Clojure to Haskell

target.clj

(defn- target
  "generate a target from a user and host string.
  If user and host are nil, evaluates to localhost.
  If host is nil, evaluates to user@localhost.
  If user is nil, evaluates to host.
  Otherwise evaluates to user@host."
  [user host]
  (let [host (if (string? host) host "localhost")
        user (if (string? user) (str user "@") "")]
    (str user host)))

target.hs

target :: String -> String -> String
target user host = user ++ "@" ++ host

{- How do I do the cases where host and user are not supplied?
   My hunch is new functions with less args, but maybe there
   is a simpler way -}

targetNoHost :: String -> String
targetNoHost user = target user "localhost"

targetNoUser :: String -> String
targetNoUser host = host

you nee a Maybe instead of a string.

target :: Maybe String -> Maybe String -> String
target :: Nothing Nothing = "localhost"
target :: Nothing Just(host) = host
target :: Just(user) Nothing = "localhost@" ++ user
target :: Just(user) Just(host) = host ++ "@" ++ user

I think this solution gives the desired API.

data NameAndHost = Name String | Host String | Both String String deriving Show

buildHostString :: NameAndHost -> String
buildHostString (Name n)   = n ++ "@localhost"
buildHostString (Host h) = "anonymous@" ++ h
buildHostString (Both n h) = n ++ "@" ++ h

This code snippet can be used as such...

*Main> buildHostString $ Name "ben"
"ben@localhost"
*Main> buildHostString $ Host "google.com"
"[email protected]"
*Main> buildHostString $ Both "ben" "google.com"
"[email protected]"