scripts: Add 'environment' command.

* guix/scripts/environment.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi ("Invoking guix environment"): New node.

Makefile.am

@@ -94,6 +94,7 @@ MODULES =					\
   guix/scripts/system.scm			\
   guix/scripts/lint.scm				\
   guix/scripts/import/nix.scm			\
+  guix/scripts/environment.scm			\
   guix.scm					\
   $(GNU_SYSTEM_MODULES)
 

doc/guix.texi

@@ -33,6 +33,12 @@ Documentation License''.
                       Managing the operating system configuration.
 @end direntry
 
+@dircategory Software development
+@direntry
+* guix environment: (guix)Invoking guix environment
+                      Building development environments with Guix.
+@end direntry
+
 @titlepage
 @title GNU Guix Reference Manual
 @subtitle Using the GNU Guix Functional Package Manager
@@ -2431,6 +2437,7 @@ programming interface of Guix in a convenient way.
 * Invoking guix hash::          Computing the cryptographic hash of a file.
 * Invoking guix refresh::       Updating package definitions.
 * Invoking guix lint::          Finding errors in package definitions.
+* Invoking guix environment::   Setting up development environments.
 @end menu
 
 @node Invoking guix build
@@ -2840,6 +2847,81 @@ and exit.
 
 @end table
 
+@node Invoking guix environment
+@section Invoking @command{guix environment}
+
+The purpose of @command{guix environment} is to assist hackers in
+creating reproducible development environments without polluting their
+package profile.  The @command{guix environment} tool takes one or more
+packages, builds all of the necessary inputs, and creates a shell
+environment to use them.
+
+The general syntax is:
+
+@example
+guix environment @var{options} @var{package}@dots{}
+@end example
+
+The following examples spawns a new shell that is capable of building
+the GNU Guile source code:
+
+@example
+guix environment guile
+@end example
+
+If the specified packages are not built yet, @command{guix environment}
+automatically builds them.  The new shell's environment is an augmented
+version of the environment that @command{guix environment} was run in.
+It contains the necessary search paths for building the given package
+added to the existing environment variables.  To create a ``pure''
+environment in which the original environment variables have been unset,
+use the @code{--pure} option.
+
+Additionally, more than one package may be specified, in which case the
+union of the inputs for the given packages are used.  For example, the
+command below spawns a shell where all of the dependencies of both Guile
+and Emacs are available:
+
+@example
+guix environment guile emacs
+@end example
+
+Sometimes an interactive shell session is not desired.  The
+@code{--exec} option can be used to specify the command to run instead.
+
+@example
+guix environment guile --exec=make
+@end example
+
+The following options are available:
+
+@table @code
+@item --expression=@var{expr}
+@itemx -e @var{expr}
+Create an environment for the package that @var{expr} evaluates to.
+
+@item --load=@var{file}
+@itemx -l @var{file}
+Create an environment for the package that the code within @var{file}
+evaluates to.
+
+@item --exec=@var{command}
+@item -E @var{command}
+Execute @var{command} in the new environment.
+
+@item --pure
+Unset existing environment variables when building the new environment.
+This has the effect of creating an environment in which search paths
+only contain package inputs.
+
+@item --search-paths
+Display the environment variable definitions that make up the
+environment.
+@end table
+
+It also supports all of the common build options that @command{guix
+build} supports (@pxref{Invoking guix build, common build options}).
+
 @c *********************************************************************
 @node GNU Distribution
 @chapter GNU Distribution

guix/scripts/environment.scm

@@ -0,0 +1,235 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 David Thompson <davet@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix scripts environment)
+  #:use-module (guix ui)
+  #:use-module (guix store)
+  #:use-module (guix derivations)
+  #:use-module (guix packages)
+  #:use-module (guix profiles)
+  #:use-module (guix utils)
+  #:use-module (guix monads)
+  #:use-module (guix build utils)
+  #:use-module (guix scripts build)
+  #:use-module (gnu packages)
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-37)
+  #:use-module (srfi srfi-98)
+  #:export (guix-environment))
+
+(define (for-each-search-path proc inputs derivations pure?)
+  "Apply PROC for each native search path in INPUTS in addition to 'PATH'.
+Use the output paths of DERIVATIONS to build each search path.  When PURE? is
+#t, the existing search path value is ignored.  Otherwise, the existing search
+path value is appended."
+  (let ((paths (map derivation->output-path derivations)))
+    (for-each (match-lambda
+               (($ <search-path-specification>
+                   variable directories separator)
+                (let* ((current (getenv variable))
+                       (path ((@@ (guix build utils) search-path-as-list)
+                              directories paths))
+                       (value (list->search-path-as-string path separator)))
+                  (proc variable
+                        (if (and current (not pure?))
+                            (string-append value separator current)
+                            value)))))
+              (cons* (search-path-specification
+                      (variable "PATH")
+                      (directories '("bin" "sbin"))
+                      (separator ":"))
+                     (delete-duplicates
+                      (append-map package-native-search-paths inputs))))))
+
+;; Protect some env vars from purification.  Borrowed from nix-shell.
+(define %precious-variables
+  '("HOME" "USER" "LOGNAME" "DISPLAY" "TERM" "TZ" "PAGER"))
+
+(define (purify-environment)
+  "Unset almost all environment variables.  A small number of variables such
+as 'HOME' and 'USER' are left untouched."
+  (for-each unsetenv
+            (remove (cut member <> %precious-variables)
+                    (match (get-environment-variables)
+                      (((names . _) ...)
+                       names)))))
+
+(define (create-environment inputs derivations pure?)
+  "Set the needed environment variables for all packages within INPUTS.  When
+PURE? is #t, unset the variables in the current environment.  Otherwise,
+augment existing enviroment variables with additional search paths."
+  (when pure? (purify-environment))
+  (for-each-search-path setenv inputs derivations pure?))
+
+(define (show-search-paths inputs derivations pure?)
+  "Display the needed search paths to build an environment that contains the
+packages within INPUTS.  When PURE? is #t, do not augment existing environment
+variables with additional search paths."
+  (for-each-search-path (lambda (variable value)
+                          (format #t "export ~a=\"~a\"~%" variable value))
+                        inputs derivations pure?))
+
+(define (show-help)
+  (display (_ "Usage: guix environment [OPTION]... PACKAGE...
+Build an environment that includes the dependencies of PACKAGE and execute a
+shell command in that environment.\n"))
+  (display (_ "
+  -e, --expression=EXPR  create environment for the package that EXPR
+                         evaluates to"))
+  (display (_ "
+  -l, --load=FILE        create environment for the package that the code within
+                         FILE evaluates to"))
+  (display (_ "
+  -E, --exec=COMMAND     execute COMMAND in new environment"))
+  (display (_ "
+  --pure                 unset existing environment variables"))
+  (display (_ "
+  --search-paths         display needed environment variable definitions"))
+  (newline)
+  (show-build-options-help)
+  (newline)
+  (display (_ "
+  -h, --help             display this help and exit"))
+  (display (_ "
+  -V, --version          display version information and exit"))
+  (show-bug-report-information))
+
+(define %default-options
+  ;; Default to opening a new shell.
+  `((exec . ,(or (getenv "SHELL") "/bin/sh"))
+    (substitutes? . #t)
+    (max-silent-time . 3600)
+    (verbosity . 0)))
+
+(define %options
+  ;; Specification of the command-line options.
+  (cons* (option '(#\h "help") #f #f
+                 (lambda args
+                   (show-help)
+                   (exit 0)))
+         (option '(#\V "version") #f #f
+                 (lambda args
+                   (show-version-and-exit "guix environment")))
+         (option '("pure") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'pure #t result)))
+         (option '(#\E "exec") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'exec arg result)))
+         (option '("search-paths") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'search-paths #t result)))
+         (option '(#\l "load") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'load arg result)))
+         (option '(#\e "expression") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'expression arg result)))
+         (option '(#\n "dry-run") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'dry-run? #t result)))
+         %standard-build-options))
+
+(define (pick-all alist key)
+  "Return a list of values in ALIST associated with KEY."
+  (define same-key? (cut eq? key <>))
+
+  (fold (lambda (pair memo)
+          (match pair
+            (((? same-key? k) . v)
+             (cons v memo))
+            (_ memo)))
+        '() alist))
+
+(define (options/resolve-packages opts)
+  "Return OPTS with package specification strings replaced by actual
+packages."
+  (map (match-lambda
+        (('package . (? string? spec))
+         `(package . ,(specification->package spec)))
+        (('expression . str)
+         (match (read/eval str)
+           ((? package? p)
+            `(package . ,p))))
+        (('load . file)
+         `(package . ,(load (string-append (getcwd) "/" file))))
+        (opt opt))
+       opts))
+
+(define (packages->transitive-inputs packages)
+  "Return a list of the transitive inputs for all PACKAGES."
+  (define (transitive-inputs package)
+    (filter-map (match-lambda
+                 ((_ (? package? package)) package)
+                 (_ #f))
+                (bag-transitive-inputs
+                 (package->bag package))))
+  (delete-duplicates
+   (append-map transitive-inputs packages)))
+
+;; TODO: Deduplicate these.
+(define show-what-to-build*
+  (store-lift show-what-to-build))
+
+(define set-build-options-from-command-line*
+  (store-lift set-build-options-from-command-line))
+
+(define (build-inputs inputs opts)
+  "Build the packages in INPUTS using the build options in OPTS."
+  (let ((substitutes? (assoc-ref opts 'substitutes?))
+        (dry-run? (assoc-ref opts 'dry-run?)))
+    (mlet* %store-monad ((drvs (sequence %store-monad
+                                         (map package->derivation inputs))))
+      (mbegin %store-monad
+        (show-what-to-build* drvs
+                             #:use-substitutes? substitutes?
+                             #:dry-run? dry-run?)
+        (if dry-run?
+            (return #f)
+            (mbegin %store-monad
+              (set-build-options-from-command-line* opts)
+              (built-derivations drvs)
+              (return drvs)))))))
+
+;; Entry point.
+(define (guix-environment . args)
+  (define (parse-options)
+    (args-fold* args %options
+                (lambda (opt name arg result)
+                  (leave (_ "~A: unrecognized option~%") name))
+                (lambda (arg result)
+                  (alist-cons 'package arg result))
+                %default-options))
+
+  (with-store store
+    (let* ((opts (parse-options))
+           (pure? (assoc-ref opts 'pure))
+           (command (assoc-ref opts 'exec))
+           (inputs (packages->transitive-inputs
+                    (pick-all (options/resolve-packages opts) 'package)))
+           (drvs (run-with-store store (build-inputs inputs opts))))
+      (cond ((assoc-ref opts 'dry-run?)
+             #t)
+            ((assoc-ref opts 'search-paths)
+             (show-search-paths inputs drvs pure?))
+            (else
+             (create-environment inputs drvs pure?)
+             (system command))))))