test case:

import std.algorithm;

void main()
{
    float[10] arr;
    arr[] = 0.0;
    map!((ref float sample){ sample = 1.0; })(arr[]);
}

D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(382): Error: nan is not an lvalue

This looks like a bug to me?

Offending line in map:
alias typeof(_fun(.ElementType!R.init)) ElementType;

So it tires to pass .init which will be nan for floats.

A quick workaround would be:
    static if (isFloatingPoint!(ElementType!R))
    {
        .ElementType!R x;
        alias typeof(_fun(x)) ElementType;
    }
    else
    {
        alias typeof(_fun(.ElementType!R.init)) ElementType;
    }

Also, my example was just a little flawed (semantically speaking), since I was missing a return statement:

    double[10] arr;
    arr[] = 1.0;
    auto result = map!((ref double sample){ return 1.0; })(arr[]);

This will then work with that change.

Ugh don't I love making silly posts like these. ref won't work for any types, not just floats, since it uses the .init property.

My real problem was that I was using assignment in my first example instead of returning a value. Using map with ref doesn't make much sense. So my complaints are void.

On 6/3/11, Andrej Mitrovic <andrej.mitrovich@gmail.com> wrote:
> Offending line in map:
> alias typeof(_fun(.ElementType!R.init)) ElementType;
>
> So it tires to pass .init which will be nan for floats.
>
> A quick workaround would be:
>     static if (isFloatingPoint!(ElementType!R))
>     {
>         .ElementType!R x;
>         alias typeof(_fun(x)) ElementType;
>     }
>     else
>     {
>         alias typeof(_fun(.ElementType!R.init)) ElementType;
>     }
>
> Also, my example was just a little flawed (semantically speaking), since I was missing a return statement:
>
>     double[10] arr;
>     arr[] = 1.0;
>     auto result = map!((ref double sample){ return 1.0; })(arr[]);
>
> This will then work with that change.
>

Andrej Mitrovic:

> A quick workaround would be:
>     static if (isFloatingPoint!(ElementType!R))
>     {
>         .ElementType!R x;
>         alias typeof(_fun(x)) ElementType;
>     }
>     else
>     {
>         alias typeof(_fun(.ElementType!R.init)) ElementType;
>     }

If you have improvements for Phobos code, then I suggest to write it in Bugzilla (or even create a pull request in git).

Bye,
bearophile

On 06/03/2011 10:15 AM, Andrej Mitrovic wrote:
> Offending line in map:
> alias typeof(_fun(.ElementType!R.init)) ElementType;

The fact that the error points at that line should be a bug. That's just an alias, nothing is evaluated at that line.

Ali