6.19.2 Using Guile Modules

To use a Guile module is to access either its public interface or a custom interface (see section General Information about Modules). Both types of access are handled by the syntactic form use-modules, which accepts one or more interface specifications and, upon evaluation, arranges for those interfaces to be available to the current module. This process may include locating and loading code for a given module if that code has not yet been loaded, following %load-path (see section Modules and the File System).

An interface specification has one of two forms. The first variation is simply to name the module, in which case its public interface is the one accessed. For example:

(use-modules (ice-9 popen))

Here, the interface specification is (ice-9 popen), and the result is that the current module now has access to open-pipe, close-pipe, open-input-pipe, and so on (see section Pipes).

Note in the previous example that if the current module had already defined open-pipe, that definition would be overwritten by the definition in (ice-9 popen). For this reason (and others), there is a second variation of interface specification that not only names a module to be accessed, but also selects bindings from it and renames them to suit the current module’s needs. For example:

(use-modules ((ice-9 popen)
              #:select ((open-pipe . pipe-open) close-pipe)
              #:renamer (symbol-prefix-proc 'unixy:)))

Here, the interface specification is more complex than before, and the result is that a custom interface with only two bindings is created and subsequently accessed by the current module. The mapping of old to new names is as follows:

(ice-9 popen) sees:             current module sees:
open-pipe                       unixy:pipe-open
close-pipe                      unixy:close-pipe

This example also shows how to use the convenience procedure symbol-prefix-proc.

You can also directly refer to bindings in a module by using the @ syntax. For example, instead of using the use-modules statement from above and writing unixy:pipe-open to refer to the pipe-open from the (ice-9 popen), you could also write (@ (ice-9 popen) open-pipe). Thus an alternative to the complete use-modules statement would be

(define unixy:pipe-open (@ (ice-9 popen) open-pipe))
(define unixy:close-pipe (@ (ice-9 popen) close-pipe))

There is also @@, which can be used like @, but does not check whether the variable that is being accessed is actually exported. Thus, @@ can be thought of as the impolite version of @ and should only be used as a last resort or for debugging, for example.

Note that just as with a use-modules statement, any module that has not yet been loaded yet will be loaded when referenced by a @ or @@ form.

You can also use the @ and @@ syntaxes as the target of a set! when the binding refers to a variable.

Scheme Procedure: symbol-prefix-proc prefix-sym

Return a procedure that prefixes its arg (a symbol) with prefix-sym.

syntax: use-modules spec …

Resolve each interface specification spec into an interface and arrange for these to be accessible by the current module. The return value is unspecified.

spec can be a list of symbols, in which case it names a module whose public interface is found and used.

spec can also be of the form:

 (MODULE-NAME [#:select SELECTION] [#:renamer RENAMER])

in which case a custom interface is newly created and used. module-name is a list of symbols, as above; selection is a list of selection-specs; and renamer is a procedure that takes a symbol and returns its new name. A selection-spec is either a symbol or a pair of symbols (ORIG . SEEN), where orig is the name in the used module and seen is the name in the using module. Note that seen is also passed through renamer.

The #:select and #:renamer clauses are optional. If both are omitted, the returned interface has no bindings. If the #:select clause is omitted, renamer operates on the used module’s public interface.

In addition to the above, spec can also include a #:version clause, of the form:

 #:version VERSION-SPEC

where version-spec is an R6RS-compatible version reference. An error will be signaled in the case in which a module with the same name has already been loaded, if that module specifies a version and that version is not compatible with version-spec. See section R6RS Version References, for more on version references.

If the module name is not resolvable, use-modules will signal an error.

syntax: @ module-name binding-name

Refer to the binding named binding-name in module module-name. The binding must have been exported by the module.

syntax: @@ module-name binding-name

Refer to the binding named binding-name in module module-name. The binding must not have been exported by the module. This syntax is only intended for debugging purposes or as a last resort.

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