packages: Add 'package-input-rewriting'.

* guix/packages.scm (package-input-rewriting): New procedure.
* tests/packages.scm ("package-input-rewriting"): New test.
* doc/guix.texi (Defining Packages): Document it.
(Package Transformation Options): Add cross-reference.

1 files changed, 41 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 5448c66664..2a7fd4d041 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -2574,6 +2574,45 @@ and operating system, such as @code{"mips64el-linux-gnu"}
 Configure and Build System}).
 @end deffn
 
+@cindex package transformations
+@cindex input rewriting
+@cindex dependency tree rewriting
+Packages can be manipulated in arbitrary ways.  An example of a useful
+transformation is @dfn{input rewriting}, whereby the dependency tree of
+a package is rewritten by replacing specific inputs by others:
+
+@deffn {Scheme Procedure} package-input-rewriting @var{replacements} @
+           [@var{rewrite-name}]
+Return a procedure that, when passed a package, replaces its direct and
+indirect dependencies (but not its implicit inputs) according to
+@var{replacements}.  @var{replacements} is a list of package pairs; the
+first element of each pair is the package to replace, and the second one
+is the replacement.
+
+Optionally, @var{rewrite-name} is a one-argument procedure that takes
+the name of a package and returns its new name after rewrite.
+@end deffn
+
+@noindent
+Consider this example:
+
+@example
+(define libressl-instead-of-openssl
+  ;; This is a procedure to replace OPENSSL by LIBRESSL,
+  ;; recursively.
+  (package-input-rewriting `((,openssl . ,libressl))))
+
+(define git-with-libressl
+  (libressl-instead-of-openssl git))
+@end example
+
+@noindent
+Here we first define a rewriting procedure that replaces @var{openssl}
+with @var{libressl}.  Then we use it to define a @dfn{variant} of the
+@var{git} package that uses @var{libressl} instead of @var{openssl}.
+This is exactly what the @option{--with-input} command-line option does
+(@pxref{Package Transformation Options, @option{--with-input}}).
+
 @menu
 * package Reference ::          The package data type.
 * origin Reference::            The origin data type.
@@ -4362,7 +4401,8 @@ This is a recursive, deep replacement.  So in this example, both
 @code{guix} and its dependency @code{guile-json} (which also depends on
 @code{guile}) get rebuilt against @code{guile-next}.
 
-However, implicit inputs are left unchanged.
+This is implemented using the @code{package-input-rewriting} Scheme
+procedure (@pxref{Defining Packages, @code{package-input-rewriting}}).
 @end table
 
 @node Additional Build Options