The last version of RASM is v0.104 (17th October 2018). This multi platform assembler/disassembler (linux, windows, but not only like MorphOS on Amiga) let you program for Amstrad CPC.
The online documentation is available.
Information by Krusty (Benediction) : For those using Visual Studio Code as Integrated Development Environment for the Amstrad CPC, you can install the z80 macro assembler extension that provides syntaxic coloration of code and some snippets for rasm. On my side it has completely replaced vim and its tailored extension. Syntax error analysis is only the missing thing.