The geiser documentation suggests that setting geiser-default-implementation is one way to prevent run-geiser from prompting for a scheme implementation. Another approach suggested by the geiser docs is to set the geiser-implementations-alist to the following value:
(((regexp "\\.scm$") guile)
((regexp "\\.ss$") racket)
((regexp "\\.rkt$") racket))
In neither case do the docs give examples of how to set. I've tried various incantations involving setq, defcustom, etc., but I continue to be prompted for the desired scheme implementation whenever I run run-geiser. The alist doesn't even evaluate properly: for one thing, the regexp function seems not to exist; for another, I'm thinking some sort of quoting is needed to prevent errors on the undefined guile/racket symbols. Would be grateful if someone could give an example of exactly what would need to be added (e.g.) to .emacs in both cases.
Would also like to understand why something like...
(setq geiser-default-implementation 'racket)
...doesn't seem to work.
You could remove the other implementations from the list of active implementations: