2.3 The defsystem grammar

system-definition := ( defsystem system-designator {option}* )

option := :components component-list
        | :pathname pathname
        | :default-component-class
        | :perform method-form 
        | :explain method-form
	| :output-files  method-form
        | :operation-done-p method-form
        | :depends-on ( {simple-component-name}* ) 
	| :serial [ t | nil ]
        | :in-order-to ( {dependency}+ )

component-list := ( {component-def}* )
                
component-def  := simple-component-name
                | ( component-type name {option}* )

component-type := :module | :file | :system | other-component-type

dependency := (dependent-op {requirement}+)
requirement := (required-op {required-component}+)
             | (feature feature-name)
dependent-op := operation-name
required-op := operation-name | feature

2.3.1 Serial dependencies

If the :serial t option is specified for a module, asdf will add dependencies for each each child component, on all the children textually preceding it. This is done as if by :depends-on.

     :components ((:file "a") (:file "b") (:file "c"))
     :serial t

is equivalent to

     :components ((:file "a")
     	     (:file "b" :depends-on ("a"))
     	     (:file "c" :depends-on ("a" "b")))

2.3.2 Source location

The :pathname option is optional in all cases for systems defined via defsystem, and in the usual case the user is recommended not to supply it.

Instead, asdf follows a hairy set of rules that are designed so that

1. find-system will load a system from disk and have its pathname default to the right place
2. this pathname information will not be overwritten with *default-pathname-defaults* (which could be somewhere else altogether) if the user loads up the .asd file into his editor and interactively re-evaluates that form.

If a system is being loaded for the first time, its top-level pathname will be set to:

• The host/device/directory parts of *load-truename*, if it is bound
• *default-pathname-defaults*, otherwise

If a system is being redefined, the top-level pathname will be

• changed, if explicitly supplied or obtained from *load-truename* (so that an updated source location is reflected in the system definition)
• changed if it had previously been set from *default-pathname-defaults*
• left as before, if it had previously been set from *load-truename* and *load-truename* is currently unbound (so that a developer can evaluate a defsystem form from within an editor without clobbering its source location)