On 08/03/2010 12:32 AM, bearophile wrote:
> Pelle:
>> I think a good thing would be NonNull!T, but I haven't managed to create
>> one. If this structure exists and becomes good practice to use, maybe we
>> can get the good syntax in D3. In 20 years or so :P
>
> Maybe we are talking about two different things, I was talking about nonnull class references/pointers, you seem to talk about nullable values :-) Both can be useful in D, but they are different things.
> Nullable values are simpler to design, they are just wrapper structs that contain a value plus a boolean, plus if you want some syntax sugar to manage them with a shorter syntax.
>
> Bye,
> bearophile

I am talking about non-nullable references indeed. I don't think I mentioned nullable types, really.

I also created this, as the simplest NotNull-type concievable:

struct NotNull(T) if(is(typeof(T.init !is null))) {
    private T _instance;

    this(T t) {
        enforce(t !is null, "Cannot create NotNull from null");
        _instance = t;
    }

    T get() {
        assert (_instance !is null,
                text("Supposed NotNull!(", T.stringof, ") is null"));
        return _instance;
    }
    alias get this;
}


This has the obvious bug in that you can declare a nonnull without an initializer and get a null from it. If we ever get @disable this(){} for structs, this struct can become better.

I'll probably try it out in some code.

Pelle:

> struct NotNull(T) if(is(typeof(T.init !is null))) {

Is this enough?
struct NotNull(T) if (is(T.init is null)) {


>      this(T t) {
>          enforce(t !is null, "Cannot create NotNull from null");

enforce() is bad, use Design by Contract instead (a precondition with an assert inside).

Bye,
bearophile

> Is this enough?
> struct NotNull(T) if (is(T.init is null)) {

Sorry, I meant:

struct NotNull(T) if (T.init is null) {

Bye,
bearophile

On 08/03/2010 01:08 AM, bearophile wrote:
> Pelle:
>
>> struct NotNull(T) if(is(typeof(T.init !is null))) {
>
> Is this enough?
> struct NotNull(T) if (is(T.init is null)) {
>
>
>>       this(T t) {
>>           enforce(t !is null, "Cannot create NotNull from null");
>
> enforce() is bad, use Design by Contract instead (a precondition with an assert inside).
>
> Bye,
> bearophile

If NotNull will be in a library, it should probably use enforce, if I have understood things correctly. External input, and all that. I think most of phobos does it like this currently.

Pelle:
> If NotNull will be in a library, it should probably use enforce, if I have understood things correctly. External input, and all that. I think most of phobos does it like this currently.

I suspect that Andrei has still to "get" DbC :-) (And your lib is not Phobos.)

Bye,
bearophile