Why Dart Types Are Optional and Unsound contains the following example:
class Mammal {}
class Cow extends Mammal { String moo() => 'moo!'; }
class Pig extends Mammal { String oink() => 'oink!'; }
Mammal mammal = new Mammal();
Cow cow = new Cow();
Pig pig = new Pig();
mammal = cow; // [1]
pig = mammal; // [2] Checks OK statically, but now pig holds a Cow.
print(pig.oink()); // [3] NoSuchMethodException if we get this far.
In
production mode, the type declaration that pig holds instances of Pig is simply ignored at [2]! IMO, this should never happen. The Dart team's reasoning is that this is still "safe", because you'll get a NoSuchMethodException at [3].
But think about it: you might get the NoSuchMethodException at a completely different place in the program! What good are type declarations/assertions when you never know whether they are true? It actually feels worse than having no type declarations at all!
Thankfully, in Dart's
checked mode,
you will get the error at [2], making this behavior less troubling. But given that the page says that
"type checks are not a major drain on performance", why have production mode at all?
My understanding was that the prime original motivation for unchecked mode was indeed performance. As our Dart implementations are getting more mature, we're starting to get a better picture of how it does actually impact performance.
I think what we're starting to see is that the performance implications aren't that bad. Given that, I think we're focusing less and less on production mode. It's still a valid option and it may make sense for some users but it may not be commonly used, sort of like -Ofast in gcc.
