Servant.Multipart

Combinator for specifying a multipart/form-data request body, typically (but not always) issued from an HTML <form>.

multipart/form-data can't be made into an ordinary content type for now in servant because it doesn't just decode the request body from some format but also performs IO in the case of writing the uploaded files to disk, e.g in /tmp, which is not compatible with servant's vision of a content type as things stand now. This also means that MultipartForm can't be used in conjunction with ReqBody in an endpoint.

The tag type parameter instructs the function to handle data either as data to be saved to temporary storage (Tmp) or saved to memory (Mem).

The a type parameter represents the Haskell type to which you are going to decode the multipart data to, where the multipart data consists in all the usual form inputs along with the files sent along through <input type="file"> fields in the form.

One option provided out of the box by this library is to decode to MultipartData.

Example:

  type API = MultipartForm Tmp (MultipartData Tmp) :> Post '[PlainText] String

  api :: Proxy API
  api = Proxy

  server :: MultipartData Tmp -> Handler String
  server multipartData = return str

    where str = "The form was submitted with "
             ++ show nInputs ++ " textual inputs and "
             ++ show nFiles  ++ " files."
          nInputs = length (inputs multipartData)
          nFiles  = length (files multipartData)
  

You can alternatively provide a FromMultipart instance for some type of yours, allowing you to regroup data into a structured form and potentially selecting a subset of the entire form data that was submitted.

Example, where we only look extract one input, username, and one file, where the corresponding input field's name attribute was set to pic:

  data User = User { username :: Text, pic :: FilePath }

  instance FromMultipart Tmp User where
    fromMultipart multipartData =
      User <$> lookupInput "username" multipartData
           <*> fmap fdPayload (lookupFile "pic" multipartData)

  type API = MultipartForm Tmp User :> Post '[PlainText] String

  server :: User -> Handler String
  server usr = return str

    where str = username usr ++ "'s profile picture"
             ++ " got temporarily uploaded to "
             ++ pic usr ++ " and will be removed from there "
             ++ " after this handler has run."
  

Note that the behavior of this combinator is configurable, by using serveWith from servant-server instead of serve, which takes an additional Context argument. It simply is an heterogeneous list where you can for example store a value of type MultipartOptions that has the configuration that you want, which would then get picked up by servant-multipart.

Important: as mentionned in the example above, the file paths point to temporary files which get removed after your handler has run, if they are still there. It is therefore recommended to move or copy them somewhere in your handler code if you need to keep the content around.

MultipartForm which can be modified with Lenient.

What servant gets out of a multipart/form-data form submission.

The type parameter tag tells if MultipartData is stored as a temporary file or stored in memory. tag is type of either Mem or Tmp.

The inputs field contains a list of textual Inputs, where each input for which a value is provided gets to be in this list, represented by the input name and the input value. See haddocks for Input.

The files field contains a list of files that were sent along with the other inputs in the form. Each file is represented by a value of type FileData which among other things contains the path to the temporary file (to be removed when your handler is done running) with a given uploaded file's content. See haddocks for FileData.

MultipartData is the type representing multipart/form-data form inputs. Sometimes you may instead want to work with a more structured type of yours that potentially selects only a fraction of the data that was submitted, or just reshapes it to make it easier to work with. The FromMultipart class is exactly what allows you to tell servant how to turn "raw" multipart data into a value of your nicer type.

  data User = User { username :: Text, pic :: FilePath }

  instance FromMultipart Tmp User where
    fromMultipart form =
      User <$> lookupInput "username" (inputs form)
           <*> fmap fdPayload (lookupFile "pic" $ files form)
  

Lookup a textual input with the given name attribute.

Lookup a file input with the given name attribute.

Global options for configuring how the server should handle multipart data.

generalOptions lets you specify mostly multipart parsing related options, such as the maximum file size, while backendOptions lets you configure aspects specific to the chosen backend. Note: there isn't anything to tweak in a memory backend (Mem). Maximum file size etc. options are in ParseRequestBodyOptions.

See haddocks for ParseRequestBodyOptions and TmpBackendOptions respectively for more information on what you can tweak.

Default configuration for multipart handling.

Uses defaultParseRequestBodyOptions and defaultBackendOptions respectively.

Tag for data stored as a temporary file

Configuration for the temporary file based backend.

You can configure the way servant-multipart gets its hands on a temporary directory (defaults to getTemporaryDirectory) as well as the filename pattern used for generating the temporary files (defaults to calling them servant-multipartXXX.buf, where XXX is some random number).

Tag for data stored in memory

Default options for the temporary file backend: getTemporaryDirectory and "servant-multipart.buf"

Representation for a textual input (any <input> type but file).

<input name="foo" value="bar" /> would appear as Input "foo" "bar".

Representation for an uploaded file, usually resulting from picking a local file for an HTML input that looks like <input type="file" name="somefile" />.

The ToMultipartSample class allows you to create sample MultipartData inputs for your type for use with Servant.Docs. This is used by the HasDocs instance for MultipartForm.

Given the example User type and FromMultipart instance above, here is a corresponding ToMultipartSample instance:

  data User = User { username :: Text, pic :: FilePath }

  instance ToMultipartSample Tmp User where
    toMultipartSamples proxy =
      [ ( "sample 1"
        , MultipartData
            [ Input "username" "Elvis Presley" ]
            [ FileData
                "pic"
                "playing_guitar.jpeg"
                "image/jpeg"
                "tmpservant-multipart000.buf"
            ]
        )
      ]