Falcon is a new compiler for Flex that is under development. It will eventually replace the legacy compiler that has been part of the Flex SDK.
Current Status
Falcon has completed a legal review at Adobe and is awaiting signature by the relevant Adobe Vice President, after which it will be donated to Apache's donation documents are complete and signed. After they have been sent to Apache and recorded, the code will be added to the repository.
The ActionScript side of Falcon is of shipping quality and is the basis for Flash Builder 4.7. The rest of Falcon (MXML, CSS, FXG, PROPERTIES) is pre-alpha quality and not ready for production use. Nevertheless, Falcon already supports enough of MXML to compile a correctly-running version of Checkinapp, the test app that is part of the SDK. (Exception: Checkinapp uses one Repeater tag, which is not yet implemented.) The main areas where Falcon needs more work are databinding, states, MXML-specific error reporting, and ASDoc generation.
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