ats2: clean up expression

pkgs/development/compilers/ats2/default.nix

···
     url = "mirror://sourceforge/ats2-lang/ATS2-Postiats-contrib-${versionPkg}.tgz" ;
     sha256 = "0kc4nx1904745c1rkj9yfbayidw7rks1mwq0lxmvsgghn98dxwjn" ;
   };
+
+  postInstallContrib = stdenv.lib.optionalString withContrib
+  ''
+    local contribDir=$out/lib/ats2-postiats-*/ ;
+    mkdir -p $contribDir ;
+    tar -xzf "${contrib}" --strip-components 1 -C $contribDir ;
+  '';
+
+  postInstallEmacs = stdenv.lib.optionalString withEmacsSupport
+  ''
+    local siteLispDir=$out/share/emacs/site-lisp/ats2 ;
+    mkdir -p $siteLispDir ;
+    install -m 0644 -v ./utils/emacs/*.el $siteLispDir ;
+  '';
 in
 
 stdenv.mkDerivation rec {
   name    = "ats2-${version}";
   version = versionPkg;
 
-    src = fetchurl {
+  src = fetchurl {
     url = "mirror://sourceforge/ats2-lang/ATS2-Postiats-${version}.tgz";
     sha256 = "140xy129fr11bdf4bj6qya9mf0fhnv2x7ksb9j46pf2yzrsrks8g";
   };
 
   buildInputs = [ gmp ];
 
-  setupHook = let
-      hookFiles = [ ./setup-hook.sh ] ++ stdenv.lib.optionals withContrib [ ./setup-contrib-hook.sh ] ;
+  setupHook = with stdenv.lib;
+    let
+      hookFiles =
+        [ ./setup-hook.sh ]
+        ++ optional withContrib ./setup-contrib-hook.sh;
     in
-      builtins.toFile "setupHook.sh" (stdenv.lib.concatMapStringsSep "\n" builtins.readFile hookFiles) ;
+      builtins.toFile "setupHook.sh"
+      (concatMapStringsSep "\n" builtins.readFile hookFiles);
 
   patches = [ ./installed-lib-directory-version.patch ];
-
-  postInstallContrib = stdenv.lib.optionalString withContrib
-  ''
-    local contribDir=$out/lib/ats2-postiats-*/ ;
-    mkdir -p $contribDir ;
-    tar -xzf "${contrib}" --strip-components 1 -C $contribDir ;
-  '';
-
-  postInstallEmacs = stdenv.lib.optionalString withEmacsSupport
-  ''
-    local siteLispDir=$out/share/emacs/site-lisp/ats2 ;
-    mkdir -p $siteLispDir ;
-    install -m 0644 -v ./utils/emacs/*.el $siteLispDir ;
-  '';
 
   postInstall = postInstallContrib + postInstallEmacs;