It strikes me that this ability to integrate arbitrary DSLs into a language in a typed fashion could be useful for a shell: if you aliased the built-ins to func

It strikes me that this ability to integrate arbitrary DSLs into a language in a typed fashion could be useful for a shell: if you aliased the built-ins to functions with parameters known to be globs, paths, filenames, regexes, etc. you might be able to add syntax highlighting or auto-completion or even just better error messages.