linux-initrd: Return file-like objects instead of monadic values.

This is an incompatible change visible to users via the 'initrd' field
of 'operating-system'.  However, assuming the user's 'initrd' value
tail-calls to 'raw-initrd' or 'base-initrd', the switch to non-monadic
style is invisible.

* gnu/system/linux-initrd.scm (expression->initrd): Use 'computed-file'
instead of 'gexp->derivation'.
(raw-initrd, base-initrd): Adjust docstring to mention non-monadic
return.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Adjust
accordingly.
* gnu/system.scm (operating-system-directory-base-entries)
(operating-system-initrd-file)
(operating-system-boot-parameters): Adjust accordingly.
* doc/guix.texi (operating-system Reference)
(Initial RAM Disk): Update.

1 files changed, 6 insertions, 8 deletions

diff --git a/gnu/system/vm.scm b/gnu/system/vm.scm
index d43b71cbaf..6064e0f899 100644
--- a/gnu/system/vm.scm
+++ b/gnu/system/vm.scm
@@ -189,14 +189,12 @@ made available under the /xchg CIFS share."
                   #~(when (zero? (system* #$user-builder))
                       (reboot))))
 
-  (mlet* %store-monad
-      ((initrd       (if initrd                   ; use the default initrd?
-                         (return initrd)
-                         (base-initrd file-systems
-                                      #:on-error 'backtrace
-                                      #:linux linux
-                                      #:linux-modules %base-initrd-modules
-                                      #:qemu-networking? #t))))
+  (let ((initrd (or initrd
+                    (base-initrd file-systems
+                                 #:on-error 'backtrace
+                                 #:linux linux
+                                 #:linux-modules %base-initrd-modules
+                                 #:qemu-networking? #t))))
 
     (define builder
       ;; Code that launches the VM that evaluates EXP.