BLUE is an acronym for Build Language User Extensible. It is a functional declarative build-system fully written in Guile.

As opposed to other build-systems, BLUE works as a library that can be used by projects to manage their builds. It is entirely self-contained and can be embedded into existing projects. It provides an optional clean and extensible CLI and extension points for external tools.

BLUE aims to reduce frictions from the build-system by providing a rich extensible API with clear error messages and documentation.