How to understand Haskell function parameters

Question

In the following function definition:

app :: Application
app _ respond = do
    putStrLn "I've done some IO here"
    respond $ responseLBS
        status200
        [("Content-Type", "text/plain")]
        "Hello, Web!"

Hello the app function doesn't seem to take arguments. Why in the above example there are two arguments?

It does Application is a type synonym for Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived: hackage.haskell.org/package/wai-3.2.2.1/docs/… — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Sep 5, 2019 at 18:22

willeM_ Van Onsem · Accepted Answer · 2019-09-05 18:25:51Z

8

Application is a type synonym. It is defined as:

type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived

It is thus a function that takes a Request, and a function that maps an Response on an IO ResponseReceived, and then produces an IO ResponseReceived. Usually such functions produce a Response that is then post-processed by the response.

answered Sep 5, 2019 at 18:25

willeM_ Van Onsem

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