build-system: Introduce "bags" as an intermediate representation.

* guix/build-system.scm (<build-system>)[build, cross-build]: Remove.
  [lower]: New field.
  (<bag>): New record type.
  (make-bag): New procedure.
* guix/packages.scm (bag-transitive-inputs, bag-transitive-build-inputs,
  bag-transitive-host-inputs, bag-transitive-target-inputs,
  package->bag): New procedures.
  (package-derivation): Use it; use the bag, apply its build procedure,
  etc.
  (package-cross-derivation): Likewise.
* gnu/packages/bootstrap.scm (raw-build, make-raw-bag): New procedure.
  (%bootstrap-guile): Use them.
* guix/build-system/trivial.scm (lower): New procedure.
  (trivial-build, trivial-cross-build): Remove 'source' parameter.  Pass
  INPUTS as is.
  (trivial-build-system): Adjust accordingly.
* guix/build-system/gnu.scm (%store, inputs-search-paths,
  standard-search-paths, expand-inputs, standard-inputs): Remove.
  (gnu-lower): New procedure.
  (gnu-build): Remove 'source' and #:implicit-inputs? parameters.
  Remove 'implicit-inputs' and 'implicit-search-paths' variables.  Get
  the source from INPUT-DRVS.
  (gnu-cross-build): Likewise.
  (standard-cross-packages): Remove call to 'standard-packages'.
  (standard-cross-inputs, standard-cross-search-paths): Remove.
  (gnu-build-system): Remove 'build' and 'cross-build'; add 'lower'.
* guix/build-system/cmake.scm (lower): New procedure.
  (cmake-build): Remove 'source' and #:cmake parameters.  Use INPUTS and
  SEARCH-PATHS as is.  Get the source from INPUTS.
* guix/build-system/perl.scm: Likewise.
* guix/build-system/python.scm: Likewise.
* guix/build-system/ruby.scm: Likewise.
* gnu/packages/cross-base.scm (cross-gcc): Change "cross-linux-headers"
  to "linux-headers".
  (cross-libc)[xlinux-headers]: Pass #:implicit-cross-inputs? #f.
  Likewise.  In 'propagated-inputs', change "cross-linux-headers" to
  "linux-headers".
* guix/git-download.scm (git-fetch): Use 'standard-packages' instead of
  'standard-inputs'.
* tests/builders.scm ("gnu-build-system"): Remove use of
  'build-system-builder'.
  ("gnu-build"): Remove 'source' and #:implicit-inputs? arguments to
  'gnu-build'.
* tests/packages.scm ("search paths"): Adjust to new build system API.
  ("package-cross-derivation, no cross builder"): Likewise.
* doc/guix.texi (Build Systems): Add paragraph on bags.

guix/build-system.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,17 +18,73 @@
 
 (define-module (guix build-system)
   #:use-module (guix records)
+  #:use-module (ice-9 match)
   #:export (build-system
             build-system?
             build-system-name
             build-system-description
-            build-system-builder
-            build-system-cross-builder))
+            build-system-lower
+
+            bag
+            bag?
+            bag-name
+            bag-build-inputs
+            bag-host-inputs
+            bag-target-inputs
+            bag-outputs
+            bag-arguments
+            bag-build
+
+            make-bag))
 
 (define-record-type* <build-system> build-system make-build-system
   build-system?
   (name        build-system-name)         ; symbol
   (description build-system-description)  ; short description
-  (build       build-system-builder)      ; (store system name source inputs)
-  (cross-build build-system-cross-builder ; (store system x-system ...)
-               (default #f)))
+  (lower       build-system-lower))       ; args ... -> bags
+
+;; "Bags" are low-level representations of "packages".  Here we use
+;; build/host/target in the sense of the GNU tool chain (info "(autoconf)
+;; Specifying Target Triplets").
+(define-record-type* <bag> bag %make-bag
+  bag?
+  (name          bag-name)               ;string
+  (build-inputs  bag-build-inputs        ;list of packages
+                 (default '()))
+  (host-inputs   bag-host-inputs         ;list of packages
+                 (default '()))
+
+  ;; "Target inputs" are packages that are built natively, but that are used
+  ;; by target programs in a cross-compilation environment.  Thus, they act
+  ;; like 'inputs' as far as search paths are concerned.  The only example of
+  ;; that is the cross-libc: it is an input of 'cross-gcc', thus built
+  ;; natively; yet, we want it to be considered as a target input for the
+  ;; purposes of $CPATH, $LIBRARY_PATH, etc.
+  (target-inputs bag-target-inputs
+                 (default '()))
+
+  (outputs       bag-outputs             ;list of strings
+                 (default '("out")))
+  (arguments     bag-arguments           ;list
+                 (default '()))
+  (build         bag-build))             ;bag -> derivation
+
+(define* (make-bag build-system name
+                   #:key source (inputs '()) (native-inputs '())
+                   (outputs '()) (arguments '())
+                   target)
+  "Ask BUILD-SYSTEM to return a 'bag' for NAME, with the given SOURCE,
+INPUTS, NATIVE-INPUTS, OUTPUTS, and additional ARGUMENTS.  If TARGET is not
+#f, it must be a string with the GNU triplet of a cross-compilation target.
+
+This is the mechanism by which a package is \"lowered\" to a bag, which is the
+intermediate representation just above derivations."
+  (match build-system
+    (($ <build-system> _ description lower)
+     (apply lower name
+            #:source source
+            #:inputs inputs
+            #:native-inputs native-inputs
+            #:outputs outputs
+            #:target target
+            arguments))))