In DMD 2.063alpha if you print a const null Nullable:


import std.stdio: writeln;
import std.typecons: Nullable;
void main() {
    auto a = Nullable!int(10);
    writeln(a);
    auto b = const(Nullable!int)(10);
    writeln(b);
    const(Nullable!int) c;
    writeln(c);
    Nullable!int d;
    writeln(d);
}


You get:


10
const(Nullable!(int))(10, false)
const(Nullable!(int))(0, true)
==> core.exception.AssertError@C:\dmd2\src\phobos\std\typecons.d(1212): Called `get' on null Nullable!int.

This could work:

    string toString() const
    {
        return this.isNull ?
            format("Nullable!%s is null", T.stringof) : text(get());
    }

How sophisticated does this have to be anyway?

(In reply to comment #2)

> How sophisticated does this have to be anyway?

Sorry, I don't remember why I have written "but it's not a good general solution:" :-)


> This could work:
> 
>     string toString() const
>     {
>         return this.isNull ?
>             format("Nullable!%s is null", T.stringof) : text(get());
>     }
> 

But I have two notes and half:

1) Maybe using a sink in toString is a bit more efficient.

2) Think about printing an array of nullables, do you like this?
[Nullable!int is null, 55, Nullable!int is null, 22]

This looks a bit better:
[Nullable!int(), 55, Nullable!int(), 22]

2b) But what's even better is something similar to Python, that uses __str__ and __repr__ if you print an item or if you print a collection of items.

So if you print a single Nullable you get:
55
or:
Nullable!int()

If you print an array/range of nullables you get:
[Nothing, 55, Nothing, 22]


D already does that in some cases:

writeln("hello");
writeln(["hello"]);

It outputs:
hello
["hello"]

In the first case it doesn't show the "".

*** Issue 10915 has been marked as a duplicate of this issue. ***

This has improved the situation:
https://github.com/D-Programming-Language/phobos/pull/2587


But the example in comment #1 still fails:


void main() {
    import std.stdio: writeln;
    import std.typecons: Nullable;
    const(Nullable!int) c;
    writeln(c);
}


core.exception.AssertError@C:\dmd2\src\phobos\std\typecons.d(1529): Called `get' on null Nullable!int.

*** Issue 10915 has been marked as a duplicate of this issue. ***

import std.stdio;
import std.typecons;

void main() {
	Nullable!string s;
	writeln(s);
}

now prints "Nullable.null".