Add simple REPL functionality

examples/CMakeLists.txt

@@ -1 +1,2 @@
 add_subdirectory(quickstart)
+add_subdirectory(repl)

examples/repl/CMakeLists.txt

@@ -0,0 +1,12 @@
+project(kalcy-repl)
+
+add_executable(${PROJECT_NAME})
+
+target_link_libraries(${PROJECT_NAME} PRIVATE
+  kalcy::kalcy
+  kalcy::kalcy-compile-options
+)
+
+target_sources(${PROJECT_NAME} PRIVATE
+  repl.cpp
+)

examples/repl/repl.cpp

@@ -0,0 +1,59 @@
+#include <kalcy/kalcy.hpp>
+#include <cassert>
+#include <format>
+#include <iomanip>
+#include <iostream>
+#include <sstream>
+
+namespace {
+struct Repl {
+	const std::string_view EXIT_TOKEN{"exit"};
+	bool verbose{}; // toggled on with '-v' and off with '-q'
+	bool is_running{true};
+
+	auto start() -> bool {
+		while (is_running) {
+			std::string text{};
+			std::cout << std::format("[{}] > ", verbose ? "verbose" : "quiet");
+			std::getline(std::cin, text);
+			// run kalcy on input expression
+			if (!run(text)) { return EXIT_FAILURE; }
+		}
+		// print epilogue
+		std::cout << std::format("\n^^ kalcy v{}\n", kalcy::version_v);
+	}
+
+	auto run(std::string_view const text) -> bool {
+		try {
+			// return false when the user enters the exit token
+			if (text == EXIT_TOKEN) {
+				is_running = false;
+			} else if (text == "-v") {
+				verbose = !verbose;
+			}  else {
+				// parse text into an expression
+				auto expr = kalcy::Parser{}.parse(text);
+				assert(expr != nullptr);
+				// evaluate parsed expression
+				// a custom Env can be used and passed, if desired
+				std::cout << kalcy::evaluate(*expr) << "\n";
+				// print AST if verbose
+				if (verbose) { std::cout << std::format("expression\t: {}\nAST\t\t: {}\n", text, kalcy::to_string(*expr)); }
+			}
+			return true;
+		} catch (kalcy::Error const& err) {
+			// print error
+			std::cerr << err.what() << "\n";
+			// highlight error location under text
+			std::cerr << " | " << text << "\n | " << err.build_highlight() << "\n";
+			return false;
+		}
+	}
+};
+} // namespace
+
+auto main(int argc, char** argv) -> int {
+
+	Repl repl{};
+	repl.start();
+}