Require - slib

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1.2 Require

SLIB creates and maintains a catalog mapping features to locations of files introducing procedures and syntax denoted by those features.

— Variable: *catalog*

Is an association list of features (symbols) and pathnames which will supply those features. The pathname can be either a string or a pair. If pathname is a pair then the first element should be a macro feature symbol, source, compiled, or one of the other cases described in Library Catalogs. The cdr of the pathname should be either a string or a list.

At the beginning of each section of this manual, there is a line like (require 'feature). The Scheme files comprising SLIB are cataloged so that these feature names map to the corresponding files.

SLIB provides a form, require, which loads the files providing the requested feature.

— Procedure: require feature

If (provided? feature) is true, then require just returns.
Otherwise, if feature is found in the catalog, then the corresponding files will be loaded and (provided?feature) will henceforth return #t. That feature is thereafter provided.
Otherwise (feature not found in the catalog), an error is signaled.

There is a related form require-if, used primarily for enabling compilers to statically include modules which would be dynamically loaded by interpreters.

— Procedure: require-if condition feature

Requires feature if condition is true.

The random module uses require-if to flag object->string as a (dynamic) required module.

     (require 'byte)
     (require 'logical)
     (require-if 'compiling 'object->string)

The batch module uses require-if to flag posix-time as a module to load if the implementation supports large precision exact integers.

     (require-if '(and bignum compiling) 'posix-time)

The catalog can also be queried using slib:in-catalog?.

— Function: slib:in-catalog? feature

Returns a CDR of the catalog entry if one was found for the symbol feature in the alist *catalog* (and transitively through any symbol aliases encountered). Otherwise, returns #f. The format of catalog entries is explained in Library Catalogs.