gexp: Add 'with-parameters'.

* guix/gexp.scm (<parameterized>): New record type.
(with-parameters): New macro.
(compile-parameterized): New gexp compiler.
* tests/gexp.scm ("with-parameters for %current-system")
("with-parameters for %current-target-system")
("with-parameters + file-append"): New tests.
* doc/guix.texi (G-Expressions): Document it.

1 files changed, 19 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index dd32b65fe0..4f8f7cfb2a 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -8022,6 +8022,25 @@ the second case, the resulting script contains a @code{(string-append
 @dots{})} expression to construct the file name @emph{at run time}.
 @end deffn
 
+@deffn {Scheme Syntax} with-parameters ((@var{parameter} @var{value}) @dots{}) @var{exp}
+This macro is similar to the @code{parameterize} form for
+dynamically-bound @dfn{parameters} (@pxref{Parameters,,, guile, GNU
+Guile Reference Manual}).  The key difference is that it takes effect
+when the file-like object returned by @var{exp} is lowered to a
+derivation or store item.
+
+A typical use of @code{with-parameters} is to force the system in effect
+for a given object:
+
+@lisp
+(with-parameters ((%current-system "i686-linux"))
+  coreutils)
+@end lisp
+
+The example above returns an object that corresponds to the i686 build
+of Coreutils, regardless of the current value of @code{%current-system}.
+@end deffn
+
 
 Of course, in addition to gexps embedded in ``host'' code, there are
 also modules containing build tools.  To make it clear that they are