guix build: '-e' can be passed a monadic thunk.

* guix/ui.scm (read/eval): New procedure.
  (read/eval-package-expression): Use it.
* guix/scripts/build.scm (derivations-from-package-expressions): Rename to...
  (derivation-from-expression): ... this.  Accept procedures, under the
  assumption that they are monadic thunk.
  (show-help): Adjust accordingly.
  (guix-build): Ditto.
* tests/guix-build.sh: Add test.
* doc/guix.texi (Invoking guix build): Augment description of '-e'.

1 files changed, 5 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index cfa5aac326..847c73ab8c 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -1483,12 +1483,16 @@ The @var{options} may be zero or more of the following:
 
 @item --expression=@var{expr}
 @itemx -e @var{expr}
-Build the package @var{expr} evaluates to.
+Build the package or derivation @var{expr} evaluates to.
 
 For example, @var{expr} may be @code{(@@ (gnu packages guile)
 guile-1.8)}, which unambiguously designates this specific variant of
 version 1.8 of Guile.
 
+Alternately, @var{expr} may refer to a zero-argument monadic procedure
+(@pxref{The Store Monad}).  The procedure must return a derivation as a
+monadic value, which is then passed through @code{run-with-store}.
+
 @item --source
 @itemx -S
 Build the packages' source derivations, rather than the packages