platform: Add discovery support.

* gnu/platform.scm (platform-modules, systems, targets,
lookup-platform-by-system, lookup-platform-by-target,
lookup-platform-by-target-or-system
platform-system->target,
platform-target->system): New procedures.
(%platforms): New variable.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

gnu/platform.scm

@@ -17,22 +17,117 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu platform)
+  #:use-module (guix discovery)
+  #:use-module (guix memoization)
   #:use-module (guix records)
+  #:use-module (guix ui)
+  #:use-module (srfi srfi-1)
   #:export (platform
             platform?
             platform-target
             platform-system
-            platform-linux-architecture))
+            platform-linux-architecture
+
+            platform-modules
+            platforms
+            lookup-platform-by-system
+            lookup-platform-by-target
+            lookup-platform-by-target-or-system
+            platform-system->target
+            platform-target->system
+
+            systems
+            targets))
 
 
 ;;;
 ;;; Platform record.
 ;;;
 
-;; Description of a platform supported by the GNU system.
+;; Description of a platform supported by GNU Guix.
+;;
+;; The 'target' field must be a valid GNU triplet as defined here:
+;; https://www.gnu.org/software/autoconf/manual/autoconf-2.68/html_node/Specifying-Target-Triplets.html.
+;; It is used for cross-compilation purposes.
+;;
+;; The 'system' field is the name of the corresponding system as defined in
+;; the (gnu packages bootstrap) module.  It can be for instance
+;; "aarch64-linux" or "armhf-linux".  It is used to emulate a different host
+;; architecture, for instance i686-linux on x86_64-linux-gnu, or armhf-linux
+;; on x86_64-linux, using the QEMU binfmt transparent emulation mechanism.
+;;
+;; The 'linux-architecture' is only relevant if the kernel is Linux.  In that
+;; case, it corresponds to the ARCH variable used when building Linux.
 (define-record-type* <platform> platform make-platform
   platform?
   (target             platform-target)               ;"x86_64-linux-gnu"
   (system             platform-system)               ;"x86_64-linux"
-  (linux-architecture platform-linux-architecture    ;"amd64"
+  (linux-architecture platform-linux-architecture    ;"x86"
                       (default #f)))
+
+;;;
+;;; Platforms.
+;;;
+
+(define (platform-modules)
+  "Return the list of platform modules."
+  (all-modules (map (lambda (entry)
+                      `(,entry . "gnu/platforms"))
+                    %load-path)
+               #:warn warn-about-load-error))
+
+(define platforms
+  ;; The list of publically-known platforms.
+  (memoize
+   (lambda ()
+     (fold-module-public-variables (lambda (obj result)
+                                     (if (platform? obj)
+                                         (cons obj result)
+                                         result))
+                                   '()
+                                   (platform-modules)))))
+
+(define (lookup-platform-by-system system)
+  "Return the platform corresponding to the given SYSTEM."
+  (find (lambda (platform)
+          (let ((s (platform-system platform)))
+            (and (string? s) (string=? s system))))
+        (platforms)))
+
+(define (lookup-platform-by-target target)
+  "Return the platform corresponding to the given TARGET."
+  (find (lambda (platform)
+          (let ((t (platform-target platform)))
+            (and (string? t) (string=? t target))))
+        (platforms)))
+
+(define (lookup-platform-by-target-or-system target-or-system)
+  "Return the platform corresponding to the given TARGET or SYSTEM."
+  (or (lookup-platform-by-target target-or-system)
+      (lookup-platform-by-system target-or-system)))
+
+(define (platform-system->target system)
+  "Return the target matching the given SYSTEM if it exists or false
+otherwise."
+  (let ((platform (lookup-platform-by-system system)))
+    (and=> platform platform-target)))
+
+(define (platform-target->system target)
+  "Return the system matching the given TARGET if it exists or false
+otherwise."
+  (let ((platform (lookup-platform-by-target system)))
+    (and=> platform platform-system)))
+
+
+;;;
+;;; Systems & Targets.
+;;;
+
+(define (systems)
+  "Return the list of supported systems."
+  (delete-duplicates
+   (filter-map platform-system (platforms))))
+
+(define (targets)
+  "Return the list of supported targets."
+  (map platform-target (platforms)))