nix: Show help when no arguments are given

Fixes #1464.

2 files changed, 4 insertions, 3 deletions

diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index df7e040875d6..0eed4945418d 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -146,7 +146,7 @@ bool Args::processArgs(const Strings & args, bool finish)
         res = true;
     }
 
-    if (finish && !expectedArgs.empty())
+    if (finish && !expectedArgs.empty() && !expectedArgs.front().optional)
         throw UsageError("more arguments are required");
 
     return res;
diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index aa11373d5f1f..ef8a7953e520 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -51,6 +51,7 @@ protected:
     {
         std::string label;
         size_t arity; // 0 = any
+        bool optional;
         std::function<void(Strings)> handler;
     };
 
@@ -165,7 +166,7 @@ public:
     /* Expect a string argument. */
     void expectArg(const std::string & label, string * dest)
     {
-        expectedArgs.push_back(ExpectedArg{label, 1, [=](Strings ss) {
+        expectedArgs.push_back(ExpectedArg{label, 1, false, [=](Strings ss) {
             *dest = ss.front();
         }});
     }
@@ -173,7 +174,7 @@ public:
     /* Expect 0 or more arguments. */
     void expectArgs(const std::string & label, Strings * dest)
     {
-        expectedArgs.push_back(ExpectedArg{label, 0, [=](Strings ss) {
+        expectedArgs.push_back(ExpectedArg{label, 0, false, [=](Strings ss) {
             *dest = ss;
         }});
     }