140 lines
5 KiB
Text
140 lines
5 KiB
Text
# File Upload (`multipart/form-data`)
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In this recipe, we will implement a web application
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with a single endpoint that can process
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`multipart/form-data` request bodies, which most
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commonly come from HTML forms that allow file upload.
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As usual, a bit of throat clearing.
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``` haskell
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE TypeOperators #-}
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{-# LANGUAGE OverloadedStrings #-}
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import Control.Concurrent
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import Control.Exception
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import Control.Monad
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import Control.Monad.IO.Class
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import Data.Text.Encoding (encodeUtf8)
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import Network.Socket (withSocketsDo)
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import Network.HTTP.Client hiding (Proxy)
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import Network.HTTP.Client.MultipartFormData
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import Network.Wai.Handler.Warp
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import Servant
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import Servant.Multipart
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import qualified Data.ByteString.Lazy as LBS
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```
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Our API consists in a single `POST` endpoint at `/`
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that takes a `multipart/form-data` request body and
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pretty-prints the data it got to stdout before returning `0`
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(because why not).
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``` haskell
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type API = MultipartForm Mem (MultipartData Mem) :> Post '[JSON] Integer
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api :: Proxy API
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api = Proxy
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```
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Because of some technicalities, multipart form data is not
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represented as a good old content type like `JSON` in servant,
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that one could use with `ReqBody`, but instead is its own
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dedicated `ReqBody`-like combinator named
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[`MultiPartForm`](https://hackage.haskell.org/package/servant-multipart-0.11/docs/Servant-Multipart.html#t:MultipartForm).
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This combinator takes two parameters. The first one is the
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"backend" to use. Currently, you only have the choice between
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`Mem` and `Tmp`. The former loads the entire input in memory,
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even the uploaded files, while `Tmp` will stream uploaded
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files to some temporary directory.
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The second parameter is the type you want the multipart data
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to be decoded to. Indeed there is a `FromMultipart` class that
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allows you to specify how to decode multipart form data from
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`MultipartData` to a custom type of yours. Here we use the
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trivial "decoding" to `MultipartData` itself, and simply
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will get our hands on the raw input. If you want to use
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a type of yours, see the documentation for
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[`FromMultipart`](https://hackage.haskell.org/package/servant-multipart-0.11/docs/Servant-Multipart.html#t:FromMultipart).
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Our only request handler has type `MultipartData Mem -> Handler Integer`.
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All it does is list the textual and file inputs that
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were sent in the multipart request body. The textual
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inputs are in the `inputs` field while the file inputs
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are in the `files` field of `multipartData`.
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``` haskell
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-- MultipartData consists in textual inputs,
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-- accessible through its "inputs" field, as well
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-- as files, accessible through its "files" field.
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upload :: Server API
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upload multipartData = do
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liftIO $ do
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putStrLn "Inputs:"
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forM_ (inputs multipartData) $ \input ->
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putStrLn $ " " ++ show (iName input)
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++ " -> " ++ show (iValue input)
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forM_ (files multipartData) $ \file -> do
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let content = fdPayload file
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putStrLn $ "Content of " ++ show (fdFileName file)
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LBS.putStr content
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return 0
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startServer :: IO ()
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startServer = run 8080 (serve api upload)
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```
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Finally, a main function that brings up our server and
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sends some test request with `http-client` (and not
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servant-client this time, has servant-multipart does not
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yet have support for client generation.
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``` haskell
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main :: IO ()
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main = withSocketsDo . bracket (forkIO startServer) killThread $ \_threadid -> do
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-- we fork the server in a separate thread and send a test
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-- request to it from the main thread.
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manager <- newManager defaultManagerSettings
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req <- parseRequest "http://localhost:8080/"
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resp <- flip httpLbs manager =<< formDataBody form req
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print resp
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where form =
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[ partBS "title" "World"
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, partBS "text" $ encodeUtf8 "Hello"
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, partFileSource "file" "./README.md"
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]
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```
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If you run this, you should get:
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``` bash
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$ cabal new-build cookbook-file-upload
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[...]
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$ dist-newstyle/build/x86_64-linux/ghc-8.2.1/cookbook-file-upload-0.1/x/cookbook-file-upload/build/cookbook-file-upload/cookbook-file-upload
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Inputs:
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"title" -> "World"
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"text" -> "Hello"
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Content of "README.md"
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# servant - A Type-Level Web DSL
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## Getting Started
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We have a [tutorial](http://docs.servant.dev/en/stable/tutorial/index.html) that
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introduces the core features of servant. After this article, you should be able
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to write your first servant webservices, learning the rest from the haddocks'
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examples.
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[...]
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Response {responseStatus = Status {statusCode = 200, statusMessage = "OK"}, responseVersion = HTTP/1.1, responseHeaders = [("Transfer-Encoding","chunked"),("Date","Fri, 08 Dec 2017 16:50:14 GMT"),("Server","Warp/3.2.13"),("Content-Type","application/json;charset=utf-8")], responseBody = "0", responseCookieJar = CJ {expose = []}, responseClose' = ResponseClose}
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```
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As usual, the code for this recipe is available in a cabal project
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[here](https://github.com/haskell-servant/servant/tree/master/doc/cookbook/file-upload).
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