[bug#29926] [PATCH 1/5] gexp: Add 'eval-gexp'.

[宋文武 <iyzsong@member.fsf.org>]

* guix/gexp.scm (eval-gexp): New procedure.
---
 guix/gexp.scm | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/guix/gexp.scm b/guix/gexp.scm
index f005c4d29..9709c5ecb 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -76,6 +76,7 @@
             gexp->derivation
             gexp->file
             gexp->script
+            eval-gexp
             text-file*
             mixed-text-file
             file-union
@@ -1167,6 +1168,21 @@ and '%load-compiled-path' to honor EXP's imported modules."
                          #:local-build? #t
                          #:substitutable? #f)))))
 
+(define* (eval-gexp exp #:optional (name "computed-value"))
+  "Return as a monadic value the EXP (a gexp) evaluate to.  This is
+implemented by building a store file NAME that contains the textual
+representation of EXP's value, and then @code{read} from it."
+  (mlet* %store-monad
+      ((drv (gexp->derivation
+             name
+             (gexp (with-output-to-file (ungexp output)
+                     (lambda ()
+                       (write (eval (quote (ungexp exp)) (current-module))))))
+             #:local-build? #t
+             #:substitutable? #f))
+       (_   (built-derivations (list drv))))
+    (return (call-with-input-file (derivation->output-path drv) read))))
+
 (define* (text-file* name #:rest text)
   "Return as a monadic value a derivation that builds a text file containing
 all of TEXT.  TEXT may list, in addition to strings, objects of any type that
-- 
2.13.3