https://d.puremagic.com/issues/show_bug.cgi?id=12489

           Summary: std.bitmanip byte swapping routines should be
                    partially instantiable
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-03-29 12:15:14 CET ---
I've had a use case for generic endianness swapping when reading file headers, and was going to use attributes. For example:

struct Header
{
    @Converter!(littleEndianToNative!ushort)
    ushort reserved1;
}

Unfortunately this doesn't work because you can't partially instantiate littleEndianToNative:

Error: template instance littleEndianToNative!ushort does not match template
declaration littleEndianToNative(T, uint n)(ubyte[n] val) if
(canSwapEndianness!T && n == T.sizeof)

It would have to be converted to a template, e.g.:

Before:

-----
T littleEndianToNative(T, size_t n)(ubyte[n] val) @safe pure nothrow
-----

After:

-----
template littleEndianToNative(T)
{
    T littleEndianToNative(size_t n)(ubyte[n] val) @safe pure nothrow
    {
        ...
    }
}
-----

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--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-03-29 12:32:30 CET ---
Here's a somewhat generic wrapper workaround:

-----
template PartialTempl(alias templ, T...)
{
    auto PartialTempl(Args...)(auto ref Args args)
    {
        return templ!T(args);
    }
}
-----

Usable as:


-----
@Converter!(PartialTempl!(littleEndianToNative, ushort))
ushort reserved1;
-----

Of course 'littleEndianToNative' and friends take a ubyte[N], not a ushort, so in my file loading routine I have to take that into account. But using attributes is pretty nice.

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX


--- Comment #2 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-03-29 21:01:05 CET ---
(In reply to comment #1)
> Here's a somewhat generic wrapper workaround:
> 
> -----
> template PartialTempl(alias templ, T...)
> {
>     auto PartialTempl(Args...)(auto ref Args args)
>     {
>         return templ!T(args);
>     }
> }
> -----

This solves it for me.

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Vladimir Panteleev <thecybershadow@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |thecybershadow@gmail.com


--- Comment #3 from Vladimir Panteleev <thecybershadow@gmail.com> 2014-03-30 14:37:22 EEST ---
(In reply to comment #0)
>     @Converter!(littleEndianToNative!ushort)
>     ushort reserved1;

I don't know the context, but there's some redundancy here already (ushort is listed twice). Any reason you can't use `@Converter!littleEndianToNative` ?

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--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-03-30 14:45:56 CEST ---
(In reply to comment #3)
> (In reply to comment #0)
> >     @Converter!(littleEndianToNative!ushort)
> >     ushort reserved1;
> 
> I don't know the context, but there's some redundancy here already (ushort is listed twice). Any reason you can't use `@Converter!littleEndianToNative` ?

Yeah you're right, I don't need to embed this information. It's really nice that I can now write:

struct Header
{
    @Converter!littleEndianToNative
    {
        ushort reserved1;

        ushort ordnum;

        ushort insnum;

        ushort patnum;

        ushort flags;

        ushort cwtv;

        ushort version_;
    }
}

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