Re: Amusing D facts: typesafe variadic arrays are lazy!

[Andrei Alexandrescu]

downs wrote:
Did you know the following code compiles?
module test;

import std.stdio;

void Assert(bool cond, string delegate()[] dgs...) {
  debug if (!cond) {
    string str;
    foreach (dg; dgs) str ~= dg();
    throw new Exception(str);
  }
}

void main() {
  Assert(false, "O hai thar! ");
}


It's true! :)

Gosh!!! What's happening over here? I even tried this:

import std.stdio;

void Assert(bool cond, string delegate()[] dgs...) {
    if (!cond) {
        string str;
        foreach (dg; dgs) str ~= dg();
        throw new Exception(str);
    }
}

string fun(string a, string b) {
    writeln("Concatenating...");
    return a ~ b;
}

void main() {
    Assert(true, fun("O hai thar! ", "wyda"));
    Assert(false, fun("O hai thar! ", "wyda"));
}

This example only prints "Concatenatning..." once, meaning that fun is 
also lazified!!!

This is very exciting! The fact that this little anomaly hasn't caused 
trouble is a good sign it could actually replace lazy!


Andrei