monad-repl: Add "build", "lower", and "verbosity" commands.

Fixes <https://issues.guix.gnu.org/56114>.
Reported by Maxime Devos <maximedevos@telenet.be>.

* guix/monad-repl.scm (%build-verbosity): New variable.
(evaluate/print-with-store): New procedure.
(run-in-store): Rewrite in terms of 'evaluate/print-with-store'.
(verbosity, lower, build): New meta-commands.
* doc/guix.texi (Using Guix Interactively): New node.
(The Store Monad): Link to it.
(Invoking guix repl): Likewise.
* doc/contributing.texi (Running Guix Before It Is Installed): Refer to
it.
(The Perfect Setup): Suggest 'guix install' rather than 'guix package -i'.

doc/contributing.texi

@@ -225,8 +225,7 @@ $ ./pre-inst-env guile -c '(use-modules (guix utils)) (pk (%current-system))'
 @noindent
 @cindex REPL
 @cindex read-eval-print loop
-@dots{} and for a REPL (@pxref{Using Guile Interactively,,, guile, Guile
-Reference Manual}):
+@dots{} and for a REPL (@pxref{Using Guix Interactively}):
 
 @example
 $ ./pre-inst-env guile
@@ -292,7 +291,7 @@ Manual}).  First, you need more than an editor, you need
 wonderful @url{https://nongnu.org/geiser/, Geiser}.  To set that up, run:
 
 @example
-guix package -i emacs guile emacs-geiser emacs-geiser-guile
+guix install emacs guile emacs-geiser emacs-geiser-guile
 @end example
 
 Geiser allows for interactive and incremental development from within

doc/guix.texi

@@ -299,6 +299,7 @@ Programming Interface
 * The Store Monad::             Purely functional interface to the store.
 * G-Expressions::               Manipulating build expressions.
 * Invoking guix repl::          Programming Guix in Guile.
+* Using Guix Interactively::    Fine-grain interaction at the REPL.
 
 Defining Packages
 
@@ -7100,6 +7101,7 @@ package definitions.
 * The Store Monad::             Purely functional interface to the store.
 * G-Expressions::               Manipulating build expressions.
 * Invoking guix repl::          Programming Guix in Guile
+* Using Guix Interactively::    Fine-grain interaction at the REPL.
 @end menu
 
 @node Package Modules
@@ -10860,8 +10862,9 @@ So, to exit the monad and get the desired effect, one must use
 @end lisp
 
 Note that the @code{(guix monad-repl)} module extends the Guile REPL with
-new ``meta-commands'' to make it easier to deal with monadic procedures:
-@code{run-in-store}, and @code{enter-store-monad}.  The former is used
+new ``commands'' to make it easier to deal with monadic procedures:
+@code{run-in-store}, and @code{enter-store-monad} (@pxref{Using Guix
+Interactively}).  The former is used
 to ``run'' a single monadic value through the store:
 
 @example
@@ -10886,6 +10889,9 @@ scheme@@(guile-user)>
 Note that non-monadic values cannot be returned in the
 @code{store-monad} REPL.
 
+Other meta-commands are available at the REPL, such as @code{,build} to
+build a file-like object (@pxref{Using Guix Interactively}).
+
 The main syntactic forms to deal with monads in general are provided by
 the @code{(guix monads)} module and are described below.
 
@@ -11778,7 +11784,8 @@ lines at the top of the script:
 @code{!#}
 @end example
 
-Without a file name argument, a Guile REPL is started:
+Without a file name argument, a Guile REPL is started, allowing for
+interactive use (@pxref{Using Guix Interactively}):
 
 @example
 $ guix repl
@@ -11834,6 +11841,130 @@ Inhibit loading of the @file{~/.guile} file.  By default, that
 configuration file is loaded when spawning a @code{guile} REPL.
 @end table
 
+@node Using Guix Interactively
+@section Using Guix Interactively
+
+The @command{guix repl} command gives you access to a warm and friendly
+@dfn{read-eval-print loop} (REPL) (@pxref{Invoking guix repl}).  If
+you're getting into Guix programming---defining your own packages,
+writing manifests, defining services for Guix System or Guix Home,
+etc.---you will surely find it convenient to toy with ideas at the REPL.
+
+If you use Emacs, the most convenient way to do that is with Geiser
+(@pxref{The Perfect Setup}), but you do not have to use Emacs to enjoy
+the REPL@.  When using @command{guix repl} or @command{guile} in the
+terminal, we recommend using Readline for completion and Colorized to
+get colorful output.  To do that, you can run:
+
+@example
+guix install guile guile-readline guile-colorized
+@end example
+
+@noindent
+... and then create a @file{.guile} in your home directory containing
+this:
+
+@lisp
+(use-modules (ice-9 readline) (ice-9 colorized))
+
+(activate-readline)
+(activate-colorized)
+@end lisp
+
+The REPL lets you evaluate Scheme code; you type a Scheme expression at
+the prompt, and the REPL prints what it evaluates to:
+
+@example
+$ guix repl
+scheme@@(guix-user)> (+ 2 3)
+$1 = 5
+scheme@@(guix-user)> (string-append "a" "b")
+$2 = "ab"
+@end example
+
+It becomes interesting when you start fiddling with Guix at the REPL.
+The first thing you'll want to do is to ``import'' the @code{(guix)}
+module, which gives access to the main part of the programming
+interface, and perhaps a bunch of useful Guix modules.  You could type
+@code{(use-modules (guix))}, which is valid Scheme code to import a
+module (@pxref{Using Guile Modules,,, guile, GNU Guile Reference
+Manual}), but the REPL provides the @code{use} @dfn{command} as a
+shorthand notation (@pxref{REPL Commands,,, guile, GNU Guile Reference
+Manual}):
+
+@example
+scheme@@(guix-user)> ,use (guix)
+scheme@@(guix-user)> ,use (gnu packages base)
+@end example
+
+Notice that REPL commands are introduced by a leading comma.  A REPL
+command like @code{use} is not valid Scheme code; it's interpreted
+specially by the REPL.
+
+Guix extends the Guile REPL with additional commands for convenience.
+Among those, the @code{build} command comes in handy: it ensures that
+the given file-like object is built, building it if needed, and returns
+its output file name(s).  In the example below, we build the
+@code{coreutils} and @code{grep} packages, as well as a ``computed
+file'' (@pxref{G-Expressions, @code{computed-file}}), and we use the
+@code{scandir} procedure to list the files in Grep's @code{/bin}
+directory:
+
+@example
+scheme@@(guix-user)> ,build coreutils
+$1 = "/gnu/store/@dots{}-coreutils-8.32-debug"
+$2 = "/gnu/store/@dots{}-coreutils-8.32"
+scheme@@(guix-user)> ,build grep
+$3 = "/gnu/store/@dots{}-grep-3.6"
+scheme@@(guix-user)> ,build (computed-file "x" #~(mkdir #$output))
+building /gnu/store/@dots{}-x.drv...
+$4 = "/gnu/store/@dots{}-x"
+scheme@@(guix-user)> ,use(ice-9 ftw)
+scheme@@(guix-user)> (scandir (string-append $3 "/bin"))
+$5 = ("." ".." "egrep" "fgrep" "grep")
+@end example
+
+At a lower-level, a useful command is @code{lower}: it takes a file-like
+object and ``lowers'' it into a derivation (@pxref{Derivations}) or a
+store file:
+
+@example
+scheme@@(guix-user)> ,lower grep
+$6 = #<derivation /gnu/store/@dots{}-grep-3.6.drv => /gnu/store/@dots{}-grep-3.6 7f0e639115f0>
+scheme@@(guix-user)> ,lower (plain-file "x" "Hello!")
+$7 = "/gnu/store/@dots{}-x"
+@end example
+
+The full list of REPL commands can be seen by typing @code{,help guix}
+and is given below for reference.
+
+@deffn {REPL command} build @var{object}
+Lower @var{object} and build it if it's not already built, returning its
+output file name(s).
+@end deffn
+
+@deffn {REPL command} lower @var{object}
+Lower @var{object} into a derivation or store file name and return it.
+@end deffn
+
+@deffn {REPL command} verbosity @var{level}
+Change build verbosity to @var{level}.
+
+This is similar to the @option{--verbosity} command-line option
+(@pxref{Common Build Options}): level 0 means total silence, level 1
+shows build events only, and higher levels print build logs.
+@end deffn
+
+@deffn {REPL command} run-in-store @var{exp}
+Run @var{exp}, a monadic expresssion, through the store monad.
+@xref{The Store Monad}, for more information.
+@end deffn
+
+@deffn {REPL command} enter-store-monad
+Enter a new REPL to evaluate monadic expressions (@pxref{The Store
+Monad}).  You can quit this ``inner'' REPL by typing @code{,q}.
+@end deffn
+
 @c *********************************************************************
 @node Utilities
 @chapter Utilities

guix/monad-repl.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2016, 2022 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,6 +21,12 @@ (define-module (guix monad-repl)
   #:use-module (guix monads)
   #:use-module (guix utils)
   #:use-module (guix packages)
+  #:use-module (guix status)
+  #:autoload   (guix gexp) (lower-object)
+  #:use-module ((guix derivations)
+                #:select (derivation?
+                          derivation->output-paths built-derivations))
+  #:use-module (ice-9 match)
   #:use-module (ice-9 pretty-print)
   #:use-module (system repl repl)
   #:use-module (system repl common)
@@ -69,16 +75,58 @@ (define (store-monad-language store)
                          #:guile-for-build guile)
                     'store-monad)))
 
+(define %build-verbosity
+  ;; Current build verbosity level.
+  1)
+
+(define* (evaluate/print-with-store mvalue #:key build?)
+  "Run monadic value MVALUE in the store monad and print its value."
+  (with-store store
+    (set-build-options store
+                       #:print-build-trace #t
+                       #:print-extended-build-trace? #t
+                       #:multiplexed-build-output? #t)
+    (with-status-verbosity %build-verbosity
+      (let* ((guile  (or (%guile-for-build)
+                         (default-guile-derivation store)))
+             (values (run-with-store store
+                       (if build?
+                           (mlet %store-monad ((obj mvalue))
+                             (if (derivation? obj)
+                                 (mbegin %store-monad
+                                   (built-derivations (list obj))
+                                   (return
+                                    (match (derivation->output-paths obj)
+                                      (((_ . files) ...) files))))
+                                 (return (list obj))))
+                           (mlet %store-monad ((obj mvalue))
+                             (return (list obj))))
+                       #:guile-for-build guile)))
+        (for-each (lambda (value)
+                    (run-hook before-print-hook value)
+                    (pretty-print value))
+                  values)))))
+
 (define-meta-command ((run-in-store guix) repl (form))
   "run-in-store EXP
 Run EXP through the store monad."
-  (with-store store
-    (let* ((guile (or (%guile-for-build)
-                      (default-guile-derivation store)))
-           (value (run-with-store store (repl-eval repl form)
-                                  #:guile-for-build guile)))
-      (run-hook before-print-hook value)
-      (pretty-print value))))
+  (evaluate/print-with-store (repl-eval repl form)))
+
+(define-meta-command ((verbosity guix) repl (level))
+  "verbosity LEVEL
+Change build verbosity to LEVEL."
+  (set! %build-verbosity (repl-eval repl level)))
+
+(define-meta-command ((lower guix) repl (form))
+  "lower OBJECT
+Lower OBJECT into a derivation or store file and return it."
+  (evaluate/print-with-store (lower-object (repl-eval repl form))))
+
+(define-meta-command ((build guix) repl (form))
+  "build OBJECT
+Lower OBJECT and build it, returning its output file name(s)."
+  (evaluate/print-with-store (lower-object (repl-eval repl form))
+                             #:build? #t))
 
 (define-meta-command ((enter-store-monad guix) repl)
   "enter-store-monad