Revised^6 Report on the Algorithmic Language Scheme

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Chapter 9
Primitive syntax

After the `import` form within a `library` form or a top-level program, the forms that constitute the body of the library or the top-level program depend on the libraries that are imported. In particular, imported syntactic keywords determine the available syntactic abstractions and whether each form is a definition or expression. A few form types are always available independent of imported libraries, however, including constant literals, variable references, procedure calls, and macro uses.

## 9.1 Primitive expression types

The entries in this section all describe expressions, which may occur in the place of <expression> syntactic variables. See also section 11.4.

### Constant literals

`<number>` syntax
`<boolean>` syntax
`<character>` syntax
`<string>` syntax
`<bytevector>` syntax

An expression consisting of a representation of a number object, a boolean, a character, a string, or a bytevector, evaluates “to itself”.

`145932 ⇒ 145932
`#t` ⇒ `#t`
"abc" ⇒ "abc"
#vu8(2 24 123) ⇒ #vu8(2 24 123)

`

As noted in section 5.10, the value of a literal expression is immutable.

### Variable references

`<variable>` syntax

An expression consisting of a variable(section 5.2) is a variable reference if it is not a macro use (see below). The value of the variable reference is the value stored in the location to which the variable is bound. It is a syntax violation to reference an unboundvariable.

The following example examples assumes the base library has been imported:

`(define x 28)
x ⇒ 28

`

### Procedure calls

`(<operator> <operand1> `...`)` syntax

A procedure call consists of expressions for the procedure to be called and the arguments to be passed to it, with enclosing parentheses. A form in an expression context is a procedure call if <operator> is not an identifier bound as a syntactic keyword (see section 9.2 below).

When a procedure call is evaluated, the operator and operand expressions are evaluated (in an unspecified order) and the resulting procedure is passed the resulting arguments.

The following examples assume the `(rnrs base (6))` library has been imported:

`(+ 3 4) ⇒ 7
((if `#f` + *) 3 4) ⇒ 12

` If the value of <operator> is not a procedure, an exception with condition type `&assertion` is raised. Also, if <operator> does not accept as many arguments as there are <operand>s, an exception with condition type `&assertion` is raised.

> Note: In contrast to other dialects of Lisp, the order of evaluation is unspecified, and the operator expression and the operand expressions are always evaluated with the same evaluation rules.

> Although the order of evaluation is otherwise unspecified, the effect of any concurrent evaluation of the operator and operand expressions is constrained to be consistent with some sequential order of evaluation. The order of evaluation may be chosen differently for each procedure call.

> Note: In many dialects of Lisp, the form `()` is a legitimate expression. In Scheme, expressions written as list/pair forms must have at least one subexpression, so `()` is not a syntactically valid expression.

## 9.2 Macros

Libraries and top-level programs can define and use new kinds of derived expressions and definitions called syntactic abstractions or macros.A syntactic abstraction is created by binding a keyword to a macro transformer or, simply, transformer. The transformer determines how a use of the macro (called a macro use) is transcribed into a more primitive form.

Most macro uses have the form:

`(<keyword> <datum> `...`)

`where <keyword> is an identifier that uniquely determines the kind of form. This identifier is called the syntactic keyword, or simply keyword, of the macro. The number of <datum>s and the syntax of each depends on the syntactic abstraction.

Macro uses can also take the form of improper lists, singleton identifiers, or `set!` forms, where the second subform of the `set!` is the keyword (see section 11.19) library section on “`make-variable-transformer`”):

`(<keyword> <datum> `...` . <datum>)
<keyword>
(set! <keyword> <datum>)

`

The `define-syntax`, `let-syntax` and `letrec-syntax` forms, described in sections 11.2.2 and 11.18, create bindings for keywords, associate them with macro transformers, and control the scope within which they are visible.

The `syntax-rules` and `identifier-syntax` forms, described in section 11.19, create transformers via a pattern language. Moreover, the `syntax-case` form, described in library chapter on “`syntax-case`”, allows creating transformers via arbitrary Scheme code.

Keywords occupy the same name space as variables. That is, within the same scope, an identifier can be bound as a variable or keyword, or neither, but not both, and local bindings of either kind may shadow other bindings of either kind.

Macros defined using `syntax-rules` and `identifier-syntax` are “hygienic” and “referentially transparent” and thus preserve Scheme’s lexical scoping [16, 15, 2, 6, 9]:

- If a macro transformer inserts a binding for an identifier (variable or keyword) not appearing in the macro use, the identifier is in effect renamed throughout its scope to avoid conflicts with other identifiers.

- If a macro transformer inserts a free reference to an identifier, the reference refers to the binding that was visible where the transformer was specified, regardless of any local bindings that may surround the use of the macro.

Macros defined using the `syntax-case` facility are also hygienic unless `datum->syntax` (see library section on “Syntax-object and datum conversions”) is used.