nix-instantiate: default to "default.nix" if no arguments are given

doc/manual/nix-instantiate.xml

@@ -50,8 +50,9 @@
 <para>The command <command>nix-instantiate</command> generates <link
 linkend="gloss-derivation">store derivations</link> from (high-level)
 Nix expressions.  It loads and evaluates the Nix expressions in each
-of <replaceable>files</replaceable>.  Each top-level expression should
-evaluate to a derivation, a list of derivations, or a set of
+of <replaceable>files</replaceable> (which defaults to
+<replaceable>./default.nix</replaceable>).  Each top-level expression
+should evaluate to a derivation, a list of derivations, or a set of
 derivations.  The paths of the resulting store derivations are printed
 on standard output.</para>
 

src/nix-instantiate/nix-instantiate.cc

@@ -148,7 +148,8 @@ void run(Strings args)
         Expr * e = parseStdin(state);
         processExpr(state, attrPaths, parseOnly, strict, autoArgs,
             evalOnly, xmlOutput, xmlOutputSourceLocation, e);
-    }
+    } else if (files.empty())
+        files.push_back("./default.nix");
 
     foreach (Strings::iterator, i, files) {
         Expr * e = state.parseExprFromFile(lookupFileArg(state, *i));