http-conduit - Appendices - Developing Web Apps with Haskell and Yesod, Second Edition (2015)

Developing Web Apps with Haskell and Yesod, Second Edition (2015)

Part IV. Appendices

Appendix D. http-conduit

Most of Yesod is about serving content over HTTP. But that’s only half the story: someone has to receive it. And even when you’re writing a web app, sometimes that someone will be you. If you want to consume content from other services or interact with RESTful APIs, you’ll need to write client code. And the recommended approach for that is http-conduit.

NOTE

This chapter is not directly connected to Yesod, and will be generally useful for anyone wanting to make HTTP requests.

Synopsis

{-# LANGUAGE OverloadedStrings #-}

importNetwork.HTTP.Conduit -- the main module

-- The streaming interface uses conduits

importData.Conduit

importData.Conduit.Binary (sinkFile)

importqualifiedData.ByteString.Lazyas L

importControl.Monad.IO.Class (liftIO)

importControl.Monad.Trans.Resource (runResourceT)

main ::IO ()

main =do

-- Simplest query: just download the information from the given URL as a

-- lazy ByteString.

simpleHttp "http://www.example.com/foo.txt" >>= L.writeFile "foo.txt"

-- Use the streaming interface instead. We need to run all of this inside a

-- ResourceT, to ensure that all our connections get properly cleaned up in

-- the case of an exception.

runResourceT $ do

-- We need a Manager, which keeps track of open connections. simpleHttp

-- creates a new manager on each run (i.e., it never reuses

-- connections).

manager <-liftIO $ newManager conduitManagerSettings

-- A more efficient version of the simpleHttp query above. First we

-- parse the URL to a request.

req <-liftIO $ parseUrl "http://www.example.com/foo.txt"

-- Now get the response

res <-http req manager

-- And finally stream the value to a file

responseBody res $$+- sinkFile "foo.txt"

-- Make it a POST request, don't follow redirects, and accept any

-- status code

let req2 =req

{ method ="POST"

, redirectCount =0

, checkStatus =\___->Nothing

}

res2 <-http req2 manager

responseBody res2 $$+- sinkFile "post-foo.txt"

Concepts

The simplest way to make a request in http-conduit is with the simpleHttp function. This function takes a String giving a URL and returns a ByteString with the contents of that URL. But under the surface, there are a few more steps:

§ A new connection Manager is allocated.

§ The URL is parsed to a Request. If the URL is invalid, then an exception is thrown.

§ The HTTP request is made, following any redirects from the server.

§ If the response has a status code outside the 200 range, an exception is thrown.

§ The response body is read into memory and returned.

§ runResourceT is called, which will free up any resources (e.g., the open socket to the server).

If you want more control over what’s going on, you can configure any of these steps (plus a few more) by explicitly creating a Request value, allocating your Manager manually, and using the http and httpLbs functions.

Request

The easiest way to create a Request is with the parseUrl function. This function will return a value in any Failure monad, such as Maybe or IO. The last of those is the most commonly used, and results in a runtime exception whenever an invalid URL is provided. However, you can use a different monad if, for example, you want to validate user input:

importNetwork.HTTP.Conduit

importSystem.Environment (getArgs)

importqualifiedData.ByteString.Lazyas L

importControl.Monad.IO.Class (liftIO)

main ::IO ()

main =do

args <-getArgs

case args of

[urlString] ->

case parseUrl urlString of

Nothing->putStrLn "Sorry, invalid URL"

Just req ->withManager $ \manager ->do

res <-httpLbs req manager

liftIO $ L.putStr $ responseBody res

_->putStrLn "Sorry, please provide exactly one URL"

The Request type is abstract, so that http-conduit can add new settings in the future without breaking the API (see Appendix C for more information). In order to make changes to individual records, you use record notation. For example, a modification to our program that issues HEADrequests and prints the response headers would be:

{-# LANGUAGE OverloadedStrings #-}

importNetwork.HTTP.Conduit

importSystem.Environment (getArgs)

importqualifiedData.ByteString.Lazyas L

importControl.Monad.IO.Class (liftIO)

main ::IO ()

main =do

args <-getArgs

case args of

[urlString] ->

case parseUrl urlString of

Nothing->putStrLn "Sorry, invalid URL"

Just req ->withManager $ \manager ->do

let reqHead =req { method ="HEAD" }

res <-http reqHead manager

liftIO $ do

print $ responseStatus res

mapM_ print $ responseHeaders res

_->putStrLn "Sorry, please provide example one URL"

There are a number of different configuration settings in the API; some noteworthy ones are:

proxy

Allows you to pass the request through the given proxy server.

redirectCount

Indicates how many redirects to follow. The default is 10.

checkStatus

Checks the status code of the return value. By default, gives an exception for any non-2XX response.

requestBody

Specifies the request body to be sent. Be sure to also update the method. For the common case of URL-encoded data, you can use the urlEncodedBody function.

Manager

The connection manager allows you to reuse connections. When making multiple queries to a single server (e.g., accessing Amazon S3), this can be critical for creating efficient code. A manager will keep track of multiple connections to a given server (taking into account ports and SSL as well), automatically reaping unused connections as needed. When you make a request, http-conduit first tries to check out an existing connection. When you’re finished with the connection (if the server allows keep-alive), the connection is returned to the manager. If anything goes wrong, the connection is closed.

To keep our code exception-safe, we use the ResourceT monad transformer. All this means for you is that your code needs to be wrapped inside a call to runResourceT, either implicitly or explicitly, and that code inside that block will need to use liftIO to perform normal IO actions.

There are two ways you can get ahold of a manager. newManager will return a manager that will not be automatically closed (you can use closeManager to do so manually), while withManager will start a new ResourceT block, allow you to use the manager, and then automatically close the ResourceT when you’re done. If you want to use a ResourceT for an entire application, and have no need to close it, you should probably use newManager.

One other thing to point out: you obviously don’t want to create a new manager for each and every request; that would defeat the whole purpose. You should create your Manager early and then share it.

Response

The Response data type contains three pieces of information: the status code, the response headers, and the response body. The first two are straightforward; let’s discuss the body.

The Response type has a type variable to allow the response body to be of multiple types. If you want to use http-conduit’s streaming interface, you want this to be a Source. For the simple interface, it will be a lazy ByteString. One thing to note is that, even though we use a lazyByteString, the entire response is held in memory. In other words, we perform no lazy I/O in this package.

NOTE

The conduit package does provide a lazy module that will allow you to read this value in lazily, but like any lazy I/O, it’s a bit unsafe, and definitely nondeterministic. If you need it, though, you can use it.

http and httpLbs

So let’s tie it together. The http function gives you access to the streaming interface (i.e., it returns a Response using a ResumableSource), while httpLbs returns a lazy ByteString. Both of these return values in the ResourceT transformer so that they can access the Manager and have connections handled properly in the case of exceptions.

NOTE

If you want to ignore the remainder of a large response body, you can connect to the sinkNull sink. The underlying connection will automatically be closed, preventing you from having to read a large response body over the network.