# On how to use the *pure* functions:

    read  :: Read a => String -> a
    reads :: Read a => ReadS a

What is this type ReadS a?  Let's ask GHCi:

    λ> :info ReadS
    type ReadS a = String -> [(a, String)]

To use read, we need to specify the type a:

    λ> read "123" :: Int
    123
    λ> read "123" :: Integer
    123
    λ> read "123" :: String
    "*** Exception: Prelude.read: no parse
    λ> read "\"123\"" :: String
    "123"
    λ> read "[12,3]" :: [Int]
    [12,3]

To use reads, we also need to specify the type a:

    λ [Prelude]> reads "123hello" :: [(Integer, String)]
    [(123,"hello")]
    λ [Prelude]> reads "123hello" :: [(String, String)]
    []

What is this?  When we evaluate

    reads x :: [(α, String)]

it returns [] if it cannot parse x as a value of type α.
Otherwise, it returns a singleton list

    [(v, rst)]

where v :: α is the value parsed from the given string x,
and rst is the remaining string.