Often, the best way to search pretty much anything is to let one of the big search engines do it: they're usually even better at it than a site's own search functionality (with the result that things like mailing lists usually don't bother to provide their own, they just invite the search engines to index the whole archive). So something like "site:mail.haskell.org inurl:haskell-cafe some search string" in google / bing / ddg / whatever (the exact syntax may vary by search engine).