To run, first get the classpath of a library that you want to explore
# macOS
coursier fetch org.typelevel:cats-core_2.12:1.4.0 -p | pbcopy
# Linux
coursier fetch org.typelevel:cats-core_2.12:1.4.0 -p | xclip
Next open sbt and run the main function.
sbt
> explorer/run <copied cats classpath> cats/Functor#
cats/Functor# => trait Functor[F[_]] extends AnyRef with Invariant[F] with Serializable { +12 decls }