Dartboard does not reject top level final variables initialised with something other than a compile-time constant

[DartBot]

This issue was originally filed by [email protected]

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The Dart draft spec version 0.01 states that "A top level variable marked final must be initialized to a compile-time constant or a compile-time error occurs." but when I run this code in Dartboard there is no compile-time error:

duplicate(x) {return '$x $x';}
final String message = duplicate("Hello, World!");
main() => print(message);

[DartBot]

This comment was originally written by [email protected]

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I'm guessing the spec is mis-worded then, because that makes 'final' relatively useless (might as well have used if-def macros).

My guess is that they are mimicing C++ in several ways.. And the following is the C++ equiv:

class Foo {
  const std::string message;
  static std::string duplicate(std::string x) { return x + " " + x; }

  Foo() : message(duplicate("Hello, World!")) { }
};

In other words, message is RUN-TIME determinable prior to the entry of the constructor (and quite possibly compile-time-optimizable, dependent on the implementation of std::string in this case).

Specifically, you need to be able to do the following in Dart:

class Foo {
  final List params;
  Foo(a,b,c) : params = const[a,b,c] { }
}

And that vector initialization is NOT compile-time determinable because a,b,c are run-time parameters.

[floitschG]

The spec is correct, and there is already a CL (changelist) under review:
http://codereview.chromium.org/8231031

As to the class Foo example: this is not supported. We discussed it for a long time, but due to its complexity decided against it. We might revisit it in the future, but for now we will try to program without it and see if there are patterns that can easily work around it.

Why is it complex. Let's assume we allow const lists in the initializer list that depend on the arguments:
class Foo {
  final List params;
  const Foo(a,b,c) : params = const[a,b,c];
}

First, note that there are two ways to invoke the constructor of "Foo". "new Foo(x,y,z)" and "const Foo(r,s,t)". r,s,t must be compile time constants (requirement of const constructions).
Clearly we could make this work for the case of "const Foo". Simply propagate r,s and t, and everything falls into place. But what about "new Foo"? If x, y, and z are not compile-time constants we would call "const[a,b,c]" with non-compile time constants, which is a big no-no.

There are some ways to work around this (like automatically changing the 'const' to 'new', or by adding a new keyword) but these solutions are hackish and we therefore decided against it.

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Set owner to [email protected].
Added Area-Compiler, Started labels.

[DartBot]

This comment was originally written by [email protected]

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r468 addresses most compile-time constant assignment issues. There are other issues that are filed for the bugs that are remaining.

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Added Fixed label.