Add (guix gexp).

* guix/gexp.scm: New file.
* tests/gexp.scm: New file.
* Makefile.am (MODULES): Add guix/gexp.scm.
  (SCM_TESTS): Add tests/gexp.scm.
* doc/guix.texi (Derivations): Add #:inputs in 'derivation' example.
  Mark 'build-expression->derivation' as deprecated, refer to
  "G-Expressions".  Remove paragraph about code strata.
  (G-Expressions): New node.

.dir-locals.el

@@ -6,6 +6,7 @@
  (scheme-mode
   .
   ((indent-tabs-mode . nil)
+   (eval . (put 'eval-when 'scheme-indent-function 1))
    (eval . (put 'test-assert 'scheme-indent-function 1))
    (eval . (put 'test-equal 'scheme-indent-function 1))
    (eval . (put 'test-eq 'scheme-indent-function 1))
@@ -31,7 +32,13 @@
    (eval . (put 'with-monad 'scheme-indent-function 1))
    (eval . (put 'mlet* 'scheme-indent-function 2))
    (eval . (put 'mlet 'scheme-indent-function 2))
-   (eval . (put 'run-with-store 'scheme-indent-function 1))))
+   (eval . (put 'run-with-store 'scheme-indent-function 1))
+
+   ;; Recognize '~' and '$', as used for gexps, as quotation symbols.  This
+   ;; notably allows '(' in Paredit to not insert a space when the preceding
+   ;; symbol is one of these.
+   (eval . (modify-syntax-entry ?~ "'"))
+   (eval . (modify-syntax-entry ?$ "'"))))
  (emacs-lisp-mode . ((indent-tabs-mode . nil)))
  (texinfo-mode    . ((indent-tabs-mode . nil)
                      (fill-column . 72))))

Makefile.am

@@ -37,6 +37,7 @@ MODULES =					\
   guix/download.scm				\
   guix/git-download.scm				\
   guix/monads.scm				\
+  guix/gexp.scm					\
   guix/profiles.scm				\
   guix/serialization.scm			\
   guix/nar.scm					\
@@ -139,6 +140,7 @@ SCM_TESTS =					\
   tests/snix.scm				\
   tests/store.scm				\
   tests/monads.scm				\
+  tests/gexp.scm				\
   tests/nar.scm					\
   tests/union.scm				\
   tests/profiles.scm

doc/guix.texi

@@ -1305,6 +1305,7 @@ package definitions.
 * The Store::                   Manipulating the package store.
 * Derivations::                 Low-level interface to package derivations.
 * The Store Monad::             Purely functional interface to the store.
+* G-Expressions::               Manipulating build expressions.
 @end menu
 
 @node Defining Packages
@@ -1762,13 +1763,21 @@ to a Bash executable in the store:
                            "echo hello world > $out\n" '())))
   (derivation store "foo"
               bash `("-e" ,builder)
+              #:inputs `((,bash) (,builder))
               #:env-vars '(("HOME" . "/homeless"))))
 @result{} #<derivation /gnu/store/@dots{}-foo.drv => /gnu/store/@dots{}-foo>
 @end lisp
 
-As can be guessed, this primitive is cumbersome to use directly.  An
+As can be guessed, this primitive is cumbersome to use directly.  A
-improved variant is @code{build-expression->derivation}, which allows
+better approach is to write build scripts in Scheme, of course!  The
-the caller to directly pass a Guile expression as the build script:
+best course of action for that is to write the build code as a
+``G-expression'', and to pass it to @code{gexp->derivation}.  For more
+information, @ref{G-Expressions}.
+
+Once upon a time, @code{gexp->derivation} did not exist and constructing
+derivations with build code written in Scheme was achieved with
+@code{build-expression->derivation}, documented below.  This procedure
+is now deprecated in favor of the much nicer @code{gexp->derivation}.
 
 @deffn {Scheme Procedure} build-expression->derivation @var{store} @
        @var{name} @var{exp} @
@@ -1816,20 +1825,6 @@ containing one file:
 @result{} #<derivation /gnu/store/@dots{}-goo.drv => @dots{}>
 @end lisp
 
-@cindex strata of code
-Remember that the build expression passed to
-@code{build-expression->derivation} is run by a separate Guile process
-than the one that calls @code{build-expression->derivation}: it is run
-by a Guile process launched by the daemon, typically in a chroot.  So,
-while there is a single language for both the @dfn{host} and the build
-side, there are really two @dfn{strata} of code: the host-side, and the
-build-side code@footnote{The term @dfn{stratum} in this context was
-coined by Manuel Serrano et al. in the context of their work on Hop.}.
-This distinction is important to keep in mind, notably when using
-higher-level constructs such as @var{gnu-build-system} (@pxref{Defining
-Packages}).  For this reason, Guix modules that are meant to be used in
-the build stratum are kept in the @code{(guix build @dots{})} name
-space.
 
 @node The Store Monad
 @section The Store Monad
@@ -1993,6 +1988,196 @@ Packages}).
 @end deffn
 
 
+@node G-Expressions
+@section G-Expressions
+
+@cindex G-expression
+@cindex build code quoting
+So we have ``derivations'', which represent a sequence of build actions
+to be performed to produce an item in the store (@pxref{Derivations}).
+Those build actions are performed when asking the daemon to actually
+build the derivations; they are run by the daemon in a container
+(@pxref{Invoking guix-daemon}).
+
+@cindex strata of code
+It should come as no surprise that we like to write those build actions
+in Scheme.  When we do that, we end up with two @dfn{strata} of Scheme
+code@footnote{The term @dfn{stratum} in this context was coined by
+Manuel Serrano et al.@: in the context of their work on Hop.}: the
+``host code''---code that defines packages, talks to the daemon,
+etc.---and the ``build code''---code that actually performs build
+actions, such as making directories, invoking @command{make}, etc.
+
+To describe a derivation and its build actions, one typically needs to
+embed build code inside host code.  It boils down to manipulating build
+code as data, and Scheme's homoiconicity---code has a direct
+representation as data---comes in handy for that.  But we need more than
+Scheme's normal @code{quasiquote} mechanism to construct build
+expressions.
+
+The @code{(guix gexp)} module implements @dfn{G-expressions}, a form of
+S-expressions adapted to build expressions.  G-expressions, or
+@dfn{gexps}, consist essentially in three syntactic forms: @code{gexp},
+@code{ungexp}, and @code{ungexp-splicing} (or simply: @code{#~},
+@code{#$}, and @code{#$@@}), which are comparable respectively to
+@code{quasiquote}, @code{unquote}, and @code{unquote-splicing}
+(@pxref{Expression Syntax, @code{quasiquote},, guile, GNU Guile
+Reference Manual}).  However, there are major differences:
+
+@itemize
+@item
+Gexps are meant to be written to a file and run or manipulated by other
+processes.
+
+@item
+When a package or derivation is unquoted inside a gexp, the result is as
+if its output file name had been introduced.
+
+@item
+Gexps carry information about the packages or derivations they refer to,
+and these dependencies are automatically added as inputs to the build
+processes that use them.
+@end itemize
+
+To illustrate the idea, here is an example of a gexp:
+
+@example
+(define build-exp
+  #~(begin
+      (mkdir #$output)
+      (chdir #$output)
+      (symlink (string-append #$coreutils "/bin/ls") 
+               "list-files")))
+@end example
+
+This gexp can be passed to @code{gexp->derivation}; we obtain a
+derivation that builds a directory containing exactly one symlink to
+@file{/gnu/store/@dots{}-coreutils-8.22/bin/ls}:
+
+@example
+(gexp->derivation "the-thing" build-exp)
+@end example
+
+As one would expect, the @code{"/gnu/store/@dots{}-coreutils"} string is
+substituted to the reference to the @var{coreutils} package in the
+actual build code, and @var{coreutils} is automatically made an input to
+the derivation.  Likewise, @code{#$output} (equivalent to @code{(ungexp
+output)}) is replaced by a string containing the derivation's output
+directory name.  The syntactic form to construct gexps is summarized
+below.
+
+@deffn {Scheme Syntax} #~@var{exp}
+@deffnx {Scheme Syntax} (gexp @var{exp})
+Return a G-expression containing @var{exp}.  @var{exp} may contain one
+or more of the following forms:
+
+@table @code
+@item #$@var{obj}
+@itemx (ungexp @var{obj})
+Introduce a reference to @var{obj}.  @var{obj} may be a package or a
+derivation, in which case the @code{ungexp} form is replaced by its
+output file name---e.g., @code{"/gnu/store/@dots{}-coreutils-8.22}.
+
+If @var{obj} is a list, it is traversed and any package or derivation
+references are substituted similarly.
+
+If @var{obj} is another gexp, its contents are inserted and its
+dependencies are added to those of the containing gexp.
+
+If @var{obj} is another kind of object, it is inserted as is.
+
+@item #$@var{package-or-derivation}:@var{output}
+@itemx (ungexp @var{package-or-derivation} @var{output})
+This is like the form above, but referring explicitly to the
+@var{output} of @var{package-or-derivation}---this is useful when
+@var{package-or-derivation} produces multiple outputs (@pxref{Packages
+with Multiple Outputs}).
+
+@item #$output[:@var{output}]
+@itemx (ungexp output [@var{output}])
+Insert a reference to derivation output @var{output}, or to the main
+output when @var{output} is omitted.
+
+This only makes sense for gexps passed to @code{gexp->derivation}.
+
+@item #$@@@var{lst}
+@itemx (ungexp-splicing @var{lst})
+Like the above, but splices the contents of @var{lst} inside the
+containing list.
+
+@end table
+
+G-expressions created by @code{gexp} or @code{#~} are run-time objects
+of the @code{gexp?} type (see below.)
+@end deffn
+
+@deffn {Scheme Procedure} gexp? @var{obj}
+Return @code{#t} if @var{obj} is a G-expression.
+@end deffn
+
+G-expressions are meant to be written to disk, either as code building
+some derivation, or as plain files in the store.  The monadic procedures
+below allow you to do that (@pxref{The Store Monad}, for more
+information about monads.)
+
+@deffn {Monadic Procedure} gexp->derivation @var{name} @var{exp} @
+       [#:system (%current-system)] [#:inputs '()] @
+       [#:hash #f] [#:hash-algo #f] @
+       [#:recursive? #f] [#:env-vars '()] [#:modules '()] @
+       [#:references-graphs #f] [#:local-build? #f] @
+       [#:guile-for-build #f]
+Return a derivation @var{name} that runs @var{exp} (a gexp) with
+@var{guile-for-build} (a derivation) on @var{system}.
+
+Make @var{modules} available in the evaluation context of @var{EXP};
+@var{MODULES} is a list of names of Guile modules from the current
+search path to be copied in the store, compiled, and made available in
+the load path during the execution of @var{exp}---e.g., @code{((guix
+build utils) (guix build gnu-build-system))}.
+
+The other arguments are as for @code{derivation}.
+@end deffn
+
+@deffn {Monadic Procedure} gexp->script @var{name} @var{exp}
+Return an executable script @var{name} that runs @var{exp} using
+@var{guile} with @var{modules} in its search path.
+
+The example below builds a script that simply invokes the @command{ls}
+command:
+
+@example
+(use-modules (guix gexp) (gnu packages base))
+
+(gexp->script "list-files"
+              #~(execl (string-append #$coreutils "/bin/ls")
+                       "ls"))
+@end example
+
+When ``running'' it through the store (@pxref{The Store Monad,
+@code{run-with-store}}), we obtain a derivation that procedures an
+executable file @file{/gnu/store/@dots{}-list-files} along these lines:
+
+@example
+#!/gnu/store/@dots{}-guile-2.0.11/bin/guile -ds
+!#
+(execl (string-append "/gnu/store/@dots{}-coreutils-8.22"/bin/ls")
+       "ls")
+@end example
+@end deffn
+
+@deffn {Monadic Procedure} gexp->file @var{name} @var{exp}
+Return a derivation that builds a file @var{name} containing @var{exp}.
+
+The resulting file holds references to all the dependencies of @var{exp}
+or a subset thereof.
+@end deffn
+
+Of course, in addition to gexps embedded in ``host'' code, there are
+also modules containing build tools.  To make it clear that they are
+meant to be used in the build stratum, these modules are kept in the
+@code{(guix build @dots{})} name space.
+
+
 @c *********************************************************************
 @node Utilities
 @chapter Utilities

guix/gexp.scm

@@ -0,0 +1,391 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix gexp)
+  #:use-module ((guix store)
+                #:select (direct-store-path?))
+  #:use-module (guix monads)
+  #:use-module ((guix derivations)
+                #:select (derivation? derivation->output-path
+                          %guile-for-build derivation))
+  #:use-module (guix packages)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-26)
+  #:use-module (ice-9 match)
+  #:export (gexp
+            gexp?
+            gexp->derivation
+            gexp->file
+            gexp->script))
+
+;;; Commentary:
+;;;
+;;; This module implements "G-expressions", or "gexps".  Gexps are like
+;;; S-expressions (sexps), with two differences:
+;;;
+;;;   1. References (un-quotations) to derivations or packages in a gexp are
+;;;      replaced by the corresponding output file name;
+;;;
+;;;   2. Gexps embed information about the derivations they refer to.
+;;;
+;;; Gexps make it easy to write to files Scheme code that refers to store
+;;; items, or to write Scheme code to build derivations.
+;;;
+;;; Code:
+
+;; "G expressions".
+(define-record-type <gexp>
+  (make-gexp references proc)
+  gexp?
+  (references gexp-references)                    ; ((DRV-OR-PKG OUTPUT) ...)
+  (proc       gexp-proc))                         ; procedure
+
+;; Reference to one of the derivation's outputs, for gexps used in
+;; derivations.
+(define-record-type <output-ref>
+  (output-ref name)
+  output-ref?
+  (name output-ref-name))
+
+(define raw-derivation
+  (store-lift derivation))
+
+(define (lower-inputs* inputs)
+  "Turn any package from INPUTS into a derivation; return the corresponding
+input list as a monadic value."
+  ;; XXX: This is like 'lower-inputs' but without the "name" part in tuples.
+  (with-monad %store-monad
+    (sequence %store-monad
+              (map (match-lambda
+                    (((? package? package) sub-drv ...)
+                     (mlet %store-monad ((drv (package->derivation package)))
+                       (return `(,drv ,@sub-drv))))
+                    (input
+                     (return input)))
+                   inputs))))
+
+(define* (gexp->derivation name exp
+                           #:key
+                           (system (%current-system))
+                           hash hash-algo recursive?
+                           (env-vars '())
+                           (modules '())
+                           (guile-for-build (%guile-for-build))
+                           references-graphs
+                           local-build?)
+  "Return a derivation NAME that runs EXP (a gexp) with GUILE-FOR-BUILD (a
+derivation) on SYSTEM.
+
+Make MODULES available in the evaluation context of EXP; MODULES is a list of
+names of Guile modules from the current search path to be copied in the store,
+compiled, and made available in the load path during the execution of
+EXP---e.g., '((guix build utils) (guix build gnu-build-system)).
+
+The other arguments are as for 'derivation'."
+  (define %modules modules)
+  (define outputs (gexp-outputs exp))
+
+  (mlet* %store-monad ((inputs   (lower-inputs* (gexp-inputs exp)))
+                       (sexp     (gexp->sexp exp #:outputs outputs))
+                       (builder  (text-file (string-append name "-builder")
+                                            (object->string sexp)))
+                       (modules  (if (pair? %modules)
+                                     (imported-modules %modules
+                                                       #:system system
+                                                       #:guile guile-for-build)
+                                     (return #f)))
+                       (compiled (if (pair? %modules)
+                                     (compiled-modules %modules
+                                                       #:system system
+                                                       #:guile guile-for-build)
+                                     (return #f)))
+                       (guile    (if guile-for-build
+                                     (return guile-for-build)
+                                     (package->derivation
+                                      (@ (gnu packages base) guile-final)
+                                      system))))
+    (raw-derivation name
+                    (string-append (derivation->output-path guile)
+                                   "/bin/guile")
+                    `("--no-auto-compile"
+                      ,@(if (pair? %modules)
+                            `("-L" ,(derivation->output-path modules)
+                              "-C" ,(derivation->output-path compiled))
+                            '())
+                      ,builder)
+                    #:outputs outputs
+                    #:env-vars env-vars
+                    #:system system
+                    #:inputs `((,guile)
+                               (,builder)
+                               ,@(if modules
+                                     `((,modules) (,compiled) ,@inputs)
+                                     inputs))
+                    #:hash hash #:hash-algo hash-algo #:recursive? recursive?
+                    #:references-graphs references-graphs
+                    #:local-build? local-build?)))
+
+(define (gexp-inputs exp)
+  "Return the input list for EXP."
+  (define (add-reference-inputs ref result)
+    (match ref
+      (((? derivation?) (? string?))
+       (cons ref result))
+      (((? package?) (? string?))
+       (cons ref result))
+      ((? gexp? exp)
+       (append (gexp-inputs exp) result))
+      (((? string? file))
+       (if (direct-store-path? file)
+           (cons ref result)
+           result))
+      ((refs ...)
+       (fold-right add-reference-inputs result refs))
+      (_
+       ;; Ignore references to other kinds of objects.
+       result)))
+
+  (fold-right add-reference-inputs
+              '()
+              (gexp-references exp)))
+
+(define (gexp-outputs exp)
+  "Return the outputs referred to by EXP as a list of strings."
+  (define (add-reference-output ref result)
+    (match ref
+      (($ <output-ref> name)
+       (cons name result))
+      ((? gexp? exp)
+       (append (gexp-outputs exp) result))
+      (_
+       result)))
+
+  (fold-right add-reference-output
+              '()
+              (gexp-references exp)))
+
+(define* (gexp->sexp exp #:key (outputs '()))
+  "Return (monadically) the sexp corresponding to EXP for the given OUTPUT,
+and in the current monad setting (system type, etc.)"
+  (define (reference->sexp ref)
+    (with-monad %store-monad
+      (match ref
+        (((? derivation? drv) (? string? output))
+         (return (derivation->output-path drv output)))
+        (((? package? p) (? string? output))
+         (package-file p #:output output))
+        (($ <output-ref> output)
+         (match (member output outputs)
+           (#f
+            (error "no such output" output))
+           (_
+            (return `((@ (guile) getenv) ,output)))))
+        ((? gexp? exp)
+         (gexp->sexp exp #:outputs outputs))
+        (((? string? str))
+         (return (if (direct-store-path? str) str ref)))
+        ((refs ...)
+         (sequence %store-monad (map reference->sexp refs)))
+        (x
+         (return x)))))
+
+  (mlet %store-monad
+      ((args (sequence %store-monad
+                       (map reference->sexp (gexp-references exp)))))
+    (return (apply (gexp-proc exp) args))))
+
+(define (canonicalize-reference ref)
+  "Return a canonical variant of REF, which adds any missing output part in
+package/derivation references."
+  (match ref
+    ((? package? p)
+     `(,p "out"))
+    ((? derivation? d)
+     `(,d "out"))
+    (((? package?) (? string?))
+     ref)
+    (((? derivation?) (? string?))
+     ref)
+    ((? string? s)
+     (if (direct-store-path? s) `(,s) s))
+    ((refs ...)
+     (map canonicalize-reference refs))
+    (x x)))
+
+(define (syntax-location-string s)
+  "Return a string representing the source code location of S."
+  (let ((props (syntax-source s)))
+    (if props
+        (let ((file   (assoc-ref props 'filename))
+              (line   (and=> (assoc-ref props 'line) 1+))
+              (column (assoc-ref props 'column)))
+          (if file
+              (simple-format #f "~a:~a:~a"
+                             file line column)
+              (simple-format #f "~a:~a" line column)))
+        "<unknown location>")))
+
+(define-syntax gexp
+  (lambda (s)
+    (define (collect-escapes exp)
+      ;; Return all the 'ungexp' present in EXP.
+      (let loop ((exp    exp)
+                 (result '()))
+        (syntax-case exp (ungexp ungexp-splicing)
+          ((ungexp _)
+           (cons exp result))
+          ((ungexp _ _)
+           (cons exp result))
+          ((ungexp-splicing _ ...)
+           (cons exp result))
+          ((exp0 exp ...)
+           (let ((result (loop #'exp0 result)))
+             (fold loop result #'(exp ...))))
+          (_
+           result))))
+
+    (define (escape->ref exp)
+      ;; Turn 'ungexp' form EXP into a "reference".
+      (syntax-case exp (ungexp ungexp-splicing output)
+        ((ungexp output)
+         #'(output-ref "out"))
+        ((ungexp output name)
+         #'(output-ref name))
+        ((ungexp thing)
+         #'thing)
+        ((ungexp drv-or-pkg out)
+         #'(list drv-or-pkg out))
+        ((ungexp-splicing lst)
+         #'lst)))
+
+    (define (substitute-references exp substs)
+      ;; Return a variant of EXP where all the cars of SUBSTS have been
+      ;; replaced by the corresponding cdr.
+      (syntax-case exp (ungexp ungexp-splicing)
+        ((ungexp _ ...)
+         (match (assoc exp substs)
+           ((_ id)
+            id)
+           (_
+            #'(syntax-error "error: no 'ungexp' substitution"
+                            #'ref))))
+        (((ungexp-splicing _ ...) rest ...)
+         (syntax-case exp ()
+           ((exp rest ...)
+            (match (assoc #'exp substs)
+              ((_ id)
+               (with-syntax ((id id))
+                 #`(append id
+                           #,(substitute-references #'(rest ...) substs))))
+              (_
+               #'(syntax-error "error: no 'ungexp-splicing' substitution"
+                               #'ref))))))
+        ((exp0 exp ...)
+         #`(cons #,(substitute-references #'exp0 substs)
+                 #,(substitute-references #'(exp ...) substs)))
+        (x #''x)))
+
+    (syntax-case s (ungexp output)
+      ((_ exp)
+       (let* ((escapes (delete-duplicates (collect-escapes #'exp)))
+              (formals (generate-temporaries escapes))
+              (sexp    (substitute-references #'exp (zip escapes formals)))
+              (refs    (map escape->ref escapes)))
+         #`(make-gexp (map canonicalize-reference (list #,@refs))
+                      (lambda #,formals
+                        #,sexp)))))))
+
+
+;;;
+;;; Convenience procedures.
+;;;
+
+(define* (gexp->script name exp
+                       #:key (modules '())
+                       (guile (@ (gnu packages base) guile-final)))
+  "Return an executable script NAME that runs EXP using GUILE with MODULES in
+its search path."
+  (mlet %store-monad ((modules  (imported-modules modules))
+                      (compiled (compiled-modules modules)))
+    (gexp->derivation name
+                      (gexp
+                       (call-with-output-file (ungexp output)
+                         (lambda (port)
+                           (format port
+                                   "#!~a/bin/guile --no-auto-compile~%!#~%"
+                                   (ungexp guile))
+                           (write
+                            '(set! %load-path
+                                   (cons (ungexp modules) %load-path))
+                            port)
+                           (write
+                            '(set! %load-compiled-path
+                                   (cons (ungexp compiled)
+                                         %load-compiled-path))
+                            port)
+                           (write '(ungexp exp) port)
+                           (chmod port #o555)))))))
+
+(define (gexp->file name exp)
+  "Return a derivation that builds a file NAME containing EXP."
+  (gexp->derivation name
+                    (gexp
+                     (call-with-output-file (ungexp output)
+                       (lambda (port)
+                         (write '(ungexp exp) port))))))
+
+
+
+;;;
+;;; Syntactic sugar.
+;;;
+
+(eval-when (expand load eval)
+  (define (read-ungexp chr port)
+    "Read an 'ungexp' or 'ungexp-splicing' form from PORT."
+    (define unquote-symbol
+      (match (peek-char port)
+        (#\@
+         (read-char port)
+         'ungexp-splicing)
+        (_
+         'ungexp)))
+
+    (match (read port)
+      ((? symbol? symbol)
+       (let ((str (symbol->string symbol)))
+         (match (string-index-right str #\:)
+           (#f
+            `(,unquote-symbol ,symbol))
+           (colon
+            (let ((name   (string->symbol (substring str 0 colon)))
+                  (output (substring str (+ colon 1))))
+              `(,unquote-symbol ,name ,output))))))
+      (x
+       `(,unquote-symbol ,x))))
+
+  (define (read-gexp chr port)
+    "Read a 'gexp' form from PORT."
+    `(gexp ,(read port)))
+
+  ;; Extend the reader
+  (read-hash-extend #\~ read-gexp)
+  (read-hash-extend #\$ read-ungexp))
+
+;;; gexp.scm ends here

tests/gexp.scm

@@ -0,0 +1,234 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (test-gexp)
+  #:use-module (guix store)
+  #:use-module (guix monads)
+  #:use-module (guix gexp)
+  #:use-module (guix derivations)
+  #:use-module ((guix packages)
+                #:select (package-derivation %current-system))
+  #:use-module (gnu packages)
+  #:use-module (gnu packages base)
+  #:use-module (gnu packages bootstrap)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-64)
+  #:use-module (rnrs io ports)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 popen))
+
+;; Test the (guix gexp) module.
+
+(define %store
+  (open-connection))
+
+;; For white-box testing.
+(define gexp-inputs (@@ (guix gexp) gexp-inputs))
+(define gexp->sexp  (@@ (guix gexp) gexp->sexp))
+
+(define guile-for-build
+  (package-derivation %store %bootstrap-guile))
+
+;; Make it the default.
+(%guile-for-build guile-for-build)
+
+(define (gexp->sexp* exp)
+  (run-with-store %store (gexp->sexp exp)
+                  #:guile-for-build guile-for-build))
+
+(define-syntax-rule (test-assertm name exp)
+  (test-assert name
+    (run-with-store %store exp
+                    #:guile-for-build guile-for-build)))
+
+
+(test-begin "gexp")
+
+(test-equal "no refs"
+  '(display "hello!")
+  (let ((exp (gexp (display "hello!"))))
+    (and (gexp? exp)
+         (null? (gexp-inputs exp))
+         (gexp->sexp* exp))))
+
+(test-equal "unquote"
+  '(display `(foo ,(+ 2 3)))
+  (let ((exp (gexp (display `(foo ,(+ 2 3))))))
+    (and (gexp? exp)
+         (null? (gexp-inputs exp))
+         (gexp->sexp* exp))))
+
+(test-assert "one input package"
+  (let ((exp (gexp (display (ungexp coreutils)))))
+    (and (gexp? exp)
+         (match (gexp-inputs exp)
+           (((p "out"))
+            (eq? p coreutils)))
+         (equal? `(display ,(derivation->output-path
+                             (package-derivation %store coreutils)))
+                 (gexp->sexp* exp)))))
+
+(test-assert "same input twice"
+  (let ((exp (gexp (begin
+                     (display (ungexp coreutils))
+                     (display (ungexp coreutils))))))
+    (and (gexp? exp)
+         (match (gexp-inputs exp)
+           (((p "out"))
+            (eq? p coreutils)))
+         (let ((e `(display ,(derivation->output-path
+                              (package-derivation %store coreutils)))))
+           (equal? `(begin ,e ,e) (gexp->sexp* exp))))))
+
+(test-assert "two input packages, one derivation, one file"
+  (let* ((drv (build-expression->derivation
+               %store "foo" 'bar
+               #:guile-for-build (package-derivation %store %bootstrap-guile)))
+         (txt (add-text-to-store %store "foo" "Hello, world!"))
+         (exp (gexp (begin
+                      (display (ungexp coreutils))
+                      (display (ungexp %bootstrap-guile))
+                      (display (ungexp drv))
+                      (display (ungexp txt))))))
+    (define (match-input thing)
+      (match-lambda
+       ((drv-or-pkg _ ...)
+        (eq? thing drv-or-pkg))))
+
+    (and (gexp? exp)
+         (= 4 (length (gexp-inputs exp)))
+         (every (lambda (input)
+                  (find (match-input input) (gexp-inputs exp)))
+                (list drv coreutils %bootstrap-guile txt))
+         (let ((e0 `(display ,(derivation->output-path
+                               (package-derivation %store coreutils))))
+               (e1 `(display ,(derivation->output-path
+                               (package-derivation %store %bootstrap-guile))))
+               (e2 `(display ,(derivation->output-path drv)))
+               (e3 `(display ,txt)))
+           (equal? `(begin ,e0 ,e1 ,e2 ,e3) (gexp->sexp* exp))))))
+
+(test-assert "input list"
+  (let ((exp   (gexp (display
+                      '(ungexp (list %bootstrap-guile coreutils)))))
+        (guile (derivation->output-path
+                (package-derivation %store %bootstrap-guile)))
+        (cu    (derivation->output-path
+                (package-derivation %store coreutils))))
+    (and (lset= equal?
+                `((,%bootstrap-guile "out") (,coreutils "out"))
+                (gexp-inputs exp))
+         (equal? `(display '(,guile ,cu))
+                 (gexp->sexp* exp)))))
+
+(test-assert "input list splicing"
+  (let* ((inputs  (list (list glibc "debug") %bootstrap-guile))
+         (outputs (list (derivation->output-path
+                         (package-derivation %store glibc)
+                         "debug")
+                        (derivation->output-path
+                         (package-derivation %store %bootstrap-guile))))
+         (exp     (gexp (list (ungexp-splicing (cons (+ 2 3) inputs))))))
+    (and (lset= equal?
+                `((,glibc "debug") (,%bootstrap-guile "out"))
+                (gexp-inputs exp))
+         (equal? (gexp->sexp* exp)
+                 `(list ,@(cons 5 outputs))))))
+
+(test-assertm "gexp->file"
+  (mlet* %store-monad ((exp -> (gexp (display (ungexp %bootstrap-guile))))
+                       (guile  (package-file %bootstrap-guile))
+                       (sexp   (gexp->sexp exp))
+                       (drv    (gexp->file "foo" exp))
+                       (out -> (derivation->output-path drv))
+                       (done   (built-derivations (list drv)))
+                       (refs   ((store-lift references) out)))
+    (return (and (equal? sexp (call-with-input-file out read))
+                 (equal? (list guile) refs)))))
+
+(test-assertm "gexp->derivation"
+  (mlet* %store-monad ((file    (text-file "foo" "Hello, world!"))
+                       (exp ->  (gexp
+                                 (begin
+                                   (mkdir (ungexp output))
+                                   (chdir (ungexp output))
+                                   (symlink
+                                    (string-append (ungexp %bootstrap-guile)
+                                                   "/bin/guile")
+                                    "foo")
+                                   (symlink (ungexp file)
+                                            (ungexp output "2nd")))))
+                       (drv     (gexp->derivation "foo" exp))
+                       (out ->  (derivation->output-path drv))
+                       (out2 -> (derivation->output-path drv "2nd"))
+                       (done    (built-derivations (list drv)))
+                       (refs    ((store-lift references) out))
+                       (refs2   ((store-lift references) out2))
+                       (guile   (package-file %bootstrap-guile "bin/guile")))
+    (return (and (string=? (readlink (string-append out "/foo")) guile)
+                 (string=? (readlink out2) file)
+                 (equal? refs (list (dirname (dirname guile))))
+                 (equal? refs2 (list file))))))
+
+(test-assertm "gexp->derivation, composed gexps"
+  (mlet* %store-monad ((exp0 -> (gexp (begin
+                                        (mkdir (ungexp output))
+                                        (chdir (ungexp output)))))
+                       (exp1 -> (gexp (symlink
+                                       (string-append (ungexp %bootstrap-guile)
+                                                      "/bin/guile")
+                                       "foo")))
+                       (exp  -> (gexp (begin (ungexp exp0) (ungexp exp1))))
+                       (drv     (gexp->derivation "foo" exp))
+                       (out ->  (derivation->output-path drv))
+                       (done    (built-derivations (list drv)))
+                       (guile   (package-file %bootstrap-guile "bin/guile")))
+    (return (string=? (readlink (string-append out "/foo"))
+                      guile))))
+
+(test-assertm "gexp->script"
+  (mlet* %store-monad ((n ->   (random (expt 2 50)))
+                       (exp -> (gexp
+                                (system*
+                                 (string-append (ungexp %bootstrap-guile)
+                                                "/bin/guile")
+                                 "-c" (object->string
+                                       '(display (expt (ungexp n) 2))))))
+                       (drv    (gexp->script "guile-thing" exp
+                                             #:guile %bootstrap-guile))
+                       (out -> (derivation->output-path drv))
+                       (done   (built-derivations (list drv))))
+    (let* ((pipe  (open-input-pipe out))
+           (str   (get-string-all pipe)))
+      (return (and (zero? (close-pipe pipe))
+                   (= (expt n 2) (string->number str)))))))
+
+(test-equal "sugar"
+  '(gexp (foo (ungexp bar) (ungexp baz "out")
+              (ungexp (chbouib 42))
+              (ungexp-splicing (list x y z))))
+  '#~(foo #$bar #$baz:out #$(chbouib 42) #$@(list x y z)))
+
+(test-end "gexp")
+
+
+(exit (= (test-runner-fail-count (test-runner-current)) 0))
+
+;; Local Variables:
+;; eval: (put 'test-assertm 'scheme-indent-function 1)
+;; End: