Re: test if the alias of a template is a literal

[Basile B. via Digitalmars-d-learn]

On Thursday, 27 October 2016 at 14:04:23 UTC, Gianni Pisetta 
wrote:
Hi all,
I have an AliasSeq composed of literal strings, variables and 
delegates. I want to build a template Optimize that will return 
an AliasSeq that have all adjacent literals concatenated into 
one. So i writed something like this:

template isStringLiteral(alias V) {
    enum bool isStringLiteral = ( is( typeof( V ) == string ) );
}

template Optimize(Os...) {
    static if ( Os.length < 2 )
        alias Optimize = Os;
    else {
        alias Optimized = Optimize!(Os[1..$]);

        static if ( isStringLiteral!(Os[0]) && 
isStringLiteral!(Optimized[0]) ) {
            enum string First = Os[0] ~ Optimized[0];
            alias Rest = Optimized[1..$];
        }
        else {
            alias First = AliasSeq!(Os[0]);
            alias Rest = Optimized;
        }
        alias Optimize = AliasSeq!(First, Rest);
    }
}

but at the moment isStringLiteral will return true even with 
variables of type string. So i searched for a metod to check if 
an alias is a literal value, but found nothing. Anyone have any 
clue on how can be done?

Thanks,
Gianni Pisetta

Hello, I think the correct isStringLiteral would be:

import
    std.meta;

template isStringLiteral(alias V)
{
    enum isCompileTime = is(typeof((){enum a = V;}));
    enum isString = is(typeof(V) == string);
    enum isStringLiteral = isCompileTime && isString;
}

unittest
{
    string a;
    enum b = "0";
    enum c = 0;
    static assert(!isStringLiteral!a);
    static assert(isStringLiteral!b);
    static assert(!isStringLiteral!c);
}

It's decomposed to show the logic:

1. If the delegate that assigns the parameter to a compile-time 
enum is valid we know that the parameter value is defined at 
compile-time.
2. check that the parameter is a string.