From 69e3ffb076931aba73b5fcc41f5264f5f62c5ce9 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Thu, 21 Apr 2016 14:58:32 +0200
Subject: [PATCH] nix --help: Show usage examples

---
 src/nix/command.cc | 15 +++++++++++++++
 src/nix/command.hh | 12 ++++++++++++
 src/nix/verify.cc  | 14 ++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/src/nix/command.cc b/src/nix/command.cc
index 986953fd8..c8d91737d 100644
--- a/src/nix/command.cc
+++ b/src/nix/command.cc
@@ -5,6 +5,21 @@ namespace nix {
 
 Commands * RegisterCommand::commands = 0;
 
+void Command::printHelp(const string & programName, std::ostream & out)
+{
+    Args::printHelp(programName, out);
+
+    auto exs = examples();
+    if (!exs.empty()) {
+        out << "\n";
+        out << "Examples:\n";
+        for (auto & ex : exs)
+            out << "\n"
+                << "  " << ex.description << "\n" // FIXME: wrap
+                << "  $ " << ex.command << "\n";
+    }
+}
+
 MultiCommand::MultiCommand(const Commands & _commands)
     : commands(_commands)
 {
diff --git a/src/nix/command.hh b/src/nix/command.hh
index a6adb5f65..34affc43d 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -11,6 +11,18 @@ struct Command : virtual Args
     virtual std::string name() = 0;
     virtual void prepare() { };
     virtual void run() = 0;
+
+    struct Example
+    {
+        std::string description;
+        std::string command;
+    };
+
+    typedef std::list<Example> Examples;
+
+    virtual Examples examples() { return Examples(); }
+
+    void printHelp(const string & programName, std::ostream & out) override;
 };
 
 class Store;
diff --git a/src/nix/verify.cc b/src/nix/verify.cc
index da1567808..fdbc2b0fd 100644
--- a/src/nix/verify.cc
+++ b/src/nix/verify.cc
@@ -36,6 +36,20 @@ struct CmdVerify : StorePathsCommand
         return "verify the integrity of store paths";
     }
 
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To verify the entire Nix store:",
+                "nix verify --all"
+            },
+            Example{
+                "To check whether each path in the closure of Firefox has at least 2 signatures:",
+                "nix verify -r -n2 --no-contents $(type -p firefox)"
+            },
+        };
+    }
+
     void run(ref<Store> store, Paths storePaths) override
     {
         restoreAffinity(); // FIXME