monads: Move '%store-monad' and related procedures where they belong.

This turns (guix monads) into a generic module for monads, and moves the
store monad and related monadic procedures in their corresponding
module.

* guix/monads.scm (store-return, store-bind, %store-monad, store-lift,
  text-file, interned-file, package-file, package->derivation,
  package->cross-derivation, origin->derivation, imported-modules,
  compiled, modules, built-derivations, run-with-store): Move to...
* guix/store.scm (store-return, store-bind, %store-monad, store-lift,
  text-file, interned-file): ... here.
  (%guile-for-build): New variable.
  (run-with-store): Moved from monads.scm.  Remove default value for
  #:guile-for-build.
* guix/packages.scm (default-guile): Export.
  (set-guile-for-build): New procedure.
  (package-file, package->derivation, package->cross-derivation,
  origin->derivation): Moved from monads.scm.
* guix/derivations.scm (%guile-for-build): Remove.
  (imported-modules): Rename to...
  (%imported-modules): ... this.
  (compiled-modules): Rename to...
  (%compiled-modules): ... this.
  (built-derivations, imported-modules, compiled-modules): New
  procedures.
* gnu/services/avahi.scm, gnu/services/base.scm, gnu/services/dbus.scm,
  gnu/services/dmd.scm, gnu/services/networking.scm,
  gnu/services/ssh.scm, gnu/services/xorg.scm, gnu/system/install.scm,
  gnu/system/linux-initrd.scm, gnu/system/shadow.scm, guix/download.scm,
  guix/gexp.scm, guix/git-download.scm, guix/profiles.scm,
  guix/svn-download.scm, tests/monads.scm: Adjust imports accordingly.
* guix/monad-repl.scm (default-guile-derivation): New procedure.
  (store-monad-language, run-in-store): Use it.
* build-aux/hydra/gnu-system.scm (qemu-jobs): Add explicit
  'set-guile-for-build' call.
* guix/scripts/archive.scm (derivation-from-expression): Likewise.
* guix/scripts/build.scm (options/resolve-packages): Likewise.
* guix/scripts/environment.scm (guix-environment): Likewise.
* guix/scripts/system.scm (guix-system): Likewise.
* doc/guix.texi (The Store Monad): Adjust module names accordingly.

14 files changed, 245 insertions, 203 deletions

diff --git a/guix/derivations.scm b/guix/derivations.scm
index b48e7e604d..4c34fcb4b8 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -28,6 +28,7 @@
   #:use-module (ice-9 vlist)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (guix monads)
   #:use-module (guix hash)
   #:use-module (guix base32)
   #:use-module (guix records)
@@ -84,11 +85,16 @@
 
             map-derivation
 
-            %guile-for-build
+            built-derivations
             imported-modules
             compiled-modules
+
             build-expression->derivation
             imported-files)
+
+  ;; Re-export it from here for backward compatibility.
+  #:re-export (%guile-for-build)
+
   #:replace (build-derivations))
 
 ;;;
@@ -895,11 +901,6 @@ recursively."
 ;;; Guile-based builders.
 ;;;
 
-(define %guile-for-build
-  ;; The derivation of the Guile to be used within the build environment,
-  ;; when using `build-expression->derivation'.
-  (make-parameter #f))
-
 (define (parent-directories file-name)
   "Return the list of parent dirs of FILE-NAME, in the order in which an
 `mkdir -p' implementation would make them."
@@ -956,11 +957,11 @@ system, imported, and appears under FINAL-PATH in the resulting store path."
   ;; up looking for the same files over and over again.
   (memoize search-path))
 
-(define* (imported-modules store modules
-                           #:key (name "module-import")
-                           (system (%current-system))
-                           (guile (%guile-for-build))
-                           (module-path %load-path))
+(define* (%imported-modules store modules
+                            #:key (name "module-import")
+                            (system (%current-system))
+                            (guile (%guile-for-build))
+                            (module-path %load-path))
   "Return a derivation that contains the source files of MODULES, a list of
 module names such as `(ice-9 q)'.  All of MODULES must be in the MODULE-PATH
 search path."
@@ -975,18 +976,18 @@ search path."
     (imported-files store files #:name name #:system system
                     #:guile guile)))
 
-(define* (compiled-modules store modules
-                           #:key (name "module-import-compiled")
-                           (system (%current-system))
-                           (guile (%guile-for-build))
-                           (module-path %load-path))
+(define* (%compiled-modules store modules
+                            #:key (name "module-import-compiled")
+                            (system (%current-system))
+                            (guile (%guile-for-build))
+                            (module-path %load-path))
   "Return a derivation that builds a tree containing the `.go' files
 corresponding to MODULES.  All the MODULES are built in a context where
 they can refer to each other."
-  (let* ((module-drv (imported-modules store modules
-                                       #:system system
-                                       #:guile guile
-                                       #:module-path module-path))
+  (let* ((module-drv (%imported-modules store modules
+                                        #:system system
+                                        #:guile guile
+                                        #:module-path module-path))
          (module-dir (derivation->output-path module-drv))
          (files      (map (lambda (m)
                             (let ((f (string-join (map symbol->string m)
@@ -1218,15 +1219,15 @@ ALLOWED-REFERENCES, and LOCAL-BUILD?."
                                       (filter-map source-path inputs)))
 
          (mod-drv  (and (pair? modules)
-                        (imported-modules store modules
-                                          #:guile guile-drv
-                                          #:system system)))
+                        (%imported-modules store modules
+                                           #:guile guile-drv
+                                           #:system system)))
          (mod-dir  (and mod-drv
                         (derivation->output-path mod-drv)))
          (go-drv   (and (pair? modules)
-                        (compiled-modules store modules
-                                          #:guile guile-drv
-                                          #:system system)))
+                        (%compiled-modules store modules
+                                           #:guile guile-drv
+                                           #:system system)))
          (go-dir   (and go-drv
                         (derivation->output-path go-drv))))
     (derivation store name guile
@@ -1255,3 +1256,17 @@ ALLOWED-REFERENCES, and LOCAL-BUILD?."
                 #:references-graphs references-graphs
                 #:allowed-references allowed-references
                 #:local-build? local-build?)))
+
+
+;;;
+;;; Monadic interface.
+;;;
+
+(define built-derivations
+  (store-lift build-derivations))
+
+(define imported-modules
+  (store-lift %imported-modules))
+
+(define compiled-modules
+  (store-lift %compiled-modules))
diff --git a/guix/download.scm b/guix/download.scm
index 4c111dd2b5..035d604aa7 100644
--- a/guix/download.scm
+++ b/guix/download.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013, 2014 Andreas Enge <andreas@enge.fr>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -21,7 +21,7 @@
   #:use-module (ice-9 match)
   #:use-module (guix derivations)
   #:use-module (guix packages)
-  #:use-module ((guix store) #:select (derivation-path? add-to-store))
+  #:use-module (guix store)
   #:use-module ((guix build download) #:prefix build:)
   #:use-module (guix monads)
   #:use-module (guix gexp)
diff --git a/guix/gexp.scm b/guix/gexp.scm
index d13e1c46da..4e8f91df1d 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -17,12 +17,9 @@
 ;;; 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 store)
   #:use-module (guix monads)
-  #:use-module ((guix derivations)
-                #:select (derivation? derivation->output-path
-                          %guile-for-build derivation))
+  #:use-module (guix derivations)
   #:use-module (guix packages)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
diff --git a/guix/git-download.scm b/guix/git-download.scm
index 94b118a7b9..490d8c319a 100644
--- a/guix/git-download.scm
+++ b/guix/git-download.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,6 +18,7 @@
 
 (define-module (guix git-download)
   #:use-module (guix gexp)
+  #:use-module (guix store)
   #:use-module (guix monads)
   #:use-module (guix records)
   #:use-module (guix packages)
diff --git a/guix/monad-repl.scm b/guix/monad-repl.scm
index 5242f5448b..ebd9151065 100644
--- a/guix/monad-repl.scm
+++ b/guix/monad-repl.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,6 +19,8 @@
 (define-module (guix monad-repl)
   #:use-module (guix store)
   #:use-module (guix monads)
+  #:use-module (guix utils)
+  #:use-module (guix packages)
   #:use-module (ice-9 pretty-print)
   #:use-module (system repl repl)
   #:use-module (system repl common)
@@ -54,20 +56,30 @@
                    #:make-default-environment
                    (language-make-default-environment scheme))))
 
+(define* (default-guile-derivation store #:optional (system (%current-system)))
+  "Return the derivation of the default "
+  (package-derivation store (default-guile) system))
+
 (define (store-monad-language)
   "Return a compiler language for the store monad."
-  (let ((store (open-connection)))
+  (let* ((store (open-connection))
+         (guile (or (%guile-for-build)
+                    (default-guile-derivation store))))
     (monad-language %store-monad
-                    (cut run-with-store store <>)
+                    (cut run-with-store store <>
+                         #:guile-for-build guile)
                     'store-monad)))
 
 (define-meta-command ((run-in-store guix) repl (form))
   "run-in-store EXP
 Run EXP through the store monad."
-  (let ((value (with-store store
-                 (run-with-store store (repl-eval repl form)))))
-    (run-hook before-print-hook value)
-    (pretty-print value)))
+  (with-store store
+    (let* ((guile (or (%guile-for-build)
+                      (default-guile-derivation store)))
+           (value (run-with-store store (repl-eval repl form)
+                                  #:guile-for-build guile)))
+      (run-hook before-print-hook value)
+      (pretty-print value))))
 
 (define-meta-command ((enter-store-monad guix) repl)
   "enter-store-monad
diff --git a/guix/monads.scm b/guix/monads.scm
index 20fee79602..7fec3d5168 100644
--- a/guix/monads.scm
+++ b/guix/monads.scm
@@ -17,9 +17,6 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix monads)
-  #:use-module (guix store)
-  #:use-module (guix derivations)
-  #:use-module (guix packages)
   #:use-module ((system syntax)
                 #:select (syntax-local-binding))
   #:use-module (ice-9 match)
@@ -49,22 +46,7 @@
             anym
 
             ;; Concrete monads.
-            %identity-monad
-
-            %store-monad
-            store-bind
-            store-return
-            store-lift
-            run-with-store
-            text-file
-            interned-file
-            package-file
-            origin->derivation
-            package->derivation
-            package->cross-derivation
-            built-derivations)
-  #:replace (imported-modules
-             compiled-modules))
+            %identity-monad))
 
 ;;; Commentary:
 ;;;
@@ -309,121 +291,4 @@ lifted in MONAD, for which PROC returns true."
   (bind   identity-bind)
   (return identity-return))
 
-
-;;;
-;;; Store monad.
-;;;
-
-;; return:: a -> StoreM a
-(define-inlinable (store-return value)
-  "Return VALUE from a monadic function."
-  ;; The monadic value is just this.
-  (lambda (store)
-    value))
-
-;; >>=:: StoreM a -> (a -> StoreM b) -> StoreM b
-(define-inlinable (store-bind mvalue mproc)
-  "Bind MVALUE in MPROC."
-  (lambda (store)
-    (let* ((value   (mvalue store))
-           (mresult (mproc value)))
-      (mresult store))))
-
-(define-monad %store-monad
-  (bind   store-bind)
-  (return store-return))
-
-
-(define (store-lift proc)
-  "Lift PROC, a procedure whose first argument is a connection to the store,
-in the store monad."
-  (define result
-    (lambda args
-      (lambda (store)
-        (apply proc store args))))
-
-  (set-object-property! result 'documentation
-                        (procedure-property proc 'documentation))
-  result)
-
-;;;
-;;; Store monad operators.
-;;;
-
-(define* (text-file name text)
-  "Return as a monadic value the absolute file name in the store of the file
-containing TEXT, a string."
-  (lambda (store)
-    (add-text-to-store store name text '())))
-
-(define* (interned-file file #:optional name
-                        #:key (recursive? #t))
-  "Return the name of FILE once interned in the store.  Use NAME as its store
-name, or the basename of FILE if NAME is omitted.
-
-When RECURSIVE? is true, the contents of FILE are added recursively; if FILE
-designates a flat file and RECURSIVE? is true, its contents are added, and its
-permission bits are kept."
-  (lambda (store)
-    (add-to-store store (or name (basename file))
-                  recursive? "sha256" file)))
-
-(define* (package-file package
-                       #:optional file
-                       #:key
-                       system (output "out") target)
-  "Return as a monadic value the absolute file name of FILE within the
-OUTPUT directory of PACKAGE.  When FILE is omitted, return the name of the
-OUTPUT directory of PACKAGE.  When TARGET is true, use it as a
-cross-compilation target triplet."
-  (lambda (store)
-    (define compute-derivation
-      (if target
-          (cut package-cross-derivation <> <> target <>)
-          package-derivation))
-
-    (let* ((system (or system (%current-system)))
-           (drv    (compute-derivation store package system))
-           (out    (derivation->output-path drv output)))
-      (if file
-          (string-append out "/" file)
-          out))))
-
-(define package->derivation
-  (store-lift package-derivation))
-
-(define package->cross-derivation
-  (store-lift package-cross-derivation))
-
-(define origin->derivation
-  (store-lift package-source-derivation))
-
-(define imported-modules
-  (store-lift (@ (guix derivations) imported-modules)))
-
-(define compiled-modules
-  (store-lift (@ (guix derivations) compiled-modules)))
-
-(define built-derivations
-  (store-lift build-derivations))
-
-(define* (run-with-store store mval
-                         #:key
-                         (guile-for-build (%guile-for-build))
-                         (system (%current-system)))
-  "Run MVAL, a monadic value in the store monad, in STORE, an open store
-connection."
-  (define (default-guile)
-    ;; Lazily resolve 'guile-final'.  This module must not refer to (gnu …)
-    ;; modules directly, to avoid circular dependencies, hence this hack.
-    (module-ref (resolve-interface '(gnu packages commencement))
-                'guile-final))
-
-  (parameterize ((%guile-for-build (or guile-for-build
-                                       (package-derivation store
-                                                           (default-guile)
-                                                           system)))
-                 (%current-system system))
-    (mval store)))
-
 ;;; monads.scm end here
diff --git a/guix/packages.scm b/guix/packages.scm
index 2a9a55e12f..909aa6d90d 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -21,6 +21,7 @@
   #:use-module (guix utils)
   #:use-module (guix records)
   #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix base32)
   #:use-module (guix derivations)
   #:use-module (guix build-system)
@@ -108,7 +109,15 @@
             bag-transitive-inputs
             bag-transitive-host-inputs
             bag-transitive-build-inputs
-            bag-transitive-target-inputs))
+            bag-transitive-target-inputs
+
+            default-guile
+
+            set-guile-for-build
+            package-file
+            package->derivation
+            package->cross-derivation
+            origin->derivation))
 
 ;;; Commentary:
 ;;;
@@ -317,7 +326,8 @@ corresponds to the arguments expected by `set-path-environment-variable'."
       ("patch" ,(ref '(gnu packages base) 'patch)))))
 
 (define (default-guile)
-  "Return the default Guile package for SYSTEM."
+  "Return the default Guile package used to run the build code of
+derivations."
   (let ((distro (resolve-interface '(gnu packages commencement))))
     (module-ref distro 'guile-final)))
 
@@ -899,3 +909,45 @@ symbolic output name, such as \"out\".  Note that this procedure calls
 `package-derivation', which is costly."
   (let ((drv (package-derivation store package system)))
     (derivation->output-path drv output)))
+
+
+;;;
+;;; Monadic interface.
+;;;
+
+(define (set-guile-for-build guile)
+  "This monadic procedure changes the Guile currently used to run the build
+code of derivations to GUILE, a package object."
+  (lambda (store)
+    (let ((guile (package-derivation store guile)))
+      (%guile-for-build guile))))
+
+(define* (package-file package
+                       #:optional file
+                       #:key
+                       system (output "out") target)
+  "Return as a monadic value the absolute file name of FILE within the
+OUTPUT directory of PACKAGE.  When FILE is omitted, return the name of the
+OUTPUT directory of PACKAGE.  When TARGET is true, use it as a
+cross-compilation target triplet."
+  (lambda (store)
+    (define compute-derivation
+      (if target
+          (cut package-cross-derivation <> <> target <>)
+          package-derivation))
+
+    (let* ((system (or system (%current-system)))
+           (drv    (compute-derivation store package system))
+           (out    (derivation->output-path drv output)))
+      (if file
+          (string-append out "/" file)
+          out))))
+
+(define package->derivation
+  (store-lift package-derivation))
+
+(define package->cross-derivation
+  (store-lift package-cross-derivation))
+
+(define origin->derivation
+  (store-lift package-source-derivation))
diff --git a/guix/profiles.scm b/guix/profiles.scm
index 44d7a314a3..921d001fa2 100644
--- a/guix/profiles.scm
+++ b/guix/profiles.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2014 Alex Kost <alezost@gmail.com>
 ;;;
@@ -25,6 +25,7 @@
   #:use-module (guix packages)
   #:use-module (guix gexp)
   #:use-module (guix monads)
+  #:use-module (guix store)
   #:use-module (ice-9 match)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 ftw)
diff --git a/guix/scripts/archive.scm b/guix/scripts/archive.scm
index 781ffc5f58..e265f82b52 100644
--- a/guix/scripts/archive.scm
+++ b/guix/scripts/archive.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -170,7 +170,10 @@ derivation of a package."
                       (package-name p))))
          (package-derivation store p system)))
     ((? procedure? proc)
-     (run-with-store store (proc) #:system system))))
+     (run-with-store store
+       (mbegin %store-monad
+         (set-guile-for-build (default-guile))
+         (proc)) #:system system))))
 
 (define (options->derivations+files store opts)
   "Given OPTS, the result of 'args-fold', return a list of derivations to
diff --git a/guix/scripts/build.scm b/guix/scripts/build.scm
index 26e9f42774..07ced30484 100644
--- a/guix/scripts/build.scm
+++ b/guix/scripts/build.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -347,12 +347,18 @@ packages."
            ((? package? p)
             `(argument . ,p))
            ((? procedure? proc)
-            (let ((drv (run-with-store store (proc) #:system system)))
+            (let ((drv (run-with-store store
+                         (mbegin %store-monad
+                           (set-guile-for-build (default-guile))
+                           (proc))
+                         #:system system)))
               `(argument . ,drv)))
            ((? gexp? gexp)
             (let ((drv (run-with-store store
-                         (gexp->derivation "gexp" gexp
-                                           #:system system))))
+                         (mbegin %store-monad
+                           (set-guile-for-build (default-guile))
+                           (gexp->derivation "gexp" gexp
+                                             #:system system)))))
               `(argument . ,drv)))))
         (opt opt))
        opts))
diff --git a/guix/scripts/environment.scm b/guix/scripts/environment.scm
index c388b0c52c..af196036d5 100644
--- a/guix/scripts/environment.scm
+++ b/guix/scripts/environment.scm
@@ -232,7 +232,10 @@ packages."
            (command (assoc-ref opts 'exec))
            (inputs (packages->transitive-inputs
                     (pick-all (options/resolve-packages opts) 'package)))
-           (drvs (run-with-store store (build-inputs inputs opts))))
+           (drvs (run-with-store store
+                   (mbegin %store-monad
+                     (set-guile-for-build (default-guile))
+                     (build-inputs inputs opts)))))
       (cond ((assoc-ref opts 'dry-run?)
              #t)
             ((assoc-ref opts 'search-paths)
diff --git a/guix/scripts/system.scm b/guix/scripts/system.scm
index 27404772b7..b0974dcfcd 100644
--- a/guix/scripts/system.scm
+++ b/guix/scripts/system.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -553,18 +553,20 @@ Build the operating system declared in FILE according to ACTION.\n"))
       (set-build-options-from-command-line store opts)
 
       (run-with-store store
-        (perform-action action os
-                        #:dry-run? dry?
-                        #:use-substitutes? (assoc-ref opts 'substitutes?)
-                        #:image-size (assoc-ref opts 'image-size)
-                        #:full-boot? (assoc-ref opts 'full-boot?)
-                        #:mappings (filter-map (match-lambda
-                                                (('file-system-mapping . m)
-                                                 m)
-                                                (_ #f))
-                                               opts)
-                        #:grub? grub?
-                        #:target target #:device device)
+        (mbegin %store-monad
+          (set-guile-for-build (default-guile))
+          (perform-action action os
+                          #:dry-run? dry?
+                          #:use-substitutes? (assoc-ref opts 'substitutes?)
+                          #:image-size (assoc-ref opts 'image-size)
+                          #:full-boot? (assoc-ref opts 'full-boot?)
+                          #:mappings (filter-map (match-lambda
+                                                  (('file-system-mapping . m)
+                                                   m)
+                                                  (_ #f))
+                                                 opts)
+                          #:grub? grub?
+                          #:target target #:device device))
         #:system system))))
 
 ;;; system.scm ends here
diff --git a/guix/store.scm b/guix/store.scm
index 571cc060d3..d3e94625a7 100644
--- a/guix/store.scm
+++ b/guix/store.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -20,6 +20,7 @@
   #:use-module (guix utils)
   #:use-module (guix config)
   #:use-module (guix serialization)
+  #:use-module (guix monads)
   #:autoload   (guix base32) (bytevector->base32-string)
   #:use-module (rnrs bytevectors)
   #:use-module (rnrs io ports)
@@ -94,6 +95,15 @@
 
             register-path
 
+            %store-monad
+            store-bind
+            store-return
+            store-lift
+            run-with-store
+            %guile-for-build
+            text-file
+            interned-file
+
             %store-prefix
             store-path?
             direct-store-path?
@@ -836,6 +846,80 @@ be used internally by the daemon's build hook."
 
 
 ;;;
+;;; Store monad.
+;;;
+
+;; return:: a -> StoreM a
+(define-inlinable (store-return value)
+  "Return VALUE from a monadic function."
+  ;; The monadic value is just this.
+  (lambda (store)
+    value))
+
+;; >>=:: StoreM a -> (a -> StoreM b) -> StoreM b
+(define-inlinable (store-bind mvalue mproc)
+  "Bind MVALUE in MPROC."
+  (lambda (store)
+    (let* ((value   (mvalue store))
+           (mresult (mproc value)))
+      (mresult store))))
+
+;; This is essentially a state monad
+(define-monad %store-monad
+  (bind   store-bind)
+  (return store-return))
+
+(define (store-lift proc)
+  "Lift PROC, a procedure whose first argument is a connection to the store,
+in the store monad."
+  (define result
+    (lambda args
+      (lambda (store)
+        (apply proc store args))))
+
+  (set-object-property! result 'documentation
+                        (procedure-property proc 'documentation))
+  result)
+
+;;
+;; Store monad operators.
+;;
+
+(define* (text-file name text)
+  "Return as a monadic value the absolute file name in the store of the file
+containing TEXT, a string."
+  (lambda (store)
+    (add-text-to-store store name text '())))
+
+(define* (interned-file file #:optional name
+                        #:key (recursive? #t))
+  "Return the name of FILE once interned in the store.  Use NAME as its store
+name, or the basename of FILE if NAME is omitted.
+
+When RECURSIVE? is true, the contents of FILE are added recursively; if FILE
+designates a flat file and RECURSIVE? is true, its contents are added, and its
+permission bits are kept."
+  (lambda (store)
+    (add-to-store store (or name (basename file))
+                  recursive? "sha256" file)))
+
+(define %guile-for-build
+  ;; The derivation of the Guile to be used within the build environment,
+  ;; when using 'gexp->derivation' and co.
+  (make-parameter #f))
+
+(define* (run-with-store store mval
+                         #:key
+                         (guile-for-build (%guile-for-build))
+                         (system (%current-system)))
+  "Run MVAL, a monadic value in the store monad, in STORE, an open store
+connection."
+  (parameterize ((%guile-for-build guile-for-build)
+                 (%current-system system))
+    (mval store)))
+
+
+;;;
 ;;; Store paths.
 ;;;
 
diff --git a/guix/svn-download.scm b/guix/svn-download.scm
index f06e449777..1c03bb9e76 100644
--- a/guix/svn-download.scm
+++ b/guix/svn-download.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014 Sree Harsha Totakura <sreeharsha@totakura.in>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -20,6 +20,7 @@
 (define-module (guix svn-download)
   #:use-module (guix records)
   #:use-module (guix gexp)
+  #:use-module (guix store)
   #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (ice-9 match)