Re: It turns out it's quite hard to have @safe pure nothrow functions. Oh, and const.

[Timon Gehr via Digitalmars-d]

On 09/14/2014 10:50 AM, Kagamin wrote:

On Sunday, 14 September 2014 at 01:38:33 UTC, Jakob Ovrum wrote:

This is necessary for `inout` to work with callback functions, such as
with internal iteration (i.e. `opApply`).

Can you write a pass and xfail test for it? This scenario looks
non-trivial.
...

struct S{
    int[] a;
    int opApply(scope int delegate(ref inout int) dg)inout{
        foreach(ref x;a) if(auto r=dg(x)) return r;
        return 0;
    }
}

void main(){
    auto s=S([1,2,3]);
    foreach(ref x;s) x++; // ok
    const(S) s2=s;
    foreach(ref x;s2){
        import std.stdio;
        writeln(x);
        // x++; error
    }
}

It can be worked around exactly the same way you would work around it
with functions that return a value - by duplicating the function. It
is essentially the same problem and thus `inout` could easily be used
to fix it.

Can you elaborate? I don't quite understand, what you mean.

inout's primary goal is to eliminate patterns similar to:

struct S{
    int[] a;
    int opApply(scope int delegate(ref int) dg){
        foreach(ref x;a) if(auto r=dg(x)) return r;
        return 0;
    }
    int opApply(scope int delegate(ref const int) dg)const{
        foreach(ref x;a) if(auto r=dg(x)) return r;
        return 0;
    }
    int opApply(scope int delegate(ref immutable int) dg)immutable{
        foreach(ref x;a) if(auto r=dg(x)) return r;
        return 0;
    }
}