[imtringued]

It's regular Groovy. Look at this unrelated library that is doing the same thing. Toggle from Groovy to see what it is doing behind the scenes. https://spreadsheet.dsl.builders/#_outlines

The first parameter in a Groovy closure it will be treated as "this" and any function you call can applied to "this" first.

sourceSets is just a function that accepts a closure whose first parameter is sourceSets (this is implementation specific). If you call main inside the closure then groovy will call sourceSets.main and set that property.

What you are probably complaining about is that parenthesis are optional in two contexts. Your function fits on a single line or your last parameter is a closure (curly braces)

You're also confused by the fact that you can pass a map as a parameter by simply doing "function key1: name1, key2: name2".