Updating Phoenix from 0.3.1 to 0.4.0

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Problem

You would like to upgrade to the latest version of Phoenix 0.4.0 from the previous version of 0.3.1. When you try to update the dependencies and try to start phoenix again (mix phoenix.start) you get the following error:

Solution

After upgrading your Elixir version to the latest (1.0.0-rc1), and updating the path to your elixir installation so that it uses the new one, clear the dependencies from your project:

Change the dependency in your mix.exs file to be as the following:

and then get the dependencies again with:

after that you should be able to start your phoenix project with tne new version.