► |  | The Racket Reference

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

▼ | 12 | Macros

12.1 | Pattern-Based Syntax Matching
12.2 | Syntax Object Content
12.3 | Syntax Object Bindings
12.4 | Syntax Transformers
12.5 | Syntax Parameters
12.6 | Local Binding with Splicing Body
12.7 | Syntax Object Properties
12.8 | Syntax Taints
12.9 | Expanding Top-Level Forms
12.10 | File Inclusion
12.11 | Syntax Utilities

► | 12.4 | Syntax Transformers

12.4.1 | require Transformers
12.4.2 | provide Transformers
12.4.3 | Keyword-Argument Conversion Introspection

On this page:

set!-transformer?
make-set!-transformer
set!-transformer-procedure
prop: set!-transformer
rename-transformer?
make-rename-transformer
rename-transformer-target
prop: rename-transformer
local-expand
syntax-local-expand-expression
local-transformer-expand
local-expand/ capture-lifts
local-transformer-expand/ capture-lifts
internal-definition-context?
syntax-local-make-definition-context
syntax-local-bind-syntaxes
internal-definition-context-seal
identifier-remove-from-definition-context
syntax-local-value
syntax-local-value/ immediate
syntax-local-lift-expression
syntax-local-lift-values-expression
syntax-local-lift-context
syntax-local-lift-module-end-declaration
syntax-local-lift-require
syntax-local-lift-provide
syntax-local-name
syntax-local-context
syntax-local-phase-level
syntax-local-module-exports
syntax-local-submodules
syntax-local-get-shadower
syntax-local-certifier
syntax-transforming?
syntax-transforming-module-expression?
syntax-local-introduce
make-syntax-introducer
make-syntax-delta-introducer
syntax-local-make-delta-introducer
syntax-local-transforming-module-provides?
syntax-local-module-defined-identifiers
syntax-local-module-required-identifiers
prop: liberal-define-context
liberal-define-context?
12.4.1`` require Transformers
expand-import
make-require-transformer
prop: require-transformer
require-transformer?
import
import-source
current-require-module-path
convert-relative-module-path
syntax-local-require-certifier
12.4.2`` provide Transformers
expand-export
pre-expand-export
make-provide-transformer
make-provide-pre-transformer
prop: provide-transformer
prop: provide-pre-transformer
provide-transformer?
provide-pre-transformer?
export
syntax-local-provide-certifier
12.4.3`` Keyword-Argument Conversion Introspection
syntax-procedure-alias-property
syntax-procedure-converted-arguments-property

top ← prev up next →

#### 12.4`` Syntax Transformers

> procedure (set!-transformer? v) → boolean?
> v : any/c

Returns #t if v is a value created by make-set!-transformer or an instance of a structure type with the prop:set!-transformer property, #f otherwise.

> procedure (make-set!-transformer proc) → set!-transformer?
> proc : (syntax? . -> . syntax?)

Creates an assignment transformer that cooperates with set!. If the result of make-set!-transformer is bound to id as a transformer binding, then proc is applied as a transformer when id is used in an expression position, or when it is used as the target of a set! assignment as (set! id expr). When the identifier appears as a set! target, the entire set! expression is provided to the transformer.

Example:
> > (let ([x 1]
> [y 2])
> (let-syntax ([x (make-set!-transformer
> (lambda (stx)
> (syntax-case stx (set!)
> ; Redirect mutation of x to y
> [(set! id v) #'(set! y v)]
> ; Normal use of x really gets x
> [id (identifier? #'id) #'x])))])
> (begin
> (set! x 3)
> (list x y))))
> '(1 3)

> procedure (set!-transformer-procedure transformer)
> → (syntax? . -> . syntax?)
> transformer : set!-transformer?

Returns the procedure that was passed to make-set!-transformer to create transformer or that is identified by the prop:set!-transformer property of transformer.

> value prop:set!-transformer : struct-type-property?

A structure type property to identify structure types that act as assignment transformers like the ones created by make-set!-transformer.

The property value must be an exact integer or procedure of one or two arguments. In the former case, the integer designates a field within the structure that should contain a procedure; the integer must be between 0 (inclusive) and the number of non-automatic fields in the structure type (exclusive, not counting supertype fields), and the designated field must also be specified as immutable.

If the property value is a procedure of one argument, then the procedure serves as a syntax transformer and for set! transformations. If the property value is a procedure of two arguments, then the first argument is the structure whose type has prop:set!-transformer property, and the second argument is a syntax object as for a syntax transformer and for set! transformations; set!-transformer-procedure applied to the structure produces a new function that accepts just the syntax object and calls the procedure associated through the property. Finally, if the property value is an integer, the target identifier is extracted from the structure instance; if the field value is not a procedure of one argument, then a procedure that always calls raise-syntax-error is used, instead.

If a value has both the prop:set!-transformer and prop:rename-transformer properties, then the latter takes precedence. If a structure type has the prop:set!-transformer and prop:procedure properties, then the former takes precedence for the purposes of macro expansion.

> procedure (rename-transformer? v) → boolean?
> v : any/c

Returns #t if v is a value created by make-rename-transformer or an instance of a structure type with the prop:rename-transformer property, #f otherwise.

Examples:
> > (rename-transformer? (make-rename-transformer #'values))
> #t
> > (rename-transformer? 'not-a-rename-transformer)
> #f

> > procedure
> > (make-rename-transformer |  | id-stx |  |  |  |
> > | [ | delta-introduce]) |  | → |  | rename-transformer?
> id-stx : syntax?
> | delta-introduce |  | : |  | (identifier? . -> . identifier?)
> |  |  | = |  | (lambda (id) id)

Creates a rename transformer that, when used as a transformer binding, acts as a transformer that inserts the identifier id-stx in place of whatever identifier binds the transformer, including in non-application positions, in set! expressions.

Such a transformer could be written manually, but the one created by make-rename-transformer triggers special cooperation with the parser and other syntactic forms when id is bound to the rename transformer:

- The parser installs a free-identifier=? and identifier-binding equivalence between id and id-stx, as long as id-stx does not have a true value for the 'not-free-identifier=? syntax property.

- A provide of id provides the binding indicated by id-stx instead of id, as long as id-stx does not have a true value for the 'not-free-identifier=? syntax property and as long as id-stx has a binding.

- If provide exports id, it uses a symbol-valued 'nominal-id property of id-stx to specify the “nominal source identifier” of the binding as reported by identifier-binding.

- If id-stx has a true value for the 'not-provide-all-defined syntax property, then id (or its target) is not exported by all-defined-out.

- The syntax-local-value and syntax-local-make-delta-introducer functions recognize rename-transformer bindings and consult their targets.

Examples:
> > (define-syntax my-or (make-rename-transformer #'or))
> > (my-or #f #t)
> #t
> > (free-identifier=? #'my-or #'or)
> #t

> procedure (rename-transformer-target transformer) → identifier?
> transformer : rename-transformer?

Returns the identifier passed to make-rename-transformer to create transformer or as indicated by a prop:rename-transformer property on transformer.

Example:
> > (rename-transformer-target (make-rename-transformer #'or))
> #<syntax:8:0 or>

> value prop:rename-transformer : struct-type-property?

A structure type property to identify structure types that act as rename transformers like the ones created by make-rename-transformer.

The property value must be an exact integer or an identifier syntax object. In the former case, the integer designates a field within the structure that should contain an identifier; the integer must be between 0 (inclusive) and the number of non-automatic fields in the structure type (exclusive, not counting supertype fields), and the designated field must also be specified as immutable.

If the property value is an identifier, the identifier serves as the target for renaming, just like the first argument to make-rename-transformer. If the property value is an integer, the target identifier is extracted from the structure instance; if the field value is not an identifier, then an identifier ? with an empty context is used, instead.

> > procedure
> > (local-expand |  | stx |  |  |  |
> > |  | context-v |  |  |  |
> > |  | stop-ids |  |  |  |
> > | [ | intdef-ctx]) |  | → |  | syntax?
> stx : syntax?
> context-v : (or/c 'expression 'top-level 'module 'module-begin list?)
> stop-ids : (or/c (listof identifier?) #f)
> | intdef-ctx |  | : |  |
> (or/c internal-definition-context?
> (and/c pair?
> (listof internal-definition-context?))
> #f)
> |  |  | = |  | #f

Expands stx in the lexical context of the expression currently being expanded. The context-v argument is used as the result of syntax-local-context for immediate expansions; a list indicates an internal-definition context, and more information on the form of the list is below.

When an identifier in stop-ids is encountered by the expander in a sub-expression, expansions stops for the sub-expression. If stop-ids is a non-empty list and does not contain just module*, then begin, quote, set!, lambda, case-lambda, let-values, letrec-values, if, begin0, with-continuation-mark, letrec-syntaxes+values, #%app, #%expression, #%top, and #%variable-reference are added to stop-ids. If #%app, #%top, or #%datum appears in stop-ids, then application, top-level variable reference, and literal data expressions without the respective explicit form are not wrapped with the explicit form. If stop-ids is #f instead of a list, then stx is expanded only as long as the outermost form of stx is a macro (i.e., expansion does not proceed to sub-expressions). A fully expanded form can include the bindings listed in Fully Expanded Programs plus the letrec-syntaxes+values form and #%expression in any expression position.

When #%plain-module-begin is not itself in stop-ids and module* is in stop-ids, then the #%plain-module-begin transformer refrains from expanding module* sub-forms. Otherwise, the #%plain-module-begin transformer detects and expands sub-forms (such as define-values) independent of the corresponding identifier’s presence in stop-ids.

When context-v is 'module-begin, and the result of expansion is a #%plain-module-begin form, then a 'submodule syntax property is added to each enclosed module form (but not module* forms) in the same way as by module expansion.

The optional intdef-ctx argument must be either #f, the result of syntax-local-make-definition-context, or a list of such results. In the latter two cases, lexical information for internal definitions is added to stx before it is expanded (in reverse order relative to the list). The lexical information is also added to the expansion result (because the expansion might introduce bindings or references to internal-definition bindings).

For a particular internal-definition context, generate a unique value and put it into a list for context-v. To allow liberal expansion of define forms, the generated value should be an instance of a structure with a true value for prop:liberal-define-context. If the internal-definition context is meant to be self-contained, the list for context-v should contain only the generated value; if the internal-definition context is meant to splice into an immediately enclosing context, then when syntax-local-context produces a list, cons the generated value onto that list.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

Examples:
> > (define-syntax-rule (do-print x ...)
> (printf x ...))
> > (define-syntax-rule (hello x)
> (do-print "hello ~a" x))
> > (define-syntax (show stx)
> (syntax-case stx ()
> [(_ x)
> (let ([partly (local-expand #'(hello x)
> 'expression
> (list #'do-print))]
> [fully (local-expand #'(hello x)
> 'expression
> #f)])
> (printf "partly expanded: ~s\n" (syntax->datum partly))
> (printf "fully expanded: ~s\n" (syntax->datum fully))
> fully)]))
> > (show 1)
> partly expanded: (do-print "hello ~a" 1)
> fully expanded: (printf "hello ~a" 1)
> hello 1

> > procedure
> > (syntax-local-expand-expression stx) |  | → |  |
> > syntax? |  | syntax?
> stx : syntax?

Like local-expand given 'expression and an empty stop list, but with two results: a syntax object for the fully expanded expression, and a syntax object whose content is opaque. The latter can be used in place of the former (perhaps in a larger expression produced by a macro transformer), and when the macro expander encounters the opaque object, it substitutes the fully expanded expression without re-expanding it; the exn:fail:syntax exception is raised if the expansion context includes bindings or marks that were not present for the original expansion, in which case re-expansion might produce different results. Consistent use of syntax-local-expand-expression and the opaque object thus avoids quadratic expansion times when local expansions are nested.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> > procedure
> > (local-transformer-expand |  | stx |  |  |  |
> > |  | context-v |  |  |  |
> > |  | stop-ids |  |  |  |
> > | [ | intdef-ctx]) |  | → |  | syntax?
> stx : syntax?
> context-v : (or/c 'expression 'top-level list?)
> stop-ids : (or/c (listof identifier?) #f)
> intdef-ctx : (or/c internal-definition-context? #f) = #f

Like local-expand, but stx is expanded as a transformer expression instead of a run-time expression. For 'expression expansion, any lifted expressions—from calls to syntax-local-lift-expression during the expansion of stx—are captured into a let-values form in the result.

> > procedure
> > (local-expand/capture-lifts |  | stx |  |  |  |
> > |  | context-v |  |  |  |
> > |  | stop-ids |  |  |  |
> > | [ | intdef-ctx |  |  |  |
> > |  | lift-ctx]) |  | → |  | syntax?
> stx : syntax?
> context-v : (or/c 'expression 'top-level 'module 'module-begin list?)
> stop-ids : (or/c (listof identifier?) #f)
> intdef-ctx : (or/c internal-definition-context? #f) = #f
> lift-ctx : any/c = (gensym 'lifts)

Like local-expand, but the result is a syntax object that represents a begin expression. Lifted expressions—from calls to syntax-local-lift-expression during the expansion of stx—appear with their identifiers in define-values forms, and the expansion of stx is the last expression in the begin. The lift-ctx value is reported by syntax-local-lift-context during local expansion. The lifted expressions are not expanded, but instead left as provided in the begin form.

> > procedure
> > (local-transformer-expand/capture-lifts |  | stx |  |  |  |
> > |  | context-v |  |  |  |
> > |  | stop-ids |  |  |  |
> > | [ | intdef-ctx |  |  |  |
> > |  | lift-ctx]) |  | → |  | syntax?
> stx : syntax?
> context-v : (or/c 'expression 'top-level list?)
> stop-ids : (or/c (listof identifier?) #f)
> intdef-ctx : (or/c internal-definition-context? #f) = #f
> lift-ctx : any/c = (gensym 'lifts)

Like local-expand/capture-lifts, but stx is expanded as a transformer expression instead of a run-time expression. Lifted expressions are reported as define-values forms (in the transformer environment).

> procedure (internal-definition-context? v) → boolean?
> v : any/c

Returns #t if v is an internal-definition context, #f otherwise.

> procedure (syntax-local-make-definition-context [intdef-ctx])
> → internal-definition-context?
> intdef-ctx : (or/c internal-definition-context? #f) = #f

Creates an opaque internal-definition context value to be used with local-expand and other functions. A transformer should create one context for each set of internal definitions to be expanded, and use it when expanding any form whose lexical context should include the definitions. After discovering an internal define-values or define-syntaxes form, use syntax-local-bind-syntaxes to add bindings to the context. Finally, the transformer must call internal-definition-context-seal after all bindings have been added; if an unsealed internal-definition context is detected in a fully expanded expression, the exn:fail:contract exception is raised.

If intdef-ctx is not #f, then the new internal-definition context extends the given one. That is, expanding in the new internal-definition context can use bindings previously introduced into intdef-ctx.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> > procedure
> > (syntax-local-bind-syntaxes |  | id-list |  |  |  |
> > |  | expr |  |  |  |
> > |  | intdef-ctx) |  | → |  | void?
> id-list : (listof identifier?)
> expr : (or/c syntax? #f)
> intdef-ctx : internal-definition-context?

Binds each identifier in id-list within the internal-definition context represented by intdef-ctx, where intdef-ctx is the result of syntax-local-make-definition-context. Supply #f for expr when the identifiers correspond to define-values bindings, and supply a compile-time expression when the identifiers correspond to define-syntaxes bindings; in the latter case, the number of values produced by the expression should match the number of identifiers, otherwise the exn:fail:contract:arity exception is raised.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (internal-definition-context-seal intdef-ctx) → void?
> intdef-ctx : internal-definition-context?

Indicates that no further bindings will be added to intdef-ctx, which must not be sealed already. See also syntax-local-make-definition-context.

> > procedure
> > (identifier-remove-from-definition-context |  | id-stx |
> > |  | intdef-ctx) |
> → identifier?
> id-stx : identifier?
> | intdef-ctx |  | : |  |
> (or/c internal-definition-context?
> (listof internal-definition-context?))

Removes intdef-ctx (or each identifier in the list) from the lexical information of id-stx. This operation is useful for correlating an identifier that is bound in an internal-definition context with its binding before the internal-definition context was created.

If simply removing the contexts produces a different binding than completely ignoring the contexts (due to nested internal definition contexts, for example), then the resulting identifier is given a syntax mark to simulate a non-existent lexical context. The intdef-ctx argument can be a list because removing internal-definition contexts one at a time can produce a different intermediate binding than removing them all at once.

> > procedure
> > (syntax-local-value |  | id-stx |  |  |  |
> > | [ | failure-thunk |  |  |  |
> > |  | intdef-ctx]) |  | → |  | any
> id-stx : syntax?
> failure-thunk : (or/c (-> any) #f) = #f
> | intdef-ctx |  | : |  |
> (or/c internal-definition-context?
> #f)
> |  | = |  | #f

Returns the transformer binding value of id-stx in either the context associated with intdef-ctx (if not #f) or the context of the expression being expanded (if intdef-ctx is #f). If intdef-ctx is provided, it must be an extension of the context of the expression being expanded.

If id-stx is bound to a rename transformer created with make-rename-transformer, syntax-local-value effectively calls itself with the target of the rename and returns that result, instead of the rename transformer.

If id-stx has no transformer binding (via define-syntax, let-syntax, etc.) in that environment, the result is obtained by applying failure-thunk if not #f. If failure-thunk is false, the exn:fail:contract exception is raised.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

Examples:
> > (define-syntax swiss-cheeses? #t)
> > (define-syntax (transformer stx)
> (if (syntax-local-value #'swiss-cheeses?)
> #''(gruyère emmental raclette)
> #''(roquefort camembert boursin)))
> > (transformer)
> '(gruyère emmental raclette)

Examples:
> > (define-syntax (transformer-2 stx)
> (syntax-local-value #'something-else (λ () (error "no binding"))))
> > (transformer-2)
> no binding

Examples:
> > (define-syntax nachos #'(printf "nachos~n"))
> > (define-syntax chips (make-rename-transformer #'nachos))
> > (define-syntax (transformer-3 stx)
> (syntax-local-value #'chips))
> > (transformer-3)
> nachos

> > procedure
> > (syntax-local-value/immediate |  | id-stx |  |  |  |
> > | [ | failure-thunk |  |  |  |
> > |  | intdef-ctx]) |  | → |  | any
> id-stx : syntax?
> failure-thunk : (or/c (-> any) #f) = #f
> | intdef-ctx |  | : |  |
> (or/c internal-definition-context?
> #f)
> |  | = |  | #f

Like syntax-local-value, but the result is normally two values. If id-stx is bound to a rename transformer, the results are the rename transformer and the identifier in the transformer. Beware that provide on an id bound to a rename transformer may export the target of the rename instead of id. See make-rename-transformer for more information. If id-stx is not bound to a rename transformer, then the results are the value that syntax-local-value would produce and #f.

If id-stx has no transformer binding, then failure-thunk is called (and it can return any number of values), or an exception is raised if failure-thunk is #f.

> procedure (syntax-local-lift-expression stx) → identifier?
> stx : syntax?

Returns a fresh identifier, and cooperates with the module, letrec-syntaxes+values, define-syntaxes, begin-for-syntax, and top-level expanders to bind the generated identifier to the expression stx.

A run-time expression within a module is lifted to the module’s top level, just before the expression whose expansion requests the lift. Similarly, a run-time expression outside of a module is lifted to a top-level definition. A compile-time expression in a letrec-syntaxes+values or define-syntaxes binding is lifted to a let wrapper around the corresponding right-hand side of the binding. A compile-time expression within begin-for-syntax is lifted to a define declaration just before the requesting expression within the begin-for-syntax.

Other syntactic forms can capture lifts by using local-expand/capture-lifts or local-transformer-expand/capture-lifts.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-lift-values-expression n stx)
> → (listof identifier?)
> n : exact-nonnegative-integer?
> stx : syntax?

Like syntax-local-lift-expression, but binds the result to n identifiers, and returns a list of the n identifiers.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-lift-context) → any/c

Returns a value that represents the target for expressions lifted via syntax-local-lift-expression. That is, for different transformer calls for which this procedure returns the same value (as determined by eq?), lifted expressions for the two transformer are moved to the same place. Thus, the result is useful for caching lift information to avoid redundant lifts.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-lift-module-end-declaration stx) → void?
> stx : syntax?

Cooperates with the module form to insert stx as a top-level declaration at the end of the module currently being expanded. If the current expression being transformed is in phase level 0 and not in the module top-level, then stx is eventually expanded in an expression context. If the current expression being transformed is in a higher phase level (i.e., nested within some number of begin-for-syntaxes within a module top-level), then the lifted declaration is placed at the very end of the module (under a suitable number of begin-for-syntaxes), instead of merely the end of the enclosing begin-for-syntax.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.
If the current expression being transformed is not within a module form (see syntax-transforming-module-expression?), then the exn:fail:contract exception is raised.

> > procedure
> > (syntax-local-lift-require |  | raw-require-spec |  |  |  |
> > |  | stx) |  | → |  | syntax?
> raw-require-spec : any/c
> stx : syntax?

Lifts a #%require form corresponding to raw-require-spec (either as a syntax object or datum) to the top-level or to the top of the module currently being expanded or to an enclosing begin-for-syntax..

The resulting syntax object is the same as stx, except that a fresh syntax mark is added. The same syntax mark is added to the lifted #%require form, so that the #%require form can bind uses of imported identifiers in the resulting syntax object (assuming that the lexical information of stx includes the binding environment into which the #%require is lifted).

If raw-require-spec and stx are part of the input to a transformer, then typically syntax-local-introduce should be applied to each before passing them to syntax-local-lift-require, and then syntax-local-introduce should be applied to the result of syntax-local-lift-require. Otherwise, marks added by the macro expander can prevent access to the new imports.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-lift-provide raw-provide-spec-stx) → void?
> raw-provide-spec-stx : syntax?

Lifts a #%provide form corresponding to raw-provide-spec-stx to the top of the module currently being expanded or to an enclosing begin-for-syntax.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.
If the current expression being transformed is not within a module form (see syntax-transforming-module-expression?), then the exn:fail:contract exception is raised.

> procedure (syntax-local-name) → any/c

Returns an inferred name for the expression position being transformed, or #f if no such name is available. A name is normally a symbol or an identifier. See also Inferred Value Names.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-context)
> → (or/c 'expression 'top-level 'module 'module-begin list?)

Returns an indication of the context for expansion that triggered a syntax transformer call. See Expansion Context for more information on contexts.

The symbol results indicate that the expression is being expanded for an expression context, a top-level context, a module context, or a module-begin context.

A list result indicates expansion in an internal-definition context. The identity of the list’s first element (i.e., its eq?ness) reflects the identity of the internal-definition context; in particular two transformer expansions receive the same first value if and only if they are invoked for the same internal-definition context. Later values in the list similarly identify internal-definition contexts that are still being expanded, and that required the expansion of nested internal-definition contexts.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-phase-level) → (or/c exact-integer? #f)

During the dynamic extent of a syntax transformer application by the expander, the result is the phase level of the form being expanded. Otherwise, the result is 0.

> procedure (syntax-local-module-exports mod-path)
> → (listof (cons/c (or/c exact-integer? #f) (listof symbol?)))
> | mod-path |  | : |  |
> (or/c module-path?
> (and/c syntax?
> (lambda (stx)
> (module-path? (syntax->datum stx)))))

Returns an association list from phase-level numbers (or #f for the label phase level) to lists of symbols, where the symbols are the names of provided bindings from mod-path at the corresponding phase level.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-submodules) → (listof symbol?)

Returns a list of submodule names that are declared via module (as opposed to module*) in the current expansion context.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-get-shadower id-stx) → identifier?
> id-stx : identifier?

Returns id-stx if no binding in the current expansion context shadows id-stx (ignoring unsealed internal-definition contexts and identifiers that had the 'unshadowable syntax property), if id-stx has no module bindings in its lexical information, and if the current expansion context is not a module context.

If a binding of inner-identifier shadows id-stx, the result is the same as (syntax-local-get-shadower inner-identifier), except that it has the location and properties of id-stx. When searching for a shadowing binding, bindings from unsealed internal-definition contexts are ignored.

Otherwise, the result is the same as id-stx with its module bindings (if any) removed from its lexical information, and the lexical information of the current module context (if any) added.

Thus, the result is an identifier corresponding to the innermost shadowing of id-stx in the current context if it is shadowed, and a module-contextless version of id-stx otherwise.

If id-stx is tainted or armed, then the resulting identifier is tainted.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-certifier [active?])
> | → |  |
> ((syntax?) (any/c (or/c procedure? #f))
> . ->* . syntax?)
> active? : boolean? = #f

For backward compatibility only; returns a procedure that returns its first argument.

> procedure (syntax-transforming?) → boolean?

Returns #t during the dynamic extent of a syntax transformer application by the expander and while a module is being visited, #f otherwise.

> procedure (syntax-transforming-module-expression?) → boolean?

Returns #t during the dynamic extent of a syntax transformer application by the expander for an expression within a module form, #f otherwise.

> procedure (syntax-local-introduce stx) → syntax?
> stx : syntax?

Produces a syntax object that is like stx, except that a syntax mark for the current expansion is added (possibly canceling an existing mark in parts of stx). See Transformer Bindings for information on syntax marks.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (make-syntax-introducer) → (syntax? . -> . syntax?)

Produces a procedure that behaves like syntax-local-introduce, but using a fresh syntax mark. Multiple applications of the same make-syntax-introducer result procedure use the same mark, and different result procedures use distinct marks.

> > procedure
> > (make-syntax-delta-introducer |  | ext-stx |
> > |  | base-stx |
> > | [ | phase-level]) |
> → (syntax? . -> . syntax?)
> ext-stx : syntax?
> base-stx : (or/c syntax? #f)
> | phase-level |  | : |  | (or/c #f exact-integer?)
> |  |  | = |  | (syntax-local-phase-level)

Produces a procedure that behaves like syntax-local-introduce, but using the syntax marks of ext-stx that are not shared with base-stx. If ext-stx does not extend the set of marks in base-stx or if base-stx is #f, and if ext-stx has a module binding in the phase level indicated by phase-level, then any marks of ext-stx that would be needed to preserve its binding are not transferred in an introduction.

This procedure is potentially useful when m-id has a transformer binding that records some orig-id, and a use of m-id introduces a binding of orig-id. In that case, the syntax marks in the use of m-id since the binding of m-id should be transferred to the binding instance of orig-id, so that it captures uses with the same lexical context as the use of m-id.

More typically, however, syntax-local-make-delta-introducer should be used, since it cooperates with rename transformers.

If ext-stx is tainted or armed, then an identifier result from the created procedure is tainted.

> procedure (syntax-local-make-delta-introducer id)
> → (identifier? . -> . identifier?)
> id : identifier?

Determines the binding of id. If the binding is not a rename transformer, the result is an introducer as created by make-syntax-delta-introducer using id and the binding of id in the environment of expansion. If the binding is a rename transformer, then the introducer is one composed with the target of the rename transformer and its binding. Furthermore, the delta-introduce functions associated with the rename transformers (supplied as the second argument to make-rename-transformer) are composed (in first-to-last order) before the introducers created with make-syntax-delta-introducer (which are composed last-to-first).

The exn:fail:contract exception is raised if id or any identifier in its rename-transformer chain has no binding.

This procedure must be called during the dynamic extent of a syntax transformer application by the expander or while a module is visited (see syntax-transforming?), otherwise the exn:fail:contract exception is raised.

> procedure (syntax-local-transforming-module-provides?) → boolean?

Returns #t while a provide transformer is running (see make-provide-transformer) or while an expand sub-form of #%provide is expanded, #f otherwise.

> procedure (syntax-local-module-defined-identifiers)
> → (and/c hash? immutable?)

Can be called only while syntax-local-transforming-module-provides? returns #t.

It returns a hash table mapping a phase-level number (such as 0) to a list of all definitions at that phase level within the module being expanded. This information is used for implementing provide sub-forms like all-defined-out.

Beware that the phase-level keys are absolute relative to the enclosing module, and not relative to the current transformer phase level as reported by syntax-local-phase-level.

> > procedure
> > (syntax-local-module-required-identifiers |  | mod-path |
> > |  | phase-level) |
> | → |  |
> (listof (cons/c (or/c exact-integer? #f)
> (listof identifier?)))
> mod-path : (or/c module-path? #f)
> phase-level : (or/c exact-integer? #f #t)

Can be called only while syntax-local-transforming-module-provides? returns #t.

It returns an association list mapping phase levels to lists of identifiers. Each list of identifiers includes all bindings imported (into the module being expanded) using the module path mod-path, or all modules if mod-path is #f. The association list includes all identifiers imported with a phase-level shift, of all shifts if phase-level is #t.

When an identifier is renamed on import, the result association list includes the identifier by its internal name. Use identifier-binding to obtain more information about the identifier.

Beware that the phase-level keys are absolute relative to the enclosing module, and not relative to the current transformer phase level as reported by syntax-local-phase-level.

> value prop:liberal-define-context : struct-type-property?
> procedure (liberal-define-context? v) → boolean?
> v : any/c

An instance of a structure type with a true value for the prop:liberal-define-context property can be used as an element of an internal-definition context representation in the result of syntax-local-context or the second argument of local-expand. Such a value indicates that the context supports liberal expansion of define forms into potentially multiple define-values and define-syntaxes forms. The 'module and 'module-body contexts implicitly allow liberal expansion.

The liberal-define-context? predicate returns #t if v is an instance of a structure with a true value for the prop:liberal-define-context property, #f otherwise.

##### 12.4.1`` require Transformers

(require racket/require-transform) | package: base

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

A transformer binding whose value is a structure with the prop:require-transformer property implements a derived require-spec for require as a require transformer.

A require transformer is called with the syntax object representing its use as a require-spec within a require form, and the result must be two lists: a list of imports and a list of import-sources.

If the derived form contains a sub-form that is a require-spec, then it can call expand-import to transform the sub-require-spec to lists of imports and import sources.

See also define-require-syntax, which supports macro-style require transformers.

> > procedure
> > (expand-import stx) |  | → |  |
> > (listof import?) |  | (listof import-source?)
> stx : syntax?

Expands the given require-spec to lists of imports and import sources. The latter specifies modules to be instantiated or visited, so the modules that it represents should be a superset of the modules represented in the former list (so that a module will be instantiated or visited even if all of imports are eventually filtered from the former list).

> procedure (make-require-transformer proc) → require-transformer?
> | proc |  | : |  |
> (syntax? . -> . (values
> (listof import?)
> (listof import-source?)))

Creates a require transformer using the given procedure as the transformer.

> value prop:require-transformer : struct-type-property?

A property to identify require transformers. The property value must be a procedure that takes the structure and returns a transformer procedure; the returned transformer procedure takes a syntax object and returns import and import-source lists.

> procedure (require-transformer? v) → boolean?
> v : any/c

Returns #t if v has the prop:require-transformer property, #f otherwise.

> > struct
> > (struct |  | import | ( | local-id
> > |  |  |  | src-sym
> > |  |  |  | src-mod-path
> > |  |  |  | mode
> > |  |  |  | req-mode
> > |  |  |  | orig-mode
> > |  |  |  | orig-stx)
> > |  | #:extra-constructor-name make-import)
> local-id : identifier?
> src-sym : symbol?
> | src-mod-path |  | : |  |
> (or/c module-path?
> (and/c syntax?
> (lambda (stx)
> (module-path? (syntax->datum stx)))))
> mode : (or/c exact-integer? #f)
> req-mode : (or/c exact-integer? #f)
> orig-mode : (or/c exact-integer? #f)
> orig-stx : syntax?

A structure representing a single imported identifier:

- local-id — the identifier to be bound within the importing module.

- src-sym — the external name of the binding as exported from its source module.

- src-mod-path — a module path (relative to the importing module) for the source of the imported binding.

- orig-stx — a syntax object for the source of the import, used for error reporting.

- mode — the phase level of the binding in the importing module.

- req-mode — the phase level shift of the import relative to the exporting module.

- orig-mode — the phase level of the binding as exported by the exporting module.

> > struct
> > (struct |  | import-source (mod-path-stx mode)
> > |  | #:extra-constructor-name make-import-source)
> | mod-path-stx |  | : |  |
> (and/c syntax?
> (lambda (x)
> (module-path? (syntax->datum x))))
> mode : (or/c exact-integer? #f)

A structure representing an imported module, which must be instantiated or visited even if no binding is imported into a module.

- mod-path-stx — a module path (relative to the importing module) for the source of the imported binding.

- mode — the phase level shift of the import.

> parameter (current-require-module-path) → (or/c #f module-path-index?)
> (current-require-module-path module-path) → void?
> module-path : (or/c #f module-path-index?)

A parameter that determines how relative require-level module paths are expanded to #%require-level module paths by convert-relative-module-path (which is used implicitly by all built-in require sub-forms).

When the value of current-require-module-path is #f, relative module paths are left as-is, which means that the require context determines the resolution of the module path.

The require form parameterizes current-require-module-path as #f while invoking sub-form transformers, while relative-in parameterizes to a given module path.

> procedure (convert-relative-module-path module-path)
> | → |  |
> (or/c module-path?
> (and/c syntax?
> (lambda (stx)
> (module-path? (syntax-e stx)))))
> | module-path |  | : |  |
> (or/c module-path?
> (and/c syntax?
> (lambda (stx)
> (module-path? (syntax-e stx)))))

Converts module-path according to current-require-module-path.

If module-path is not relative or if the value of current-require-module-path is #f, then module-path is returned. Otherwise, module-path is converted to an absolute module path that is equivalent to module-path relative to the value of current-require-module-path.

> procedure (syntax-local-require-certifier)
> | → |  |
> ((syntax?) (or/c #f (syntax? . -> . syntax?))
> . ->* . syntax?)

For backward compatibility only; returns a procedure that returns its first argument.

##### 12.4.2`` provide Transformers

(require racket/provide-transform) | package: base

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

A transformer binding whose value is a structure with the prop:provide-transformer property implements a derived provide-spec for provide as a provide transformer. A provide transformer is applied as part of the last phase of a module’s expansion, after all other declarations and expressions within the module are expanded.

A transformer binding whose value is a structure with the prop:provide-pre-transformer property implements a derived provide-spec for provide as a provide pre-transformer. A provide pre-transformer is applied as part of the first phase of a module’s expansion. Since it is used in the first phase, a provide pre-transformer can use functions such as syntax-local-lift-expression to introduce expressions and definitions in the enclosing module.

An identifier can have a transformer binding to a value that acts both as a provide transformer and provide pre-transformer. The result of a provide pre-transformer is not automatically re-expanded, so a provide pre-transformer can usefully expand to itself in that case.

A transformer is called with the syntax object representing its use as a provide-spec within a provide form and a list of symbols representing the export modes specified by enclosing provide-specs. The result of a provide transformer must be a list of exports, while the result of a provide pre-transformer is a syntax object to be used as a provide-spec in the last phase of module expansion.

If a derived form contains a sub-form that is a provide-spec, then it can call expand-export or pre-expand-export to transform the sub-provide-spec sub-form.

See also define-provide-syntax, which supports macro-style provide transformers.

> procedure (expand-export stx modes) → (listof export?)
> stx : syntax?
> modes : (listof (or/c exact-integer? #f))

Expands the given provide-spec to a list of exports. The modes list controls the expansion of sub-provide-specs; for example, an identifier refers to a binding in the phase level of the enclosing provide form, unless the modes list specifies otherwise. Normally, modes is either empty or contains a single element.

> procedure (pre-expand-export stx modes) → syntax?
> stx : syntax?
> modes : (listof (or/c exact-integer? #f))

Expands the given provide-spec at the level of provide pre-transformers. The modes argument is the same as for expand-export.

> procedure (make-provide-transformer proc) → provide-transformer?
> | proc |  | : |  |
> (syntax? (listof (or/c exact-integer? #f))
> . -> . (listof export?))
> (make-provide-transformer proc pre-proc)
> → (and/c provide-transformer? provide-pre-transformer?)
> | proc |  | : |  |
> (syntax? (listof (or/c exact-integer? #f))
> . -> . (listof export?))
> | pre-proc |  | : |  |
> (syntax? (listof (or/c exact-integer? #f))
> . -> . syntax?)

Creates a provide transformer (i.e., a structure with the prop:provide-transformer property) using the given procedure as the transformer. If a pre-proc is provided, then the result is also a provide pre-transformer.

> procedure (make-provide-pre-transformer pre-proc)
> → provide-pre-transformer?
> | pre-proc |  | : |  |
> (syntax? (listof (or/c exact-integer? #f))
> . -> . syntax?)

Like make-provide-transformer, but for a value that is a provide pre-transformer, only.

> value prop:provide-transformer : struct-type-property?

A property to identify provide transformers. The property value must be a procedure that takes the structure and returns a transformer procedure; the returned transformer procedure takes a syntax object and mode list and returns an export list.

> value prop:provide-pre-transformer : struct-type-property?

A property to identify provide pre-transformers. The property value must be a procedure that takes the structure and returns a transformer procedure; the returned transformer procedure takes a syntax object and mode list and returns a syntax object.

> procedure (provide-transformer? v) → boolean?
> v : any/c

Returns #t if v has the prop:provide-transformer property, #f otherwise.

> procedure (provide-pre-transformer? v) → boolean?
> v : any/c

Returns #t if v has the prop:provide-pre-transformer property, #f otherwise.

> > struct
> > (struct |  | export (local-id out-sym mode protect? orig-stx)
> > |  | #:extra-constructor-name make-export)
> local-id : identifier?
> out-sym : symbol?
> mode : (or/c exact-integer? #f)
> protect? : any/c
> orig-stx : syntax?

A structure representing a single imported identifier:

- local-id — the identifier that is bound within the exporting module.

- out-sym — the external name of the binding.

- orig-stx — a syntax object for the source of the export, used for error reporting.

- protect? — indicates whether the identifier should be protected (see Code Inspectors).

- mode — the phase level of the binding in the exporting module.

> procedure (syntax-local-provide-certifier)
> | → |  |
> ((syntax?) (or/c #f (syntax? . -> . syntax?))
> . ->* . syntax?)

For backward compatibility only; returns a procedure that returns its first argument.

##### 12.4.3`` Keyword-Argument Conversion Introspection

(require racket/keyword-transform) | package: base

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

> procedure (syntax-procedure-alias-property stx)
> | → |  |
> (or/c #f
> (letrec ([val? (recursive-contract
> (or/c (cons/c identifier? identifier?)
> (cons/c val? val?)))])
> val?))
> stx : syntax?
> procedure (syntax-procedure-converted-arguments-property stx)
> | → |  |
> (or/c #f
> (letrec ([val? (recursive-contract
> (or/c (cons/c identifier? identifier?)
> (cons/c val? val?)))])
> val?))
> stx : syntax?

Reports the value of a syntax property that can be attached to an identifier by the expansion of a keyword-application form. See lambda for more information about the property.

The property value is normally a pair consisting of the original identifier and an identifier that appears in the expansion. Property-value merging via syntax-track-origin can make the value a pair of such values, and so on.

top ← prev up next →