map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn] map f [x1, x2, ...] == [f x1, f x2, ...]
>>> map (+1) [1, 2, 3] [2,3,4]
map id = cat map (g . f) = map f >-> map g
>>> let message = pack "I am not angry. Not at all." >>> T.map (\c -> if c == '.' then '!' else c) message "I am not angry! Not at all!"
unstream . map f . stream = map f
>>> HashSet.map show (HashSet.fromList [1,2,3]) HashSet.fromList ["1","2","3"]