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About Deferred

Sometimes I would like to include code in my project that is only available if certain libraries are loaded. Since I don't want to require these libraries as dependencies, but can't refer to their symbols either without them being loaded, things get a bit messy. This library attempts to provide a much more convenient way.

How To

Deferred uses two components: a dispatch reader macro on #^ and two macros, WITH-DEFERRED-LIBRARY or WHEN-PACKAGES. #^ is used to circumvent the reader problem of being unable to refer to symbols in packages that don't exist at read-time. The macro is used to then acquire access to the symbol's environment to figure out the proper transformation for the symbol into proper values.

WITH-DEFERRED-LIBRARY is used to specify deferred libraries that are needed at execution time. Meaning the required library can be loaded only just when the code block is executed.

(with-deferred-library (:drakma)
  (#^http-request "http://google.com"))

Deferred symbols can be used in function calls, with FUNCTION and QUOTE and as values (f.e. special variables). Currently unsupported are deferred SETF functions as it's impossible to properly defer a SETF-expander until execution-time.

If instead you are looking for code that is only compiled when certain libraries are already loaded before yours, there is WHEN-PACKAGES. This macro only outputs code when the specified list of packages can be found, otherwise it expands to NIL. This is useful if you don't want to encounter the edge-cases and performance penalties of WITH-DEFERRED-LIBRARY and can depend on the fact that if the deferred feature is needed, its dependencies are loaded before your system.

(defun frillneckedlizard ()
  (error "NOT IMPLEMENTED!"))

(when-packages (:drakma)
  (defun frillneckedlizard ()
    (#^drakma:http-request "http://frillneckedlizard.moe")))

If the default dispatch reader macro has been overwritten in your readtable for one reason or another, you can use (named-readtables:in-readtable :deferred).