gistfile1.txt

void main()
{
    int a = 42;

    import std.stdio;
    import std.conv : text;

    //
    // Want to create the string "a is 42"
    //
    writeln("Looking for 'a is 42':");

    string attempt1 = text(mixin(interp("a is $(a)")));
    writefln("attempt1 '%s'", attempt1); // NOPE
    //assert(attempt1 == "a is 42"); // Fails

    string attempt2 = text(interp("a is $(a)"));
    writefln("attempt2 '%s'", attempt2); // NOPE
    //assert(attempt2 == "a is 42"); // Fails

    string attempt3 = text(mixin("a is $(a)".interp));
    writefln("attempt3 '%s'", attempt3); // NOPE
    //assert(attempt3 == "a is 42"); // Fails
}

//
//
// TODO: move what's below into std/experimental/typecons.d
//
//

// !!!!!!!!!!!!!!!!!!!!!!!
// !!!!!!!!!!!!!!!!!!!!!!!
// !!!!!!!!!!!!!!!!!!!!!!!
// !!!!!!!!!!!!!!!!!!!!!!!
// NEED TO PUBLICLY IMPORT THE `tuple` SYMBOL BECAUSE THE CODE GENERATED BY
// THE interpolate FUNCTION REQUIRES IT.
// !!!!!!!!!!!!!!!!!!!!!!!
// !!!!!!!!!!!!!!!!!!!!!!!
// !!!!!!!!!!!!!!!!!!!!!!!
// !!!!!!!!!!!!!!!!!!!!!!!
public import std.typecons : tuple;

/**
Returns code meant to be used inside a "mixin" that creates a tuple.
*/
string interpolate(string str) pure @safe
{
    return "tuple(" ~ interpolateHelper1(str) ~ ")";
}

/// ditto
alias interp = interpolate; // optional shorthand

private string interpolateHelper1(string str) pure @safe
{
    import std.string : indexOf;

    auto dollarIndex = str.indexOf("$");
    if (dollarIndex < 0)
        return `"` ~ str ~ `"`;
    else if (dollarIndex == 0)
        return interpolateHelper2(str[1 .. $]);
    else
        return `"` ~ str[0 .. dollarIndex] ~ `", ` ~ interpolateHelper2(str[dollarIndex + 1 .. $]);
}
// str starts with a dollar expression (just after the '$' character)
private string interpolateHelper2(string str) pure @safe
{
    if (str[0] == '(')
    {
        uint depth = 1;
        size_t i = 1;
        for (;; i++)
        {
            if (i >= str.length)
                throw new InterpolateException("unterminted $(...) expression");
            if (str[i] == ')')
            {
                depth--;
                if (depth == 0)
                    break;
            }
            else if (str[i] == '(')
            {
                depth++;
            }
            // TODO: handle parens inside comments/string literals, etc
            //       need a simple lexer that ignore comments/string literals
        }
        if (i == str.length - 1)
            return str[1 .. i];
        return str[1 .. i] ~ `,` ~ interpolateHelper1(str[i + 1 .. $]);
    }
    else if (str[0] == '$')
        throw new InterpolateException("interpolate $$ not implemented");
    else
        throw new InterpolateException("interpolated expression $ currently requires parens $(...)");
}

class InterpolateException : Exception
{
    this(string msg) pure @safe
    {
        super(msg);
    }
}