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Hello. It is fascinating to see string interpolation in D. Let me try to spread some light on it; I hope my thoughts will be useful.
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First of all, I’d like to notice that in the DIP1027 variant of the code we see:
>auto fmt = arg[0];(
argis undeclared identifier here; I presumeargswas meant.) There is a problem: this line is executed at CTFE, but it cannot accessargs, which is a runtime parameter ofexeci. For this to work, the format string should go to a template parameter, and interpolated expressions should go to runtime parameters. How can DIP1027 accomplish this? -
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Note that nested istrings are not supported.
To clarify: “not supported” means one cannot write
db.execi(i"SELECT field FROM items WHERE server = $(i"europe$(number)")");Instead, you have to be more explicit about what you want the inner string to become. This is legal:
db.execi(i"SELECT field FROM items WHERE server = $(i"europe$(number)".text)");However, it is not hard to adjust
execiso that it fully supports nested istrings:struct Span { size_t i, j; bool topLevel; } enum segregatedInterpolations(Args...) = { Span[ ] result; size_t processedTill; size_t depth; static foreach (i, T; Args) static if (is(T == InterpolationHeader)) { if (!depth++) { result ~= Span(processedTill, i, true); processedTill = i; } } else static if (is(T == InterpolationFooter)) if (!--depth) { result ~= Span(processedTill, i + 1); processedTill = i + 1; } return result; }(); auto execi(Args...)(Sqlite db, InterpolationHeader header, Args args, InterpolationFooter footer) { import std.conv: text, to; import arsd.sqlite; // sqlite lets you do ?1, ?2, etc enum string query = () { string sql; int number; static foreach (span; segregatedInterpolations!Args) static if (span.topLevel) { static foreach (T; Args[span.i .. span.j]) static if (is(T == InterpolatedLiteral!str, string str)) sql ~= str; else static if (is(T == InterpolatedExpression!code, string code)) sql ~= "?" ~ to!string(++number); } return sql; }(); auto statement = Statement(db, query); int number; static foreach (span; segregatedInterpolations!Args) static if (span.topLevel) { static foreach (arg; args[span.i .. span.j]) static if (!isInterpolatedMetadata!(typeof(arg))) statement.bind(++number, arg); } else // Convert a nested interpolation to string with `.text`. statement.bind(++number, args[span.i .. span.j].text); return statement.execute(); }Here, we just invoke
.texton nested istrings. A more advanced implementation would allocate a buffer and reuse it. It could even be@nogcif it wanted. -
DIP1036 appeals more to me because it passes rich, high-level information about parts of the string. With DIP1027, on the other hand, we have to extract that information ourselves by parsing the string character by character. But the compiler already tokenized the string; why do we have to do it again? (And no, lower level doesn’t imply broader possibilities here.)
It may have another implication: looping over characters might put current CTFE engine in trouble if strings are large. Much more iterations need to be executed, and more memory is consumed in the process. We certainly need numbers here, but I thought it was important to at least bring attention to this point.
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What I don’t like in both DIPs is a rather arbitrary selection of meta characters:
$,$$and%s. In regular strings, all of them are just normal characters; in istrings, they gain special meaning.I suppose a cleaner way would be to use
\(...)syntax (like in Swift). Soi"a \(x) b"interpolatesxwhile"a \(x) b"is an immediate syntax error. First, it helps to catch bugs caused by missingi. Second, the question, how do we escape$, gets the most straightforward answer: we don’t.A downside is that parentheses will always be required with this syntax. But the community preferred them anyway even with
$.
