Just because it's a slow Thursday on this forum

[John Colvin via Digitalmars-d]

On Monday, 8 February 2016 at 13:37:19 UTC, Andrei Alexandrescu 
wrote:
> On 2/7/16 7:11 PM, John Colvin wrote:
>> alias dump = dumpTo!stdout;
>> alias errDump = dumpTo!stderr;
>
> I'm hoping for something with a simpler syntax, a la 
> dump!(stdout, "x") where stdout is optional. -- Andrei

How about this, which allows you to specify variables as alias 
parameters (i.e. without strings) as well. It could be a lot 
neater if a static assert is used in the body instead of using 
template constraints, but obviously that has its downsides.

import std.stdio : File;
import std.traits : isSomeString;
import std.meta : allSatisfy;

private template isAlias(a ...)
if (a.length == 1)
{
     enum isAlias = __traits(compiles, { alias b = a[0]; })
         && is(typeof(a[0]));
}

private template isStringValue(a ...)
if (a.length == 1)
{
     enum isStringValue = isSomeString!(typeof(a[0]));
}

private template isStringOrAlias(a ...)
if (a.length == 1)
{
     /* can't use templateOr in the dump template constraints
      * because `Error: template instance F!(a) cannot use local 
'a'
      * as parameter to non-global template templateOr(T...)` */
      * enum isStringOrAlias = isAlias!a || isStringValue!a;
}

mixin template dump(alias file, Args ...)
if (is(typeof(file) == File) && Args.length > 0
     && allSatisfy!(isStringOrAlias, Args))
{
     auto _unused_dump = {
         import std.traits : Select;
         // can put expressions directly in Select with
         // 
https://github.com/D-Programming-Language/phobos/pull/3978
         enum sep = ", ";
         enum term = "\n";
         foreach (i, arg; Args)
         {
             static if (isSomeString!(typeof(arg)))
                 file.write(arg, " = ", mixin(arg),
                     Select!(i < Args.length - 1, sep, term));
             else
                 file.write(__traits(identifier, Args[i]), " = ", 
arg,
                     Select!(i < Args.length - 1, sep, term));
         }
         return false;
     }();
}

mixin template dump(Args ...)
if (Args.length > 0
     && allSatisfy!(isStringOrAlias, Args))
{
     import std.stdio : stdout;
     mixin .dump!(stdout, Args);
}

unittest
{
     import std.stdio;
     int a = 3, b = 4;
     mixin dump!q{ a + b };
     mixin dump!(stderr, "a - b");

     mixin dump!a;
     mixin dump!(stderr, a, b);
}