Display

// `PrintLine("{}", value)` prints numbers, text and booleans because each of those types
// implements one interface, `Display` from the Text package. A type of your own prints the same
// way as soon as it implements `Display` too.
//
// `Display` asks for a single method, `WriteDisplay`. It is handed a writer, which stands for
// wherever the text is going, and it writes the value's text into it. Writing can fail (the
// console may be closed, a buffer may be full), so the method returns `! FormatError`, and a
// failure from any step is passed straight on with `?` or `return`.
import Format::WriteFormat;
import Io::PrintLine;
import Text::{ Display, FormatError, FormatSpec, TextWriter, WriteBytes };

// An amount of money, kept as whole cents so nothing is lost to rounding.
struct Money {
    cents: int64;
}

extend Money : Display {
    // `spec` carries what the placeholder asked for, such as the width in `{:>8}`. This
    // implementation ignores it, which a type is free to do.
    func WriteDisplay(self: &Money, writer: &var TextWriter, spec: FormatSpec) -> ! FormatError {
        var cents = self.cents;
        if cents < 0 {
            // One write that may fail. `?` hands a failure to the caller and stops here.
            WriteBytes(writer, "-")?;
            cents = -cents;
        }
        // `WriteFormat` is PrintLine's twin: the same placeholders, written into the writer.
        // `{:02}` pads the cents to two digits with zeros, so 5 cents shows as `05`.
        return WriteFormat(writer, "${}.{:02}", cents / 100, cents % 100);
    }
}

func Main() -> int {
    let price = Money { cents: 1205 };
    let refund = Money { cents: -350 };
    let coin = Money { cents: 5 };

    // Now Money goes into a placeholder like any number would.
    PrintLine("price:  {}", price);
    PrintLine("refund: {}", refund);
    PrintLine("coin:   {}", coin);
    PrintLine("{} back from {}", refund, price);

    // Take away the `extend` and every line above is refused: "argument 2 to 'PrintLine' has
    // type 'Money', but variadic parameter 'args' requires 'Display'".
    return 0;
}