Add subscript operator to Expression to access sub-expressions by name (

#60)

* Add another subscript operator to expression to access sub-expressions by name.

* explicitly include optional for windows.

* clang-format run.

* Add test for subscript operators.

* Forgot the clang-format run again ...

* Simplify subscript test

include/peg_parser/interpreter.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <iterator>
+#include <optional>
 #include <type_traits>
 
 #include "parser.h"
@@ -58,6 +59,14 @@ namespace peg_parser {
       Expression operator[](size_t idx) const {
         return interpreter.interpret(syntaxTree->inner[idx]);
       }
+      std::optional<Expression> operator[](std::string_view name) const {
+        auto it = std::find_if(syntaxTree->inner.begin(), syntaxTree->inner.end(),
+                               [name](auto st) { return st->rule->name == name; });
+        if (it != syntaxTree->inner.end()) {
+          return interpreter.interpret(*it);
+        }
+        return {};
+      }
       iterator begin() const { return iterator(*this, 0); }
       iterator end() const { return iterator(*this, size()); }
 

test/source/parser.cpp

@@ -284,3 +284,14 @@ TEST_CASE("Documentation Example") {
   REQUIRE(g.run("1 - 2*3/2 + 4") == Approx(2));
   REQUIRE(g.run("1 + 2 * (3+4)/ 2 - 3") == Approx(5));
 }
+
+TEST_CASE("Subscript Operators") {
+  ParserGenerator<bool> program;
+  program["Word"] << "[a-z]+";
+  program["Yell"] << "[A-Z]+";
+  program["Start"] << "Word | Yell" >> [](auto e) { return bool(e["Yell"]); };
+  program.setStart(program["Start"]);
+
+  REQUIRE(!program.run("hello"));
+  REQUIRE(program.run("HELLO"));
+}