libcmd: turn autoArgs into a map that points to std::variant

This is a little more elegant and easier to reason about than prefixing
strings with whatever type the rest of the string is.

Change-Id: I7769535303dcb9f67b79e89bef162beec990e2a0

lix/libcmd/common-eval-args.cc

@@ -44,7 +44,7 @@ MixEvalArgs::MixEvalArgs()
          .labels = {"name", "expr"},
          .handler = {[&](std::string name, std::string expr) {
              checkValidNixIdentifier(name);
-             autoArgs[name] = 'E' + expr;
+             autoArgs[name] = ExprArgument(expr);
          }}}
     );
 
@@ -55,7 +55,7 @@ MixEvalArgs::MixEvalArgs()
         .labels = {"name", "string"},
         .handler = {[&](std::string name, std::string s) {
             checkValidNixIdentifier(name);
-            autoArgs[name] = 'S' + s;
+            autoArgs[name] = StringArgument(s);
         }},
     });
 
@@ -182,11 +182,18 @@ MixEvalArgs::MixEvalArgs()
 Bindings * MixEvalArgs::getAutoArgs(Evaluator & state)
 {
     auto res = state.buildBindings(autoArgs.size());
-    for (auto & i : autoArgs) {
-        Value v = i.second[0] == 'E'
-            ? state.evalLazily(state.parseExprFromString(i.second.substr(1), CanonPath::fromCwd()))
-            : Value{NewValueAs::string, ((std::string_view) i.second).substr(1)};
-        res.insert(state.symbols.create(i.first), v);
+    for (auto & [name, value] : autoArgs) {
+        Value v = std::visit(
+            overloaded{
+                [&](StringArgument & str) -> Value { return {NewValueAs::string, (std::string_view) str.value}; },
+                [&](ExprArgument & e) -> Value {
+                    return state.evalLazily(state.parseExprFromString(e.expr, CanonPath::fromCwd()));
+                }
+            },
+            value
+        );
+
+        res.insert(state.symbols.create(name), v);
     }
     return res.finish();
 }

lix/libcmd/common-eval-args.hh

@@ -14,6 +14,15 @@ class EvalState;
 class Bindings;
 struct SourcePath;
 
+struct StringArgument
+{
+    std::string value;
+};
+struct ExprArgument
+{
+    std::string expr;
+};
+
 struct MixEvalArgs : virtual Args, virtual MixRepair
 {
     static constexpr auto category = "Common evaluation options";
@@ -27,7 +36,7 @@ struct MixEvalArgs : virtual Args, virtual MixRepair
     std::optional<std::string> evalStoreUrl;
 
 private:
-    std::map<std::string, std::string> autoArgs;
+    std::map<std::string, std::variant<StringArgument, ExprArgument>> autoArgs;
 };
 
 /** @brief Resolve an argument that is generally a file, but could be something that