Ganymede is an Io interpreter, originally based on Martin "vague" Sandin's Amalthea, which is an Io interpreter written in OCaml. There are some differences since Amalthea is based on the description of Io found in Raphael Finkel's 1996 book "Advanced Programming Language Design" (APLD), whereas Ganymede is more faithful to Raph Levien's original 1989 paper.