gnu: Add guile-srfi-159.

* gnu/packages/guile-xyz.scm (guile-srfi-159): New variable.

gnu/packages/guile-xyz.scm

@@ -84,6 +84,7 @@ (define-module (gnu packages guile-xyz)
   #:use-module (guix packages)
   #:use-module (guix download)
   #:use-module (guix git-download)
+  #:use-module (guix hg-download)
   #:use-module (guix build-system gnu)
   #:use-module (guix build-system guile)
   #:use-module (guix utils)
@@ -2274,3 +2275,33 @@ (define-public guile-gi
 libraries, such as GTK+3.  Its README comes with the disclaimer: This is
 pre-alpha code.")
       (license license:gpl3+))))
+
+(define-public guile-srfi-159
+  (let ((commit "1bd98abda2ae4ef8f36761a167903e55c6bda7bb")
+        (revision "0"))
+    (package
+      (name "guile-srfi-159")
+      (version (git-version "0" revision commit))
+      (home-page "https://bitbucket.org/bjoli/guile-srfi-159")
+      (source (origin
+                (method hg-fetch)
+                (uri (hg-reference (changeset commit)
+                                   (url home-page)))
+                (sha256
+                 (base32
+                  "1zw6cmcy7xdbfiz3nz9arqnn7l2daidaps6ixkcrc9b6k51fdv3p"))
+                (file-name (git-file-name name version))))
+      (build-system guile-build-system)
+      (arguments
+       ;; The *-impl.scm files are actually included from module files; they
+       ;; should not be compiled separately, but they must be installed.
+       '(#:not-compiled-file-regexp "-impl\\.scm$"))
+      (inputs
+       `(("guile" ,guile-2.2)))
+      (synopsis "Formatting combinators for Guile")
+      (description
+       "The @code{(srfi-159)} module and its sub-modules implement the
+formatting combinators specified by
+@uref{https://srfi.schemers.org/srfi-159/srfi-159.html, SRFI-159}.  These are
+more expressive and flexible than the traditional @code{format} procedure.")
+      (license license:bsd-3))))