2.2 Importing and Exporting: require and provide
Imports: require in Guide: PLT Scheme introduces require.
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In a top-level context, require instantiates modules (see Modules and Module-Level Variables). In a module context, require visits modules (see Module Phases and Visits). In both contexts, require introduces bindings into a namespace or a module (see Introducing Bindings). A require form in a expression context or internal-definition context is a syntax error.
A require-spec designates a particular set of identifiers to be bound in the importing context. Each identifier is mapped to a particular export of a particular module; the identifier to bind may be different from the symbolic name of the originally exported identifier. Each identifier also binds at a particular phase level.
The syntax of require-spec can be extended via define-require-syntax, but the pre-defined forms are as follows.
module-path
Imports all exported bindings from the named module, using the export identifiers as the local identifiers. (See below for information on module-path.) The lexical context of the module-path form determines the context of the introduced identifiers.
(only-in require-spec id-maybe-renamed )
Like require-spec, but constrained to those exports for which the identifiers to bind match id-maybe-renamed: as id or as orig-id in [orig-id bind-id]. If the id or orig-id of any id-maybe-renamed is not in the set that require-spec describes, a syntax error is reported.
(except-in require-spec id )
Like require-spec, but omitting those imports for which ids are the identifiers to bind; if any id is not in the set that require-spec describes, a syntax error is reported.
(prefix-in prefix-id require-spec)
Like require-spec, but adjusting each identifier to be bound by prefixing it with prefix-id. The lexical context of the prefix-id is ignored, and instead preserved from the identifiers before prefixing.
(rename-in require-spec [orig-id bind-id] )
Like require-spec, but replacing the identifier to bind orig-id with bind-id; if any orig-id is not in the set that require-spec describes, a syntax error is reported.
(combine-in require-spec )
The union of the require-specs.
(only-meta-in phase-level require-spec )
Like the combination of require-specs, but removing any binding that is not for phase-level, where #f for phase-level corresponds to the label phase level.
(for-meta phase-level require-spec )
Like the combination of require-specs, but constrained each binding specified by each require-spec is shifted by phase-level. The label phase level corresponds to #f, and a shifting combination that involves #f produces #f.
(for-syntax require-spec )
Same as (for-meta 1 require-spec ).
(for-template require-spec )
Same as (for-meta -1 require-spec ).
(for-label require-spec )
Same as (for-meta #f require-spec ).
derived-require-spec
See define-require-syntax for information on expanding the set of require-spec forms.
Module Paths in Guide: PLT Scheme introduces module paths.
A module-path identifies a module, either through a concrete name in the form of an identifier, or through an indirect name that can trigger automatic loading of the module declaration. Except for the id case below, the actual resolution is up to the current module name resolver (see current-module-name-resolver), and the description below corresponds to the default module name resolver.
(quote id)
Refers to a module previously declared interactively with the name id.
rel-string
A path relative to the containing source (as determined by current-load-relative-directory or current-directory). Regardless of the current platform, rel-string is always parsed as a Unix-format relative path: / is the path delimiter (multiple adjacent /s are treated as a single delimiter), .. accesses the parent directory, and . accesses the current directory. The path cannot be empty or contain a leading or trailing slash, path elements before than the last one cannot include a file suffix (i.e., a . in an element other than . or ..), and the only allowed characters are ASCII letters, ASCII digits, -, +, _, ., /, and %. Furthermore, a % is allowed only when followed by two lowercase hexadecimal digits, and the digits must form a number that is not the ASCII value of a letter, digit, -, +, or _.
The % provision is intended to support a one-to-one encoding of arbitrary strings as path elements (after UTF-8 encoding). Such encodings are not decoded to arrive at a filename, but instead preserved in the file access.
(lib rel-string )
A path to a module installed into a collection (see Libraries and Collections). The rel-strings in lib are constrained similar to the plain rel-string case, with the additional constraint that a rel-string cannot contain . or .. directory indicators.
The specific interpretation of the path depends on the number and shape of the rel-strings:
If a single rel-string is provided, and if it consists of a single element (i.e., no /) with no file suffix (i.e., no .), then rel-string names a collection, and "main.ss" is the library file name.
If a single rel-string is provided, and if it consists of multiple /-separated elements, then each element up to the last names a collection, subcollection, etc., and the last element names a file. If the last element has no file suffix, ".ss" is added.
If a single rel-string is provided, and if it consists of a single element with a file suffix (i.e, with a .), then rel-string names a file within the "mzlib" collection. (This convention is for compatibility with older version of PLT Scheme.)
Otherwise, when multiple rel-strings are provided, the first rel-string is effectively moved after the others, and all rel-strings are appended with / separators. The resulting path names a collection, then subcollection, etc., ending with a file name. No suffix is added automatically. (This convention is for compatibility with older version of PLT Scheme.)
id
A shorthand for a lib form with a single rel-string whose characters are the same as in the symbolic form of id. In addition to the constraints of a lib rel-string, id must not contain ..
(file string)
Similar to the plain rel-string case, but string is a path – possibly absolute – using the current platform’s path conventions and expand-user-path.
(planet id)
(planet string)
(planet rel-string (user-string pkg-string vers)
rel-string )
Specifies a library available via the PLaneT server.
The first form is a shorthand for the last one, where the id’s character sequence must match the following ‹spec› grammar:
‹spec›
::=
‹owner› / ‹pkg› ‹lib›
‹owner›
::=
‹elem›
‹pkg›
::=
‹elem› | ‹elem› : ‹version›
‹version›
::=
‹int› | ‹int› : ‹minor›
‹minor›
::=
‹int› | <= ‹int› | >= ‹int› | = ‹int›
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‹int› - ‹int›
‹lib›
::=
‹empty› | / ‹path›
‹path›
::=
‹elem› | ‹elem› / ‹path›
and where an ‹elem› is a non-empty sequence of characters that are ASCII letters, ASCII digits, -, +, _, or % followed by lowercase hexadecimal digits (that do not encode one of the other allowed characters), and an ‹int› is a non-empty sequence of ASCII digits. As this shorthand is expended, a ".plt" extension is added to ‹pkg›, and a ".ss" extension is added to "path"; if no ‹path› is included, "main.ss" is used in the expansion.
A (planet string) form is like a (planet id) form with the identifier converted to a string, except that the string can optionally end with a file extension (i.e., a .) for a ‹path›.
In the more general last form of a planet module path, the rel-strings are similar to the lib form, except that the (user-string pkg-string vers) names a PLaneT-based package instead of a collection. A version specification can include an optional major and minor version, where the minor version can be a specific number or a constraint: (nat nat) specifies an inclusive range, (= nat) specifies an exact match, (+ nat) specifies a minimum version and is equivalent to just nat, and (- nat) specifies a maximum version. The =, +, and - identifiers in a minor-version constraint are recognized symbolically.
No identifier can be bound multiple times in a given phase level by an import, unless all of the bindings refer to the same original definition in the same module. In a module context, an identifier can be either imported or defined for a given phase level, but not both.
Exports: provide in Guide: PLT Scheme introduces provide.
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Declares exports from a module. A provide form must appear in a module context or a module-begin context.
A provide-spec indicates one or more bindings to provide. For each exported binding, the external name is a symbol that can be different from the symbolic form of the identifier that is bound within the module. Also, each export is drawn from a particular phase level and exported at the same phase level.
The syntax of provide-spec can be extended via define-provide-syntax, but the pre-defined forms are as follows.
id
Exports id, which must be bound within the module (i.e., either defined or imported) at the relevant phase level. The symbolic form of id is used as the external name, and the symbolic form of the defined or imported identifier must match (otherwise, the external name could be ambiguous).
Exports all identifiers that are defined at phase level 0 or phase level 1 within the exporting module, and that have the same lexical context as the (all-defined-out) form. The external name for each identifier is the symbolic form of the identifier. Only identifiers accessible from the lexical context of the (all-defined-out) form are included; that is, macro-introduced imports are not re-exported, unless the (all-defined-out) form was introduced at the same time.
(all-from-out module-path )
Exports all identifiers that are imported into the exporting module using a require-spec built on each module-path (see Importing and Exporting: require and provide) with no phase-level shift. The symbolic name for export is derived from the name that is bound within the module, as opposed to the symbolic name of the export from each module-path. Only identifiers accessible from the lexical context of the module-path are included; that is, macro-introduced imports are not re-exported, unless the module-path was introduced at the same time.
(rename-out [orig-id export-id] )
Exports each orig-id, which must be bound within the module at phase level 0. The symbolic name for each export is export-id instead orig-d.
(except-out provide-spec provide-spec )
Like the first provide-spec, but omitting the bindings listed in each subsequent provide-spec. If one of the latter bindings is not included in the initial provide-spec, a syntax error is reported. The symbolic export name information in the latter provide-specs is ignored; only the bindings are used.
(prefix-out prefix-id provide-spec)
Like provide-spec, but with each symbolic export name from provide-spec prefixed with prefix-id.
(struct-out id)
Exports the bindings associated with a structure type id. Typically, id is bound with (define-struct id ....) or (define-struct (id super-id) ....); more generally, id must have a transformer binding of structure-type information at phase level 0; see Structure Type Transformer Binding. Furthermore, for each identifier mentioned in the structure-type information, the enclosing module must define or import one identifier that is free-identifier=?. If the structure-type information includes a super-type identifier, and if the identifier has a transformer binding of structure-type information, the accessor and mutator bindings of the super-type are not included by struct-out for export.
(combine-out provide-spec )
The union of the provide-specs.
(protect-out provide-spec )
Like the union of the provide-specs, except that the exports are protected; see Code Inspectors. The provide-spec must specify only bindings that are defined within the exporting module.
(for-meta phase-level provide-spec )
Like the union of the provide-specs, but adjusted to apply to phase level specified by phase-level (where #f corresponds to the label phase level). In particular, an id or rename-out form as a provide-spec refers to a binding at phase-level, an all-define-out exports only phase-level definitions, and an all-from-out exports bindings imported with a shift by phase-level.
(for-syntax provide-spec )
Same as (for-meta 1 provide-spec ).
(for-template provide-spec )
Same as (for-meta -1 provide-spec ).
(for-label provide-spec )
Same as (for-meta #f provide-spec ).
derived-provide-spec
See define-provide-syntax for information on expanding the set of provide-spec forms.
Each export specified within a module must have a distinct symbolic export name, though the same binding can be specified with the multiple symbolic names.
(for-meta require-spec ) |
(for-syntax require-spec ) |
(for-template require-spec ) |
(for-label require-spec ) |
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The primitive import form, to which require expands. A raw-require-spec is similar to a require-spec in a require form, except that the syntax is more constrained, not composable, and not extensible. Also, sub-form names like for-syntax and lib are recognized symbolically, instead of via bindings. Although not formalized in the grammar above, a just-meta form cannot appear within a just-meta form.
Each raw-require-spec corresponds to the obvious require-spec, but the rename sub-form has the identifiers in reverse order compared to rename-in.
For most raw-require-specs, the lexical context of the raw-require-spec determines the context of introduced identifiers. The exception is the rename sub-form, where the lexical context of the local-id is preserved.
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The primitive export form, to which provide expands. A raw-module-path is as for #%require. A protect sub-form cannot appear within a protect sub-form.
Like #%require, the sub-form keywords for #%provide are recognized symbolically, and nearly every raw-provide-spec has an obvious equivalent provide-spec via provide, with the exception of the struct and expand sub-forms.
A (struct struct-id (field-id )) sub-form expands to struct-id, make-struct-id, struct:struct-id, struct-id?, struct-id-field-id for each field-id, and set-struct-id-field-id! for each field-id. The lexical context of the struct-id is used for all generated identifiers.
Unlike #%require, the #%provide form is macro-extensible via an explicit expand sub-form; the (id . datum) part is locally expanded as an expression (even though it is not actually an expression), stopping when a begin form is produced; if the expansion result is (begin raw-provide-spec ), it is spliced in place of the expand form, otherwise a syntax error is reported. The expand sub-form is not normally used directly; it provides a hook for implementing provide and provide transformers.
The all-from and all-from-except forms re-export only identifiers that are accessible in lexical context of the all-from or all-from-except form itself. That is, macro-introduced imports are not re-exported, unless the all-from or all-from-except form was introduced at the same time. Similarly, all-defined and its variants export only definitions accessible from the lexical context of the phaseless-spec form.
2.2.1 Additional require Forms
The bindings documented in this section are provided by the scheme/require library, not scheme/base or scheme.
The following forms support more complex selection and manipulation of sets of imported identifiers. Note that a require form is expanded before it is used, which means that requiring the library itself should be a separate form. For example, use
(require scheme/require) |
(require (matching-identifiers-in #rx"foo" "foo.ss")) |
instead of
(require scheme/require |
(matching-identifiers-in #rx"foo" "foo.ss")) |
(matching-identifiers-in regexp require-spec) |
Like require-spec, but including only imports whose names match regexp. The regexp must be a literal regular expression (see Regular Expressions).
(subtract-in require-spec subtracted-spec ) |
Like require-spec, but omitting those imports that would be imported by one of the subtracted-specs.
(filtered-in proc-expr require-spec) |
The proc-expr should evaluate to a single-argument procedure, which is applied on each of the names (as strings) that are to be required according to require-spec. For each name, the procedure should return either a string (possibly different if you want it renamed), or #f to exclude the name. (Note that proc-expr is a syntax-time expression.)
For example,
(lambda (name) |
(and (regexp-match? #rx"^[a-z-]+$" name) |
#rx"-" (string-titlecase name) ""))) |
scheme/base)) |
will get the scheme/base bindings that match the regexp, and renamed to use “caml case”.
2.2.2 Additional provide Forms
The bindings documented in this section are provided by the scheme/provide library, not scheme/base or scheme.
(matching-identifiers-out regexp provide-spec) |
Like provide-spec, but omitting the export of each binding with an external name that matches regexp. The regexp must be a literal regular expression (see Regular Expressions).
(filtered-out proc-expr provide-spec) |
The proc-expr should evaluate to a single-argument procedure, which is applied on each of the names (as strings) that are to be provided according to provide-spec. For each name, the procedure should return either a string (possibly different if you want it renamed), or #f to exclude the name. (Note that proc-expr is a syntax-time expression.)
For example,
(lambda (name) |
(and (regexp-match? #rx"^[a-z-]+$" name) |
#rx"-" (string-titlecase name) ""))) |
(all-defined-out))) |
will provide all defined bindings that match the regexp, and renamed to use “caml case”.