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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2023 Pierre-Henry Fröhring <contact@phfrohring.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/>.
+
+;; Commentary:
+;;
+;; Code:
+
+(define-module (guix build mix-build-system)
+  #:use-module ((guix build gnu-build-system) #:prefix gnu:)
+  #:use-module (guix build utils)
+  #:use-module (ice-9 ftw)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 regex)
+  #:use-module (ice-9 string-fun)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-71)
+  #:export (mix-build
+            %standard-phases))
+
+;; The Elixir version is constant as soon as it is computable from the current
+;; execution.  It is a X.Y string where X and Y are respectively the major and
+;; minor version number of the Elixir used in the build.
+(define %elixir-version (make-parameter "X.Y"))
+
+(define* (elixir-libdir path #:optional (version (%elixir-version)))
+  "Return the path where all libraries under PATH for a specified Elixir
+VERSION are installed."
+  (string-append path "/lib/elixir/" version))
+
+(define* (strip-prefix name #:optional (prefix "elixir-"))
+  "Return NAME without the prefix PREFIX."
+  (if (string-prefix? prefix name)
+      (string-drop name (string-length prefix))
+      name))
+
+(define (mix-build-dir mix-build-root mix-env)
+  "Return the directory where build artifacts are to be installed according to
+en environment MIX-ENV in the current directory.  MIX-BUILD-ROOT depends on the
+package arguments.  See: https://hexdocs.pm/mix/1.15/Mix.html#module-environment-variables"
+  (string-append mix-build-root "/" mix-env "/lib"))
+
+(define (elixir-version inputs)
+  "Return an X.Y string where X and Y are respectively the major and minor version number of PACKAGE.
+Example: /gnu/store/…-elixir-1.14.0 → 1.14"
+  ((compose
+    (cute string-join <> ".")
+    (cute take <> 2)
+    (cute string-split <> #\.)
+    strip-prefix
+    strip-store-file-name)
+   (assoc-ref inputs "elixir")))
+
+(define* (unpack #:key source mix-path #: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."
+  (let ((gnu-unpack (assoc-ref gnu:%standard-phases 'unpack)))
+    (gnu-unpack #:source source)
+    (when (file-exists? "contents.tar.gz")
+      (invoke "tar" "xvf" "contents.tar.gz"))))
+
+(define (list-directories dir)
+  "List absolute paths of directories directly under the directory DIR."
+  (map (cute string-append dir "/" <>)
+       (scandir dir (lambda (filename)
+                      (and (not (member filename '("." "..")))
+                           (directory-exists? (string-append dir "/" filename)))))))
+
+(define* (set-mix-env #:key inputs mix-path mix-exs #:allow-other-keys)
+  "Set environment variables.
+See: https://hexdocs.pm/mix/1.15.7/Mix.html#module-environment-variables"
+  (setenv "MIX_ARCHIVES" "archives")
+  (setenv "MIX_BUILD_ROOT" "_build")
+  (setenv "MIX_DEPS_PATH" "deps")
+  (setenv "MIX_EXS" mix-exs)
+  (setenv "MIX_HOME" (getcwd))
+  (setenv "MIX_PATH" (or mix-path ""))
+  (setenv "MIX_REBAR3" (string-append (assoc-ref inputs "rebar3") "/bin/rebar3")))
+
+(define* (set-elixir-version #:key inputs #:allow-other-keys)
+  "Store the version number of the Elixir input in a parameter."
+  (%elixir-version (elixir-version inputs))
+  (format #t "Elixir version: ~a~%" (%elixir-version)))
+
+(define* (build #:key mix-environments #:allow-other-keys)
+  "Builds the Mix project."
+  (for-each (lambda (mix-env)
+              (setenv "MIX_ENV" mix-env)
+              (invoke "mix" "compile" "--no-deps-check"))
+            mix-environments))
+
+(define* (check #:key (tests? #t) #:allow-other-keys)
+  "Test the Mix project."
+  (if tests?
+      (invoke "mix" "test" "--no-deps-check")
+      (format #t "tests? = ~a~%" tests?)))
+
+(define* (remove-mix-dirs . _)
+  "Remove all .mix/ directories.
+We do not want to copy them to the installation directory."
+  (for-each delete-file-recursively
+            (find-files "." (file-name-predicate "\\.mix$") #:directories? #t)))
+
+(define (package-name->elixir-name name+ver)
+  "Convert the Guix package NAME-VER to the corresponding Elixir name-version
+format.  Example: elixir-a-pkg-1.2.3 -> a_pkg"
+  ((compose
+    (cute string-join <> "_")
+    (cute drop-right <> 1)
+    (cute string-split <> #\-))
+   (strip-prefix name+ver)))
+
+(define* (install #:key
+                  inputs
+                  outputs
+                  name
+                  build-per-environment
+                  #:allow-other-keys)
+  "Install build artifacts in the store."
+  (let* ((lib-name (package-name->elixir-name name))
+         (lib-dir (string-append (elixir-libdir (assoc-ref outputs "out")) "/" lib-name))
+         (root (getenv "MIX_BUILD_ROOT"))
+         (env (if build-per-environment "prod" "shared")))
+    (mkdir-p lib-dir)
+    (copy-recursively (string-append (mix-build-dir root env) "/" lib-name) lib-dir
+                      #:follow-symlinks? #t)))
+
+(define %standard-phases
+  (modify-phases gnu:%standard-phases
+    (delete 'bootstrap)
+    (delete 'configure)
+    (add-after 'install-locale 'set-mix-env set-mix-env)
+    (add-after 'set-mix-env 'set-elixir-version set-elixir-version)
+    (replace 'unpack unpack)
+    (replace 'build build)
+    (replace 'check check)
+    (add-before 'install 'remove-mix-dirs remove-mix-dirs)
+    (replace 'install install)))
+
+(define* (mix-build #:key inputs (phases %standard-phases)
+                    #:allow-other-keys #:rest args)
+  "Build the given Mix package, applying all of PHASES in order."
+  (apply gnu:gnu-build #:inputs inputs #:phases phases args))
+
+;;; mix-build-system.scm ends here