The parser and AST lowering is written in scheme. I can't really comment on all the design decisions involved, since I joined the project at a later stage, but

The parser and AST lowering is written in scheme. I can't really comment on all the design decisions involved, since I joined the project at a later stage, but personally I'm rather happy with it and I think especially people new to programming have an easier time with a syntax like this than with S-expressions.