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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2016, 2018 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2019 Björn Höfling <bjoern.hoefling@bjoernhoefling.de>
+;;; Copyright © 2020 Hartmut Goebel <h.goebel@crazy-compilers.com>
+;;;
+;;; 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 build rebar3-build-system)
+  #:use-module ((guix build gnu-build-system) #:prefix gnu:)
+  #:use-module ((guix build utils) #:hide (delete))
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 ftw)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:export (%standard-phases
+            rebar3-build))
+
+;;
+;; Builder-side code of the standard build procedure for Erlang packages using
+;; rebar3.
+;;
+;; TODO: Think about whether bindir ("ebin"), libdir ("priv") and includedir
+;; "(include") need to be configurable
+
+(define %erlang-libdir "/lib/erlang/lib")
+
+(define* (erlang-depends #:key inputs #:allow-other-keys)
+  (define input-directories
+    (match inputs
+      (((_ . dir) ...)
+       dir)))
+  (mkdir-p "_checkouts")
+
+  (for-each
+   (lambda (input-dir)
+     (let ((elibdir (string-append input-dir %erlang-libdir)))
+       (when (directory-exists? elibdir)
+         (for-each
+          (lambda (dirname)
+            (symlink (string-append elibdir "/" dirname)
+                     (string-append "_checkouts/" dirname)))
+          (list-directories elibdir)))))
+   input-directories)
+  #t)
+
+(define* (unpack #:key source #:allow-other-keys)
+  "Unpack SOURCE in the working directory, and change directory within the
+source.  When SOURCE is a directory, copy it in a sub-directory of the current
+working directory."
+  ;; archives from hexpm typicalls do not contain a directory level
+  ;; TODO: Check if archive contains a directory level
+  (mkdir "source")
+  (chdir "source")
+  (if (file-is-directory? source)
+      (begin
+        ;; Preserve timestamps (set to the Epoch) on the copied tree so that
+        ;; things work deterministically.
+        (copy-recursively source "."
+                          #:keep-mtime? #t))
+      (begin
+        (if (string-suffix? ".zip" source)
+            (invoke "unzip" source)
+            (invoke "tar" "xvf" source))))
+  #t)
+
+(define* (build #:key (make-flags '()) (build-target "compile")
+                #:allow-other-keys)
+  (apply invoke `("rebar3" ,build-target ,@make-flags)))
+
+(define* (check #:key target (make-flags '()) (tests? (not target))
+                (test-target "eunit")
+                #:allow-other-keys)
+  (if tests?
+      (apply invoke `("rebar3" ,test-target ,@make-flags))
+      (format #t "test suite not run~%"))
+  #t)
+
+(define (erlang-package? name)
+  "Check if NAME correspond to the name of an Erlang package."
+  (string-prefix? "erlang-" name))
+
+(define (package-name-version->erlang-name name+ver)
+  "Convert the Guix package NAME-VER to the corresponding Erlang name-version
+format.  Essentially drop the prefix used in Guix and replace dashes by
+underscores."
+  (let* ((name- (package-name->name+version name+ver)))
+    (string-join
+     (string-split
+      (if (erlang-package? name-)  ; checks for "erlang-" prefix
+          (string-drop name- (string-length "erlang-"))
+          name-)
+      #\-)
+     "_")))
+
+(define (list-directories directory)
+  "Return file names of the sub-directory of DIRECTORY."
+  (scandir directory
+           (lambda (file)
+             (and (not (member file '("." "..")))
+                  (file-is-directory? (string-append directory "/" file))))))
+
+(define* (install #:key name outputs
+                  (pkg-name (package-name-version->erlang-name name))
+                  #:allow-other-keys)
+  (let* ((out (assoc-ref outputs "out"))
+         (build-dir "_build/default/lib")
+         (pkg-dir (string-append out %erlang-libdir "/" pkg-name)))
+    (for-each
+     (lambda (pkg)
+       (for-each
+        (lambda (dirname)
+          (let ((src-dir (string-append build-dir "/" pkg "/" dirname))
+                (dst-dir (string-append pkg-dir "/" dirname)))
+            (when (file-exists? src-dir)
+              (copy-recursively src-dir dst-dir #:follow-symlinks? #t))
+            (false-if-exception
+             (delete-file (string-append dst-dir "/.gitignore")))))
+        '("ebin" "include" "priv")))
+     (list-directories build-dir))
+    (false-if-exception
+     (delete-file (string-append pkg-dir "/priv/Run-eunit-loop.expect")))
+    #t))
+
+(define %standard-phases
+  (modify-phases gnu:%standard-phases
+    (replace 'unpack unpack)
+    (delete 'bootstrap)
+    (delete 'configure)
+    (add-before 'build 'erlang-depends erlang-depends)
+    (replace 'build build)
+    (replace 'check check)
+    (replace 'install install)))
+
+(define* (rebar3-build #:key inputs (phases %standard-phases)
+                       #:allow-other-keys #:rest args)
+  "Build the given Erlang package, applying all of PHASES in order."
+  (apply gnu:gnu-build #:inputs inputs #:phases phases args))