config :: [(String, [(String, String)])]; config = [ ("database", [("path", "var/app/db"), ("encoding", "euc-jp")]), ("urlmapper", [("cgiurl", "/app"), ("rewrite", "True")]), ("template", [("path", "/var/app/template")]) ]
main = print $ case (lookup "database" config) of Just entries -> lookup "encoding" entries Nothing -> Nothing
↓
main = print $ lookup "database" config >>= lookup "encoding"
↓
main = print $ return config >>= lookup "database" >>= lookup "encoding"
