► |  | The Racket Reference

1 | Language Model
2 | Syntactic Forms
3 | Datatypes
4 | Structures
5 | Classes and Objects
6 | Units
7 | Contracts
8 | Pattern Matching
9 | Control Flow
10 | Concurrency and Parallelism
11 | Macros
12 | Input and Output
13 | Reflection and Security
14 | Operating System
15 | Memory Management
16 | Unsafe Operations
17 | Running Racket
| Bibliography
| Index

▼ | 11 | Macros

11.1 | Pattern-Based Syntax Matching
11.2 | Syntax Object Content
11.3 | Syntax Object Bindings
11.4 | Syntax Transformers
11.5 | Syntax Parameters
11.6 | Local Binding with Splicing Body
11.7 | Syntax Object Properties
11.8 | Syntax Taints
11.9 | Expanding Top-Level Forms
11.10 | File Inclusion
11.11 | Syntax Utilities

► | 11.5 | Syntax Parameters

11.5.1 | Syntax Parameter Inspection

On this page:

define-syntax-parameter
syntax-parameterize
11.5.1`` Syntax Parameter Inspection
syntax-parameter-value
make-parameter-rename-transformer

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#### 11.5`` Syntax Parameters

(require racket/stxparam)

The bindings documented in this section are provided by the racket/stxparam library, not racket/base or racket.

> > syntax

> > (define-syntax-parameter id expr)

Binds id as syntax to a syntax parameter. The expr is an expression in the transformer environment that serves as the default value for the syntax parameter. The value is typically obtained by a transformer using syntax-parameter-value.

The id can be used with syntax-parameterize or syntax-parameter-value (in a transformer). If expr produces a procedure of one argument or a make-set!-transformer result, then id can be used as a macro. If expr produces a make-rename-transformer result, then id can be used as a macro that expands to a use of the target identifier, but syntax-local-value of id does not produce the target’s value.

Examples:

> > (define-syntax-parameter current-class #f)

> > (define-syntax-parameter yield (make-rename-transformer #'abort))

> > (define-syntax-parameter define/public
> (λ (stx)
> (raise-syntax-error #f "use of a class keyword not in a class" stx)))

> > (begin-for-syntax (displayln (syntax-parameter-value #'current-class)))
> #f

> > (yield 5)
> 5

> > syntax

> > (syntax-parameterize ([id expr] ...) body-expr ...+)

> > > See also splicing-syntax-parameterize.

Each id must be bound to a syntax parameter using define-syntax-parameter. Each expr is an expression in the transformer environment. During the expansion of the body-exprs, the value of each expr is bound to the corresponding id.

If an expr produces a procedure of one argument or a make-set!-transformer result, then its id can be used as a macro during the expansion of the body-exprs. If expr produces a make-rename-transformer result, then id can be used as a macro that expands to a use of the target identifier, but syntax-local-value of id does not produce the target’s value.

Examples:

> > (define-syntax-parameter abort (syntax-rules ()))

> > (define-syntax forever
> (syntax-rules ()
> [(forever body ...)
> (call/cc (lambda (abort-k)
> (syntax-parameterize
> ([abort (syntax-rules () [(_) (abort-k)])])
> (let loop () body ... (loop)))))]))

> > (define-syntax-parameter it (syntax-rules ()))

> > (define-syntax aif
> (syntax-rules ()
> [(aif test then else)
> (let ([t test])
> (syntax-parameterize ([it (syntax-id-rules () [_ t])])
> (if t then else)))]))

##### 11.5.1`` Syntax Parameter Inspection

(require racket/stxparam-exptime)

> > procedure

> > (syntax-parameter-value id-stx) → any

> id-stx : syntax?

This procedure is intended for use in a transformer environment, where id-stx is an identifier bound in the normal environment to a syntax parameter. The result is the current value of the syntax parameter, as adjusted by syntax-parameterize form.

This binding is provided for-syntax by racket/stxparam, since it is normally used in a transformer. It is provided normally by racket/stxparam-exptime.

> > procedure

> > (make-parameter-rename-transformer id-stx) → any

> id-stx : syntax?

This procedure is intended for use in a transformer, where id-stx is an identifier bound to a syntax parameter. The result is a transformer that behaves as id-stx, but that cannot be used with syntax-parameterize or syntax-parameter-value.

Using make-parameter-rename-transformer is analogous to defining a procedure that calls a parameter. Such a procedure can be exported to others to allow access to the parameter value, but not to change the parameter value. Similarly, make-parameter-rename-transformer allows a syntax parameter to be used as a macro, but not changed.

The result of make-parameter-rename-transformer is not treated specially by syntax-local-value, unlike the result of make-rename-transformer.

This binding is provided for-syntax by racket/stxparam, since it is normally used in a transformer. It is provided normally by racket/stxparam-exptime.

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