ensure that the corresponding argument is of the specified type. It is the implementation’s responsibility to check for that type.

A parameter called list means that it is the programmer’s responsibility to pass an argument that is a list (see section 11.9). It is the implementation’s responsibility to check that the argument is appropriately structured for the operation to perform its function, to the extent that this is possible and reasonable. The implementation must at least check that the argument is either an empty list or a pair.

Descriptions of procedures may express other restrictions on the arguments of a procedure. Typically, such a restriction is formulated as a phrase of the form “x must be a `...`” (or otherwise using the word “must”).

## 6.3 Implementation responsibilities

In addition to the restrictions implied by naming conventions, an entry may list additional explicit restrictions. These explicit restrictions usually describe both the programmer’s responsibilities, who must ensure that the subforms of a form are appropriate, or that an appropriate argument is passed, and the implementation’s responsibilities, which must check that subform adheres to the specified restrictions (if macro expansion terminates), or if the argument is appropriate. A description may explicitly list the implementation’s responsibilities for some arguments or subforms in a paragraph labeled “Implementation responsibilities”. In this case, the responsibilities specified for these subforms or arguments in the rest of the description are only for the programmer. A paragraph describing implementation responsibility does not affect the implementation’s responsibilities for checking subforms or arguments not mentioned in the paragraph.

## 6.4 Other kinds of entries

If category is something other than “syntax” and “procedure”, then the entry describes a non-procedural value, and the category describes the type of that value. The header line

`&who` condition type
indicates that `&who` is a condition type. The header line

`unquote` auxiliary syntax
indicates that `unquote` is a syntax binding that may occur only as part of specific surrounding expressions. Any use as an independent syntactic construct or identifier is a syntax violation. As with “syntax” entries, some “auxiliary syntax” entries carry a suffix (`expand`), specifying that the syntactic keyword of the construct is exported with level 1.

## 6.5 Equivalent entries

The description of an entry occasionally states that it is the same as another entry. This means that both entries are equivalent. Specifically, it means that if both entries have the same name and are thus exported from different libraries, the entries from both libraries can be imported under the same name without conflict.

## 6.6 Evaluation examples

The symbol “⇒” used in program examples can be read “evaluates to”. For example,

`(* 5 8) ⇒ 40

`

means that the expression `(* 5 8)` evaluates to the object `40`. Or, more precisely: the expression given by the sequence of characters “`(* 5 8)`” evaluates, in an environment that imports the relevant library, to an object that may be represented externally by the sequence of characters “`40`”. See section 4.3 for a discussion of external representations of objects.

The “⇒” symbol is also used when the evaluation of an expression causes a violation. For example,

`(integer->char `#`xD800) `&assertion` exception

` means that the evaluation of the expression `(integer->char `#`xD800)` must raise an exception with condition type `&assertion`.

Moreover, the “⇒” symbol is also used to explicitly say that the value of an expression in unspecified. For example:

`(eqv? "" "") ⇒ unspecified

`

Mostly, examples merely illustrate the behavior specified in the entry. In some cases, however, they disambiguate otherwise ambiguous specifications and are thus normative. Note that, in some cases, specifically in the case of inexact number objects, the return value is only specified conditionally or approximately. For example:

`(atan -inf.0)
⇒ -1.5707963267948965 ; approximately

`

## 6.7 Naming conventions

By convention, the names of procedures that store values into previously allocated locations (see section 5.10) usually end in “`!`”.

By convention, “`->`” appears within the names of procedures that take an object of one type and return an analogous object of another type. For example, `list->vector` takes a list and returns a vector whose elements are the same as those of the list.

By convention, the names of predicates—procedures that always return a boolean value—end in “`?`” when the name contains any letters; otherwise, the predicate’s name does not end with a question mark.

By convention, the components of compound names are separated by “`-`” In particular, prefixes that are actual words or can be pronounced as though they were actual words are followed by a hyphen, except when the first character following the hyphen would be something other than a letter, in which case the hyphen is omitted. Short, unpronounceable prefixes (“`fx`” and “`fl`”) are not followed by a hyphen.

By convention, the names of condition types start with “`&`”.