gexp: Add ‘assume-source-relative-file-name’.

* guix/gexp.scm (assume-source-relative-file-name): New macro.
(local-file): Use assume-source-relative-file-name to look up a non-literal
file relative to the current source directory.
* doc/guix.texi (G-expressions): Document it.
* tests/gexp.scm ("local-file, non-literal source relative file name"):
New test.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Change-Id: I18573c8c7a6c87e8351b34412f9d26bb23b068b4

3 files changed, 25 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 16c697586ac..cb8efa1e50d 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -12251,6 +12251,11 @@ Wrapping is done like this:
 (local-file (assume-valid-file-name alice-key-file-path))
 @end lisp
 
+@var{file} can be wrapped in the @code{assume-source-relative-file-name}
+syntactic keyword.  When this is done, the file name will be looked up
+relative to the source file where it appears even when it is not a
+string literal.
+
 This is the declarative counterpart of the @code{interned-file} monadic
 procedure (@pxref{The Store Monad, @code{interned-file}}).
 @end deffn
diff --git a/guix/gexp.scm b/guix/gexp.scm
index 74b4c49f906..871e59cfdce 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -52,6 +52,7 @@
             gexp-input-native?
 
             assume-valid-file-name
+            assume-source-relative-file-name
             local-file
             local-file?
             local-file-file
@@ -485,6 +486,12 @@ the given file name is valid, even if it's not a string literal, and thus not
 warn about it."
   file)
 
+(define-syntax-rule (assume-source-relative-file-name file)
+  "This is a syntactic keyword to tell 'local-file' that it can assume that
+the given file is relative to the source directory, even if it's not a string
+literal."
+  file)
+
 (define-syntax local-file
   (lambda (s)
     "Return an object representing local file FILE to add to the store; this
@@ -503,13 +510,19 @@ where FILE is the entry's absolute file name and STAT is the result of
 This is the declarative counterpart of the 'interned-file' monadic procedure.
 It is implemented as a macro to capture the current source directory where it
 appears."
-    (syntax-case s (assume-valid-file-name)
+    (syntax-case s (assume-valid-file-name assume-source-relative-file-name)
       ((_ file rest ...)
        (string? (syntax->datum #'file))
        ;; FILE is a literal, so resolve it relative to the source directory.
        #'(%local-file file
                       (delay (absolute-file-name file (current-source-directory)))
                       rest ...))
+      ((_ (assume-source-relative-file-name file) rest ...)
+       ;; FILE is not a literal, but the user requested we look it up
+       ;; relative to the current source directory.
+       #'(%local-file file
+                      (delay (absolute-file-name file (current-source-directory)))
+                      rest ...))
       ((_ (assume-valid-file-name file) rest ...)
        ;; FILE is not a literal, so resolve it relative to the current
        ;; directory.  Since the user declared FILE is valid, do not pass
diff --git a/tests/gexp.scm b/tests/gexp.scm
index b35bfc920f2..ab99e19daa2 100644
--- a/tests/gexp.scm
+++ b/tests/gexp.scm
@@ -251,6 +251,12 @@
       (let ((file (local-file (string-copy "../base32.scm"))))
         (local-file-absolute-file-name file)))))
 
+(test-equal "local-file, non-literal source relative file name"
+  (current-filename)
+  (let ((file (local-file (assume-source-relative-file-name
+                           (string-append "gexp" ".scm")))))
+    (local-file-absolute-file-name file)))
+
 (test-assert "local-file, relative file name, within gexp"
   (let* ((file     (search-path %load-path "guix/base32.scm"))
          (interned (add-to-store %store "base32.scm" #f "sha256" file)))