They used qualified imports:

    import qualified Data.ByteString as ByteString
    import qualified Data.Map.Strict as Map

So everything from “Data.ByteString” has to be prefixed with “ByteString.”

In real-world code, depending on conventions, you’d probably add unqualified imports for the type names to avoid the needless duplication:

    import Data.ByteString (ByteString)
    import Data.Map.Strict (Map)
    import qualified Data.ByteString as ByteString
    import qualified Data.Map.Strict as Map

    type Chan = Map Int ByteString

    pushMsg =
      …
        let inserted = Map.insert …