laziness

[@gavinking] As suggested by @RossTate in #3594, we could define a class Evaluable<Value> or whatever, as a subtype of Iterable<Value>, abbreviated to {Value}, and instantiated using the syntax {val}, allowing support for lazy evaluation.

For example:

void debug({String} text) {
    if (logging.debugEnabled) {
        logging.debug(text.evaluate);
    }
}

Which would allow lazy evaluation of the log message:

debug({ "New user created: "  + user.name });

We would also slightly massage the semantics of named argument lists to allow the following:

debug { "New user created: "  + user.name };

This idea would impact { ... }-style iterable enumerations in general, since they would come with the expectation that the constituent expressions are always evaluated lazily. This is actually more consistent, since that’s also the expectation when a comprehension appears inside { ... }.

[Migrated from ceylon/ceylon-spec#491]

[jvasileff]

I suppose this ancient issue was resolved in favor of https://github.com/ceylon/ceylon/issues/3597#issuecomment-156646707? The following program completes successfully on the JVM & Dart, and as of 1.3.2, JS:

shared void run() {
    String overflow() => overflow();
    void nolog(Boolean level, String template, {String*} args) {
        if (level) {
            printAll { template, *args };
        }
    }
    nolog { false; "abc"; overflow(), overflow() };
}

Shall we close it?

[gavinking]

Yeah, sure.