import std;

​

enum Currency : string

{

    USD = "USD",

    EUR = "EUR",

    GBP = "GBP",

    JPY = "JPY",

}

​

struct Instrument

{

    Currency bid;

    Currency ask;

​

    static immutable USD_JPY = Instrument(Currency.USD, Currency.JPY);

}

​

struct Price

{

    Instrument instrument;

    ulong value;

}

​

class MinRecorder

{

@nogc nothrow pure @safe:

​

    // prices from local scoped array buffers.

    // I want to restrict prices reference to scope.

    void update(scope const(Price)[] prices) scope

    {

        foreach (price; prices)

        {

            update(price);

        }

    }

​

    // I thought price parameter need `scope` when called by scoped array elements.

    // But it can remove `scope` attribute currently.

    void update( /* scope */ ref const(Price) price) scope

    {

        if (minPrice.isNull || price.value < minPrice.get.value)

        {

            minPrice = price;

        }

    }

​

    Nullable!Price minPrice;

}

​

class MinPointerRecorder

{

@nogc nothrow pure @safe:

​

    void update(scope const(Price)[] prices) scope

    {

        foreach (price; prices)

        {

            update(price);

        }

    }

​

    void update( /* scope */ ref const(Price) price) scope

    {

        if (!minPrice || price.value < minPrice.value)

        {

            // @safe BUG?

            // compile error at dmd v2.093

            // Error: cannot take address of parameter price

            minPrice = &price;

        }

    }

​

    const(Price)* minPrice;

}

​

void main() @safe

{

    scope recorder = new MinRecorder();

​

    // received prices in local buffers.

    scope prices = [

        Price(Instrument.USD_JPY, 300), Price(Instrument.USD_JPY, 100),

        Price(Instrument.USD_JPY, 200),

    ];

    recorder.update(prices);

    writefln("%s", recorder.minPrice);

}