gexp: Add 'let-system'.

* guix/gexp.scm (<system-binding>): New record type.
(let-system): New macro.
(system-binding-compiler): New procedure.
(default-expander): Add 'self-quoting?' case.
(self-quoting?): New procedure.
(lower-inputs): Add 'filterm'.  Pass the result of
'mapm/accumulate-builds' through FILTERM.
(gexp->sexp)[self-quoting?]: Remove.
* tests/gexp.scm ("let-system", "let-system, target")
("let-system, ungexp-native, target")
("let-system, nested"): New tests.
* doc/guix.texi (G-Expressions): Document it.

.dir-locals.el

@@ -85,6 +85,7 @@
    (eval . (put 'with-imported-modules 'scheme-indent-function 1))
    (eval . (put 'with-extensions 'scheme-indent-function 1))
    (eval . (put 'with-parameters 'scheme-indent-function 1))
+   (eval . (put 'let-system 'scheme-indent-function 1))
 
    (eval . (put 'with-database 'scheme-indent-function 2))
    (eval . (put 'call-with-transaction 'scheme-indent-function 2))

doc/guix.texi

@@ -8123,6 +8123,32 @@ the second case, the resulting script contains a @code{(string-append
 @dots{})} expression to construct the file name @emph{at run time}.
 @end deffn
 
+@deffn {Scheme Syntax} let-system @var{system} @var{body}@dots{}
+@deffnx {Scheme Syntax} let-system (@var{system} @var{target}) @var{body}@dots{}
+Bind @var{system} to the currently targeted system---e.g.,
+@code{"x86_64-linux"}---within @var{body}.
+
+In the second case, additionally bind @var{target} to the current
+cross-compilation target---a GNU triplet such as
+@code{"arm-linux-gnueabihf"}---or @code{#f} if we are not
+cross-compiling.
+
+@code{let-system} is useful in the occasional case where the object
+spliced into the gexp depends on the target system, as in this example:
+
+@example
+#~(system*
+   #+(let-system system
+       (cond ((string-prefix? "armhf-" system)
+              (file-append qemu "/bin/qemu-system-arm"))
+             ((string-prefix? "x86_64-" system)
+              (file-append qemu "/bin/qemu-system-x86_64"))
+             (else
+              (error "dunno!"))))
+   "-net" "user" #$image)
+@end example
+@end deffn
+
 @deffn {Scheme Syntax} with-parameters ((@var{parameter} @var{value}) @dots{}) @var{exp}
 This macro is similar to the @code{parameterize} form for
 dynamically-bound @dfn{parameters} (@pxref{Parameters,,, guile, GNU

guix/gexp.scm

@@ -37,6 +37,7 @@ (define-module (guix gexp)
             gexp?
             with-imported-modules
             with-extensions
+            let-system
 
             gexp-input
             gexp-input?
@@ -195,7 +196,9 @@ (define (default-expander thing obj output)
     ((? derivation? drv)
      (derivation->output-path drv output))
     ((? string? file)
-     file)))
+     file)
+    ((? self-quoting? obj)
+     obj)))
 
 (define (register-compiler! compiler)
   "Register COMPILER as a gexp compiler."
@@ -327,6 +330,52 @@ (define-gexp-compiler raw-derivation-file-compiler <raw-derivation-file>
                     (derivation-file-name lowered)
                     lowered)))
 
+
+;;;
+;;; System dependencies.
+;;;
+
+;; Binding form for the current system and cross-compilation target.
+(define-record-type <system-binding>
+  (system-binding proc)
+  system-binding?
+  (proc system-binding-proc))
+
+(define-syntax let-system
+  (syntax-rules ()
+    "Introduce a system binding in a gexp.  The simplest form is:
+
+  (let-system system
+    (cond ((string=? system \"x86_64-linux\") ...)
+          (else ...)))
+
+which binds SYSTEM to the currently targeted system.  The second form is
+similar, but it also shows the cross-compilation target:
+
+  (let-system (system target)
+    ...)
+
+Here TARGET is bound to the cross-compilation triplet or #f."
+    ((_ (system target) exp0 exp ...)
+     (system-binding (lambda (system target)
+                       exp0 exp ...)))
+    ((_ system exp0 exp ...)
+     (system-binding (lambda (system target)
+                       exp0 exp ...)))))
+
+(define-gexp-compiler system-binding-compiler <system-binding>
+  compiler => (lambda (binding system target)
+                (match binding
+                  (($ <system-binding> proc)
+                   (with-monad %store-monad
+                     ;; PROC is expected to return a lowerable object.
+                     ;; 'lower-object' takes care of residualizing it to a
+                     ;; derivation or similar.
+                     (return (proc system target))))))
+
+  ;; Delegate to the expander of the object returned by PROC.
+  expander => #f)
+
 
 ;;;
 ;;; File declarations.
@@ -706,6 +755,15 @@ (define (gexp-extensions gexp)
 list."
   (gexp-attribute gexp gexp-self-extensions))
 
+(define (self-quoting? x)
+  (letrec-syntax ((one-of (syntax-rules ()
+                            ((_) #f)
+                            ((_ pred rest ...)
+                             (or (pred x)
+                                 (one-of rest ...))))))
+    (one-of symbol? string? keyword? pair? null? array?
+            number? boolean? char?)))
+
 (define* (lower-inputs inputs
                        #:key system target)
   "Turn any object from INPUTS into a derivation input for SYSTEM or a store
@@ -714,8 +772,11 @@ (define* (lower-inputs inputs
   (define (store-item? obj)
     (and (string? obj) (store-path? obj)))
 
+  (define filterm
+    (lift1 (cut filter ->bool <>) %store-monad))
+
   (with-monad %store-monad
-    (mapm/accumulate-builds
+    (>>= (mapm/accumulate-builds
           (match-lambda
             (((? struct? thing) sub-drv ...)
              (mlet %store-monad ((obj (lower-object
@@ -727,10 +788,16 @@ (define (store-item? obj)
                                              sub-drv)))
                             (derivation-input drv outputs)))
                          ((? store-item? item)
-                     item)))))
+                          item)
+                         ((? self-quoting?)
+                          ;; Some inputs such as <system-binding> can lower to
+                          ;; a self-quoting object that FILTERM will filter
+                          ;; out.
+                          #f)))))
             (((? store-item? item))
              (return item)))
-     inputs)))
+          inputs)
+         filterm)))
 
 (define* (lower-reference-graphs graphs #:key system target)
   "Given GRAPHS, a list of (FILE-NAME INPUT ...) lists for use as a
@@ -1146,15 +1213,6 @@ (define* (gexp->sexp exp #:key
                      (target (%current-target-system)))
   "Return (monadically) the sexp corresponding to EXP for the given OUTPUT,
 and in the current monad setting (system type, etc.)"
-  (define (self-quoting? x)
-    (letrec-syntax ((one-of (syntax-rules ()
-                              ((_) #f)
-                              ((_ pred rest ...)
-                               (or (pred x)
-                                   (one-of rest ...))))))
-      (one-of symbol? string? keyword? pair? null? array?
-              number? boolean? char?)))
-
   (define* (reference->sexp ref #:optional native?)
     (with-monad %store-monad
       (match ref

tests/gexp.scm

@@ -321,6 +321,60 @@ (define (match-input thing)
        (string=? result
                  (string-append (derivation->output-path drv)
                                 "/bin/touch"))))))
+(test-equal "let-system"
+  (list `(begin ,(%current-system) #t) '(system-binding) '()
+        'low '() '())
+  (let* ((exp #~(begin
+                  #$(let-system system system)
+                  #t))
+         (low (run-with-store %store (lower-gexp exp))))
+    (list (lowered-gexp-sexp low)
+          (match (gexp-inputs exp)
+            (((($ (@@ (guix gexp) <system-binding>)) "out"))
+             '(system-binding))
+            (x x))
+          (gexp-native-inputs exp)
+          'low
+          (lowered-gexp-inputs low)
+          (lowered-gexp-sources low))))
+
+(test-equal "let-system, target"
+  (list `(list ,(%current-system) #f)
+        `(list ,(%current-system) "aarch64-linux-gnu"))
+  (let ((exp #~(list #$@(let-system (system target)
+                          (list system target)))))
+    (list (gexp->sexp* exp)
+          (gexp->sexp* exp "aarch64-linux-gnu"))))
+
+(test-equal "let-system, ungexp-native, target"
+  `(here it is: ,(%current-system) #f)
+  (let ((exp #~(here it is: #+@(let-system (system target)
+                                 (list system target)))))
+    (gexp->sexp* exp "aarch64-linux-gnu")))
+
+(test-equal "let-system, nested"
+  (list `(system* ,(string-append "qemu-system-" (%current-system))
+                  "-m" "256")
+        '()
+        '(system-binding))
+  (let ((exp #~(system*
+                #+(let-system (system target)
+                    (file-append (@@ (gnu packages virtualization)
+                                     qemu)
+                                 "/bin/qemu-system-"
+                                 system))
+                "-m" "256")))
+    (list (match (gexp->sexp* exp)
+            (('system* command rest ...)
+             `(system* ,(and (string-prefix? (%store-prefix) command)
+                             (basename command))
+                       ,@rest))
+            (x x))
+          (gexp-inputs exp)
+          (match (gexp-native-inputs exp)
+            (((($ (@@ (guix gexp) <system-binding>)) "out"))
+             '(system-binding))
+            (x x)))))
 
 (test-assert "ungexp + ungexp-native"
   (let* ((exp    (gexp (list (ungexp-native %bootstrap-guile)