@@ -449,6 +449,7 @@ Packaging Guidelines
* Version Numbers:: When the name is not enough.
* Synopses and Descriptions:: Helping users find the right package.
* Snippets versus Phases:: Whether to use a snippet, or a build phase.
+* Cyclic Module Dependencies:: Going full circle.
* Emacs Packages:: Your Elisp fix.
* Python Modules:: A touch of British comedy.
* Perl Modules:: Little pearls.
@@ -720,6 +721,61 @@ Snippets versus Phases
using build phases. Refer to the @code{origin} record documentation for
more information (@pxref{origin Reference}).
+@node Cyclic Module Dependencies
+@subsection Cyclic Module Dependencies
+
+While there cannot be circular dependencies between packages, Guile's
+lax module loading mechanism allows circular dependencies between Guile
+modules, which doesn't cause problems as long as the following
+conditions are followed for two modules part of a dependency cycle:
+
+@cindex rules to cope with circular module dependencies
+@enumerate
+@item
+Macros are not shared between the co-dependent modules
+@item
+Top-level variables are only referenced in delayed (@i{thunked}) package
+fields: @code{arguments}, @code{native-inputs}, @code{inputs},
+@code{propagated-inputs} or @code{replacement}
+@item
+Procedures referencing top-level variables from another module are not
+called at the top level of a module themselves.
+@end enumerate
+
+Straying away from the above rules may work while there are no
+dependency cycles between modules, but given such cycles are confusing
+and difficult to troubleshoot, it is best to follow the rules to avoid
+introducing problems down the line.
+
+@noindent
+Here is a common trap to avoid:
+
+@lisp
+(define-public avr-binutils
+ (package
+ (inherit (cross-binutils "avr"))
+ (name "avr-binutils")))
+@end lisp
+
+In the above example, the @code{avr-binutils} package was defined in the
+module @code{(gnu packages avr)}, and the @code{cross-binutils}
+procedure in @code{(gnu packages cross-base)}. Because the
+@code{inherit} field is not delayed (thunked), it is evaluated at the
+top level at load time, which is problematic in the presence of module
+dependency cycles. This could be resolved by turning the package into a
+procedure instead, like:
+
+@lisp
+(define (make-avr-binutils)
+ (package
+ (inherit (cross-binutils "avr"))
+ (name "avr-binutils")))
+@end lisp
+
+Care would need to be taken to ensure the above procedure is only ever
+used in a package delayed fields or within another procedure also not
+called at the top level.
+
@node Emacs Packages
@subsection Emacs Packages