11.4 Syntax Transformers
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.
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 also causes the parser to install a
free-identifier=? and identifier-binding
equivalence, as long as id-stx does not have a true value for
the 'not-free-identifier=? syntax property.
Also, if id-stx has a true value for the
'not-provide-all-defined syntax property and
it is bound as a module-level transformer, the bound identifier is not
exported by all-defined-out; the provide form
otherwise uses a symbol-valued 'nominal-id property
of id-stx to specify the “nominal source identifier” of the
binding. Finally, the rename transformer cooperates specially with
syntax-local-value and
syntax-local-make-delta-introducer.
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.
When an identifier in stop-ids is encountered by the expander
in a subexpression, expansions stops for the subexpression. If
stop-ids is a non-empty list, 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.
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).
Expansion of stx can use certificates for the expression
already being expanded (see Syntax Certificates), and inactive
certificates associated with stx are activated for
stx (see Syntax Certificates). Furthermore, if the
transformer is defined within a module (i.e., the current expansion
was triggered by a use of a module-defined identifier with a
transformer binding) or if the current expression is being
expanded for the body of a module, then the expansion of stx
can use any identifier defined by the module.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
Examples: |
|
|
|
| > (show 1) |
partly expanded syntax is (do-print hello ~a 1) | expanded syntax is (printf hello ~a 1) | hello 1 |
|
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, otherwise the exn:fail:contract exception is raised.
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.
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).
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, otherwise the exn:fail:contract exception is raised.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
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.
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.
Resolving id-stx can use certificates for the expression
being transformed (see Syntax Certificates) as well as inactive
certificates associated with id-stx (see
Syntax Certificates). Furthermore, if the transformer is defined
within a module (i.e., the current transformation was triggered by a
use of a module-defined identifier) or if the current expression is
being expanded for the body of a module, then resolving
id-stx can access any identifier defined by the module.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
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.
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-for-syntax
declaration just before the requesting expression.
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, otherwise the exn:fail:contract exception is raised.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
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, otherwise the exn:fail:contract exception is raised.
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 not in the module top-level, then
stx is
eventually expanded in an expression context.
If the current expression being transformed is not
within a
module form, or if it is not a run-time expression,
then the
exn:fail:contract exception is raised.
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,
wrapping it with
for-meta if the current expansion context is
not
phase level 0.
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).
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
Lifts a
#%provide form corresponding to
raw-provide-spec-stx to the top of the module currently being
expanded.
If the current expression being transformed is not
within a
module form, or if it is not a run-time expression,
then the
exn:fail:contract exception is raised.
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, otherwise the exn:fail:contract exception is raised.
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, otherwise the exn:fail:contract exception is raised.
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.
Returns three lists of symbols that represent the
provided
bindings of the module named by
mod-path. The first list
corresponds to the
phase level 0 exports of the module, the
second list corresponds to the
phase level -1 exports of the
module, and the last list corresponds to the
label phase level
exports of the module.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
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.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
Returns a procedure that captures any certificates currently available
for
syntax-local-value or
local-expand. The
procedure accepts one to three arguments:
stx (required),
key (optional), and
intro (optional). The
procedure’s result is a syntax object like
stx, except that
it includes the captured certificates as inactive (see
Syntax Certificates) if
active? is
#f (the default)
or active otherwise. If
key is supplied and not
#f,
it is associated with each captured certificate for later use through
syntax-recertify. If
intro is supplied, and if it
is not
#f (the default), then it must be a procedure created
by
make-syntax-introducer, in which case the certificate
applies only to parts of
stx that are marked as introduced by
intro.
Supply #t for active? when the syntax to be
certified can be safely used in any context by any party, and where
access to the syntax object should not confer any additional
access. Supply #f for active? when the syntax to be
certified is not accessible to parties that might abuse the access
that the certificate provides, and when the certified syntax
eventually appears (via macro expansion) within a larger expression
from which it cannot be safely extracted by other parties.
This procedure must be called during the
dynamic extent of a syntax transformer application by the
expander, otherwise the exn:fail:contract exception is raised.
Returns
#t during the dynamic extent of a
syntax
transformer application by the expander,
#f otherwise.
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, otherwise the exn:fail:contract exception is raised.
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.
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, otherwise the exn:fail:contract exception is raised.
It returns two lists of identifiers corresponding to all definitions
within the module being expanded. This information is used for
implementing provide sub-forms like all-defined-out.
The first result list corresponds to phase 0 (i.e., normal)
definitions, and the second corresponds to phase -1 (i.e.,
for-syntax) definitions.
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.
11.4.1 require Transformers
A transformer binding whose value is a structure with the
prop:require-transformer property implements a derived
require-spec for require.
The 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.
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).
Creates a
require transformer (i.e., a structure with the
prop:require-transformer property) using the given procedure
as the transformer.
A property to identify
require transformers. The property
value must be a procedure that takes a syntax object and returns
import and import-source lists.
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.
A structure representing an imported module, which must be
instantiated or
visited even if no binding is imported
into a module.
11.4.2 provide Transformers
A transformer binding whose value is a structure with the
prop:provide-transformer property implements a derived
provide-spec for provide.
The 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 must be a list of
exports.
If the derived form contains a sub-form that is a
provide-spec, then it can call expand-export to
transform the sub-provide-spec to a list of exports.
See also define-provide-syntax, which supports macro-style
provide transformers.
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
phase level 0 binding unless the
modes list specifies
otherwise. Normally,
modes is either empty or contains a
single element.
Creates a
provide transformer (i.e., a structure with the
prop:provide-transformer property) using the given procedure
as the transformer.
A property to identify
provide transformers. The property
value must be a procedure that takes a syntax object and mode list and
returns an export list.
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.