LiveCode is both an open-source and proprietary cross-platform rapid application development language.
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Inside LiveCode's IDE you can build for OSX, Linux and Windows, and (making use of Xcode and the Android SDK) for iOS and Android too
Pro Fast to get results
Pro Extremely simple to get started
Single-click install of entire development environment, and all-in-one IDE makes it easy to go from visually layout out a user interface to writing code to power it to creating your own single-file applications.
Pro Approachable yet capable language
Pro GUI-driven development
In LiveCode, you start by dropping GUI elements into place, then add the code that makes them function. This lets beginners see visual results right away, which is often more exciting (especially for young students). It also lets you focus on the user interaction before you get into the nitty-gritty of code implementation.
Pro Highest ROI compared to other popular cross-platform development tools
Fluid production - from prototyping to delivery in one toolkit. With a high-level language that includes GUI controls as native elements, code more directly expresses the end-user experience. And the platform coverage is awesome, nearly unmatched.
Con Poor ecosystem
Not a mainstream programming language.