import: Add JSON importer.

* doc/guix.texi (Invoking guix import): Document it.
* guix/scripts/import/json.scm: New file.
* Makefile.am (MODULES): Add it.
* guix/scripts/import.scm (importers): Add json importer.

Makefile.am

@@ -174,6 +174,7 @@ MODULES =					\
   guix/scripts/import/gnu.scm			\
   guix/scripts/import/nix.scm			\
   guix/scripts/import/hackage.scm		\
+  guix/scripts/import/json.scm  		\
   guix/scripts/import/elpa.scm  		\
   guix/scripts/import/texlive.scm  		\
   guix/scripts/environment.scm			\

doc/guix.texi

@@ -5899,6 +5899,56 @@ CTAN while fetching the sources from the directory
 guix import texlive --archive=generic ifxetex
 @end example
 
+@item json
+@cindex JSON, import
+Import package metadata from a local JSON file@footnote{This
+functionality requires Guile-JSON to be installed.
+@xref{Requirements}.}.  Consider the following example package
+definition in JSON format:
+
+@example
+@{
+  "name": "hello",
+  "version": "2.10",
+  "source": "mirror://gnu/hello/hello-2.10.tar.gz",
+  "build-system": "gnu",
+  "home-page": "https://www.gnu.org/software/hello/",
+  "synopsis": "Hello, GNU world: An example GNU package",
+  "description": "GNU Hello prints a greeting.",
+  "license": "GPL-3.0+",
+  "native-inputs": ["gcc@@6"]
+@}
+@end example
+
+The field names are the same as for the @code{<package>} record
+(@xref{Defining Packages}).  References to other packages are provided
+as JSON lists of quoted package specification strings such as
+@code{guile} or @code{guile@@2.0}.
+
+The importer also supports a more explicit source definition using the
+common fields for @code{<origin>} records:
+
+@example
+@{
+  @dots{}
+  "source": @{
+    "method": "url-fetch",
+    "uri": "mirror://gnu/hello/hello-2.10.tar.gz",
+    "sha256": @{
+      "base32": "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"
+    @}
+  @}
+  @dots{}
+@}
+@end example
+
+The command below reads metadata from the JSON file @code{hello.json}
+and outputs a package expression:
+
+@example
+guix import json hello.json
+@end example
+
 @item nix
 Import metadata from a local copy of the source of the
 @uref{http://nixos.org/nixpkgs/, Nixpkgs distribution}@footnote{This

guix/scripts/build.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -31,9 +32,11 @@
 
   #:use-module (guix monads)
   #:use-module (guix gexp)
+  #:autoload   (json) (json-string->scm)
   #:autoload   (guix http-client) (http-fetch http-get-error?)
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 rdelim)
   #:use-module (ice-9 vlist)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
@@ -55,6 +58,10 @@
             register-root
             register-root*))
 
+;; Lazy reference to import utils to avoid cycle
+(define (lazy-util sym)
+  (module-ref (resolve-interface '(guix import utils)) sym))
+
 (define %default-log-urls
   ;; Default base URLs for build logs.
   '("http://hydra.gnu.org/log"))
@@ -606,7 +613,16 @@ build---packages, gexps, derivations, and so on."
                        (else
                         (list (specification->package spec)))))
                 (('file . file)
-                 (ensure-list (load* file (make-user-module '()))))
+                 (if (string-suffix? ".json" file)
+                     (begin
+                       ;; Load (json) lazily to avoid hard dependency.
+                       ;; TODO: doesn't work
+                       (let* ((json (json-string->scm
+                                     (with-input-from-file file read-string)))
+                              (pkg ((lazy-util 'data->guix-package)
+                                    ((lazy-util 'hash-table->alist) json))))
+                         (ensure-list pkg)))
+                     (ensure-list (load* file (make-user-module '())))))
                 (('expression . str)
                  (ensure-list (read/eval str)))
                 (('argument . (? derivation? drv))

guix/scripts/import.scm

@@ -74,7 +74,7 @@ rather than \\n."
 ;;;
 
 (define importers '("gnu" "nix" "pypi" "cpan" "hackage" "stackage" "elpa" "gem"
-                    "cran" "crate" "texlive"))
+                    "cran" "crate" "texlive" "json"))
 
 (define (resolve-importer name)
   (let ((module (resolve-interface

guix/scripts/import/json.scm

@@ -0,0 +1,102 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 Eric Bavier <bavier@member.fsf.org>
+;;; Copyright © 2015, 2017 Ricardo Wurmus <rekado@elephly.net>
+;;;
+;;; 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 import json)
+  #:use-module (json)
+  #:use-module (guix ui)
+  #:use-module (guix utils)
+  #:use-module (guix scripts)
+  #:use-module (guix import utils)
+  #:use-module (guix import print)
+  #:use-module (guix scripts import)
+  #:use-module (guix packages)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-9 gnu)
+  #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-37)
+  #:use-module (srfi srfi-41)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 format)
+  #:export (guix-import-json))
+
+
+;;;
+;;; Command-line options.
+;;;
+
+(define %default-options
+  '())
+
+(define (show-help)
+  (display (G_ "Usage: guix import json PACKAGE-FILE
+Import and convert the JSON package definition in PACKAGE-FILE.\n"))
+  (display (G_ "
+  -h, --help             display this help and exit"))
+  (display (G_ "
+  -V, --version          display version information and exit"))
+  (newline)
+  (show-bug-report-information))
+
+(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 import json")))
+         %standard-import-options))
+
+
+;;;
+;;; Entry point.
+;;;
+
+(define (guix-import-json . args)
+  (define (parse-options)
+    ;; Return the alist of option values.
+    (args-fold* args %options
+                (lambda (opt name arg result)
+                  (leave (G_ "~A: unrecognized option~%") name))
+                (lambda (arg result)
+                  (alist-cons 'argument arg result))
+                %default-options))
+
+  (let* ((opts (parse-options))
+         (args (filter-map (match-lambda
+                            (('argument . value)
+                             value)
+                            (_ #f))
+                           (reverse opts))))
+    (match args
+      ((file-name)
+       (catch 'json-invalid
+         (lambda ()
+           (let ((json (json-string->scm
+                        (with-input-from-file file-name read-string))))
+             ;; TODO: also print define-module boilerplate
+             (package->code (alist->package (hash-table->alist json)))))
+         (lambda _
+           (leave (G_ "invalid JSON in file '~a'~%") file-name))))
+      (()
+       (leave (G_ "too few arguments~%")))
+      ((many ...)
+       (leave (G_ "too many arguments~%"))))))