Is there a way to make sure a delegate only reads state, without changing it? I tried annotating the delegate as const, but that does not seem to work.

```D
void main()
{
    import std.stdio : writeln;
    auto r = [0,1,2,3];

    auto f = delegate() const // Compiles even though we are changing r
    {
        import std.array : popFront;
        r.popFront;
    };

    r.writeln; // [0,1,2,3]
    f();
    r.writeln; // [1,2,3]
}
```

On Monday, 4 April 2016 at 08:10:10 UTC, Edwin van Leeuwen wrote:
> Is there a way to make sure a delegate only reads state, without changing it? I tried annotating the delegate as const, but that does not seem to work.
>

Note that annotating with pure also doesn't help. As a result we can have a pure delegate that returns different results every time it is called.

```D
void main()
{
    import std.stdio : writeln;
    auto r = [0,1,2,3];

    auto f = delegate() const pure
    {
        import std.array : front, popFront;
        r.popFront;
        return r.front;
    };

    r.writeln; // [0,1,2,3]
    auto f1 = f();
    r.writeln; // [1,2,3]
    assert( f() == f1 ); // Throws
}
```

On Monday, 4 April 2016 at 08:10:10 UTC, Edwin van Leeuwen wrote:
> Is there a way to make sure a delegate only reads state, without changing it? I tried annotating the delegate as const, but that does not seem to work.
> ```

Yeah this is a nasty old issue. The underlying problem is that a delegate's function and context pointers are completely untyped.

https://issues.dlang.org/show_bug.cgi?id=1983

On Monday, 4 April 2016 at 11:32:23 UTC, Rene Zwanenburg wrote:
> https://issues.dlang.org/show_bug.cgi?id=1983

Bug 1983 is about usage of delegates after creation, restrictions during creation are enforced. AIU, OP wants to have const check during creation.

On Monday, 4 April 2016 at 11:32:23 UTC, Rene Zwanenburg wrote:
> On Monday, 4 April 2016 at 08:10:10 UTC, Edwin van Leeuwen wrote:
>> Is there a way to make sure a delegate only reads state, without changing it? I tried annotating the delegate as const, but that does not seem to work.
>> ```
>
> Yeah this is a nasty old issue. The underlying problem is that a delegate's function and context pointers are completely untyped.
>
> https://issues.dlang.org/show_bug.cgi?id=1983

Thanks for the reference, hopefully this will be resolved at some point :)

On Monday, 4 April 2016 at 11:39:55 UTC, Kagamin wrote:
> On Monday, 4 April 2016 at 11:32:23 UTC, Rene Zwanenburg wrote:
>> https://issues.dlang.org/show_bug.cgi?id=1983
>
> Bug 1983 is about usage of delegates after creation, restrictions during creation are enforced. AIU, OP wants to have const check during creation.

I think the underlying issue is the same. The problem seems to be that:
> Unfortunately, there is no way to declare a const delegate (by which I mean, a delegate whose context pointer is typed const).

I actually discovered the problem, due to the hole it leaves in the const system, where I got different results calling a const method multiple times. The const method in question called a delegate that changed its context pointer, resulting in changes during calls.