What we want for Map is definitely not that, but rather ??? :: Ord k => k -> Map k v -> Maybe (k, v) In each case we look up a key to retrieve an "entry" (whether we want that terminology or not). In the case of a Map, the entry is a key-value pair; in the case of a Set it is merely a key. On Jul 4, 2016 11:48 AM, "Nicolas Godbout" <nicolas.godbout@gmail.com> wrote: Correction, there are several errors in the signatures at the end of my last message. > lookup??? :: Eq a => a -> [a] -> Maybe a > lookup??? :: Ord a => a -> Set a -> Maybe a Also, copying the pattern for maps gives a slightly stranger signature > lookup??? :: (Ord k, Eq a) => (k,a) -> Map k a -> Maybe (k,a) which combines a key lookup with an equality test on the value. Nicolas. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries