6.1 Using Rassumfrassum ¶

Rassumfrassum is an LSP server multiplexer program that fits the bill. Like most language servers, it must be installed separately since it is not bundled with Emacs (at time of writing). The installation is similar to installing any other language server, and usually amounts to making sure the program executable is somewhere in PATH or exec-path.

The Rassumfrassum program, invoked via the rass command, works by spawning multiple LSP server subprocesses and aggregating their capabilities, requests, and responses into a single unified LSP interface. From Eglot’s perspective, it appears to be communicating with a single server.

To use Rassumfrassum with Eglot, you can start it interactively with a prefix argument to eglot and specify the rass command followed by the actual servers you want to use, separated by --:

C-u M-x eglot RET rass -- clangd -- codebook-lsp serve RET

This starts clangd for C++ language support and codebook-lsp for spell-checking in the same buffer.

For Python, you might use:

C-u M-x eglot RET rass -- ty server -- ruff server RET

or simply C-u M-x eglot RET rass python, using the “preset” feature. This combines ty for type checking with ruff for linting and formatting.

These configurations can be integrated into the eglot-server-programs variable (see Setting Up LSP Servers) for automatic use:

(with-eval-after-load 'eglot
 (add-to-list 'eglot-server-programs
 '(c-ts-base-mode . ("rass" "--" "clangd" "--"
 "codebook-lsp" "serve")))
 (add-to-list 'eglot-server-programs
 '(python-mode . ("rass" "--" "ty" "server" "--"
 "ruff" "server"))))

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