Add (guix monads).

* guix/monads.scm: New file.
* tests/monads.scm: New file.
* Makefile.am (MODULES): Add guix/monads.scm.
  (SCM_TESTS): Add tests/monads.scm.
* doc/guix.texi (The Store Monad): New node.
  (The Store): Reference it.

tests/monads.scm

@@ -0,0 +1,163 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2013 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-monads)
+  #:use-module (guix store)
+  #:use-module (guix monads)
+  #:use-module (guix derivations)
+  #:use-module ((guix packages)
+                #:select (package-derivation %current-system))
+  #:use-module (gnu packages)
+  #:use-module (gnu packages bootstrap)
+  #:use-module (ice-9 match)
+  #:use-module (rnrs io ports)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-64))
+
+;; Test the (guix store) module.
+
+(define %store
+  (open-connection))
+
+;; Make sure we build everything by ourselves.
+(set-build-options %store #:use-substitutes? #f)
+
+(define %monads
+  (list %identity-monad %store-monad))
+
+(define %monad-run
+  (list identity
+        (cut run-with-store %store <>)))
+
+
+(test-begin "monads")
+
+;; The 3 "monad laws": <http://www.haskell.org/haskellwiki/Monad_laws>.
+
+(test-assert "left identity"
+  (every (lambda (monad run)
+           (let ((number (random 777)))
+             (with-monad monad
+               (define (f x)
+                 (return (* (1+ number) 2)))
+
+               (= (run (>>= (return number) f))
+                  (run (f number))))))
+         %monads
+         %monad-run))
+
+(test-assert "right identity"
+  (every (lambda (monad run)
+           (with-monad monad
+             (let ((number (return (random 777))))
+               (= (run (>>= number return))
+                  (run number)))))
+         %monads
+         %monad-run))
+
+(test-assert "associativity"
+  (every (lambda (monad run)
+           (with-monad monad
+             (define (f x)
+               (return (+ 1 x)))
+             (define (g x)
+               (return (* 2 x)))
+
+             (let ((number (return (random 777))))
+               (= (run (>>= (>>= number f) g))
+                  (run (>>= number (lambda (x) (>>= (f x) g))))))))
+         %monads
+         %monad-run))
+
+(test-assert "lift"
+  (every (lambda (monad run)
+           (let ((f (lift1 1+ monad)))
+             (with-monad monad
+               (let ((number (random 777)))
+                 (= (run (>>= (return number) f))
+                    (1+ number))))))
+         %monads
+         %monad-run))
+
+(test-assert "mlet* + text-file + package-file"
+  (run-with-store %store
+    (mlet* %store-monad ((guile (package-file %bootstrap-guile "bin/guile"))
+                         (file  (text-file "monadic" guile)))
+      (return (equal? (call-with-input-file file get-string-all)
+                      guile)))
+    #:guile-for-build (package-derivation %store %bootstrap-guile)))
+
+(test-assert "mlet* + derivation-expression"
+  (run-with-store %store
+    (mlet* %store-monad ((guile  (package-file %bootstrap-guile "bin/guile"))
+                         (gdrv   (package->derivation %bootstrap-guile))
+                         (exp -> `(let ((out (assoc-ref %outputs "out")))
+                                    (mkdir out)
+                                    (symlink ,guile
+                                             (string-append out "/guile-rocks"))))
+                         (drv    (derivation-expression "rocks" (%current-system)
+                                                        exp `(("g" ,gdrv))))
+                         (out -> (derivation->output-path drv))
+                         (built? (built-derivations (list drv))))
+      (return (and built?
+                   (equal? guile
+                           (readlink (string-append out "/guile-rocks"))))))
+    #:guile-for-build (package-derivation %store %bootstrap-guile)))
+
+(test-assert "mapm"
+  (every (lambda (monad run)
+           (with-monad monad
+             (equal? (run (mapm monad (lift1 1+ monad) (map return (iota 10))))
+                     (map 1+ (iota 10)))))
+         %monads
+         %monad-run))
+
+(test-assert "sequence"
+  (every (lambda (monad run)
+           (let* ((input (iota 100))
+                  (order '()))
+             (define (frob i)
+               ;; The side effect here is used to keep track of the order in
+               ;; which monadic values are bound.
+               (set! order (cons i order))
+               i)
+
+             (and (equal? input
+                          (run (sequence monad
+                                         (map (lift1 frob monad) input))))
+
+                  ;; Make sure this is from left to right.
+                  (equal? order (reverse input)))))
+         %monads
+         %monad-run))
+
+(test-assert "listm"
+  (every (lambda (monad run)
+           (run (with-monad monad
+                  (let ((lst (listm monad
+                                    (return 1) (return 2) (return 3))))
+                    (mlet monad ((lst lst))
+                      (return (equal? '(1 2 3) lst)))))))
+         %monads
+         %monad-run))
+
+(test-end "monads")
+
+
+(exit (= (test-runner-fail-count (test-runner-current)) 0))