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Enabling type checks

Posted by Seth Ladd

Adding types to your Dart program helps express your intent more clearly to your fellow developers. The Dart platform can also use those type annotations to catch errors and warn you of problems from mismatches or bugs from incorrect types. Turning on "checked mode" will enable these type checks, catching your bugs early and providing valuable feedback during development.

The Dart VM can run in two modes: checked and production. Checked mode turns on type assertions, and reports on type mismatches and bugs. Production mode runs with the type assertions turned off, which may result in a speed boost.

We highly encourage you to run in checked mode during any and all development. In checked mode, you will see errors like "type 'OneByteString' is not assignable to type 'num' of 'a'" if there are type mismatches:

To enabled checked mode, use these methods:
  • for the VM: --enable_type_checks --enable_asserts
  • for Dartium:  DART_FLAGS='--enable_type_checks --enable_asserts' path/chrome
  • for frog: --enable_type_checks
You can download Dartium, and find the VM and fog in the SDK.

Jim Hugunin, an engineer on the Dart project, has this perspective:

I really like both static and dynamically typed languages.  Up until joining the Dart team, my preferred development experience was a mix of Python and C# - with a lot of work invested to make that pleasant on both ends.  I find dynamic typing much more productive for the early stages of development and I find static typing much more productive when working on more mature code with larger teams.

I much prefer Python over "looser" dynamic languages like JS, Perl or TCL because it is more agressive about providing runtime errors when I do something stupid.  Simple things like o.x throwing an exception when o has no member named 'x' or 2 + "2" throwing an exception rather than trying to guess if the result should be "22" or 4.  These runtime type errors do slightly reduce my flexibility, but the errors that they find early in my code are always worth the tradeoff.

What gets me most excited about Dart as a language is its mix of these two programming paradigms that lets me move between them as fluidly as possible.  However, if I don't enable runtime type checks, I often feel that Dart is more like TCL or Perl rather than my friendly Python.
In summary, please use checked mode when developing Dart code. Checked mode combined with type annotations will help you catch errors and bugs early, always a good thing.