A sink consumes a stream of data, and produces a result. A sink must always produce a result, and must always produce a single result. This is encoded in the types themselves.

There is a Monad instance for sink, making it simple to compose multiple sinks together into a larger sink. You can also use the built-in sink functions to perform most of your work. Like sources, you’ll rarely need to dive into the inner workings. Let’s start off with an example: getting lines from a stream of Chars (we’ll assume Unix line endings for simplicity).

import Data.Conduit
import qualified Data.Conduit.List as CL

-- Get a single line from the stream.
sinkLine :: Resource m => Sink Char m String
sinkLine = sinkState
    id -- initial state, nothing at the beginning of the line