What is MPS build language?
Build Language is an extensible build automation DSL for defining builds in a declarative way. Generated into Ant, it leverages Ant execution power while keeping your sources clean and free from clutter and irrelevant details. Organized as a stack of MPS languages with ANT at the bottom, it allows each part of your build procedure to be expressed at a different abstraction level. Building a complex artifact (like an MPS plug-in) could be specified in just one line of code, if you follow the language conventions, but, at the same time, nothing prevents you from diving deeper and customize the details like file management or manifest properties.
As with many of build automation tools project definition is the core of the script. Additionally and unlike most of the other tools, Build Language gives you full control over the output directory layout. The expected build result is defined separately in the build script and not as a part of some (third-party) plugin.
Every build script is made up of three parts. The first is dependencies, something required that comes already built. Think of libraries or third-party languages, for example. Next is the project structure. It contains declarations of everything you have in your repository and what is going to be build, as well as the required build parameters. Note that declaring an item here does not trigger its build unless it is needed, i.e. refered to from the last part of the script - the output layout. The output could be as straightforward as a set of plain folders and copied files, or much more complex with yipped artifacts such as packaged plug-ins or MPS languages. For example, to build a jar file out of Java sources you need to declare a Java module in the project structure and the respective jar file with a reference to the module in the output layout.
Thanks to MPS, Build Language comes with concise textual notation and an excellent editing experience, including completion and on-the-fly validation. Extension languages (or plugins if we stick to the terminology of the other build tools) add additional abstractions on top of the language. In our experience, it is quite an easy process to create a new one compared to developing Maven or Gradle plugins.