legis/examples/calculator.lgi

use library std.console;
use library std.text;


/// The mathematical operation the user has chosen.
choices Operation {
	Add;
	Subtract;
	Multiply;
	Divide;
}


/// Asks the user which operation to perform.
function choose_operation() returns Result<Operation, Text> {
	std.console.println("");
	std.console.println("Choose an operation");
	std.console.println("  +  Add");
	std.console.println("  -  Subtract");
	std.console.println("  *  Multiply");
	std.console.println("  /  Divide");
	std.console.print("> ");

	let response = pass up std.console.read_line()
		.or_fail("Could not read from the keyboard");

	match std.text.trim(response) {
		is "+": give back Operation.Add;
		is "-": give back Operation.Subtract;
		is "*": give back Operation.Multiply;
		is "/": give back Operation.Divide;
		otherwise:
			give back fail("Please enter +, -, *, or /");
	}
}


/// Asks the user for a decimal number.
function choose_number(prompt: Text) returns Result<Decimal, Text> {
	std.console.print("{prompt}: ");

	let response = pass up std.console.read_line()
		.or_fail("Could not read from the keyboard");

	let cleaned = std.text.trim(response);

	when std.text.parse<Decimal>(cleaned) {
		succeeds with number:
			give back number;
		fails:
			give back fail("'{cleaned}' is not a valid number");
	}
}


/// Carries out the calculation.
/// Division by zero gives back a failure rather than crashing.
pure function calculate(
	first_number:  Decimal,
	operation:     Operation,
	second_number: Decimal
) returns Result<Decimal, Text> {
	match operation {
		is Operation.Add:
			give back first_number + second_number;
		is Operation.Subtract:
			give back first_number - second_number;
		is Operation.Multiply:
			give back first_number * second_number;
		is Operation.Divide:
			if second_number == 0.0 {
				give back fail("Division by zero is not allowed");
			}
			give back first_number / second_number;
	}
}


/// Runs one calculation and gives back whether to keep going.
function run_calculator() returns Bool {

	let first_number = when choose_number("First number") {
		succeeds with value: value;
		fails with problem: {
			std.console.println(problem);
			give back true;
		}
	};

	let chosen_operation = when choose_operation() {
		succeeds with value: value;
		fails with problem: {
			std.console.println(problem);
			give back true;
		}
	};

	let second_number = when choose_number("Second number") {
		succeeds with value: value;
		fails with problem: {
			std.console.println(problem);
			give back true;
		}
	};

	when calculate(first_number, chosen_operation, second_number) {
		succeeds with answer: {
			let symbol = match chosen_operation {
				is Operation.Add:      "+";
				is Operation.Subtract: "-";
				is Operation.Multiply: "*";
				is Operation.Divide:   "/";
			};
			std.console.println("");
			std.console.println("{first_number} {symbol} {second_number} = {answer}");
		}
		fails with reason:
			std.console.println("Problem: {reason}");
	}

	std.console.println("");
	std.console.print("Another calculation? (y/n): ");

	let response = when std.console.read_line() {
		succeeds with value: std.text.trim(value).to_lower();
		fails: give back false;
	};

	give back response == "y" || response == "yes";
}


/// Entry point — runs the calculator until the user says to stop.
function main() {
	std.console.println("Legis Calculator");
	std.console.println("----------------");

	vary running = true;

	repeat while running {
		running = run_calculator();
	}

	std.console.println("");
	std.console.println("Goodbye.");
}