Support for Servant-Multipart

[rsoeldner]

I would like to use servant-multipart with reflex for file upload. Could this be easily added ?

[imalsogreg]

A couple things make this tricky to support

1. servant-multipart packages both the API type and the servant-server instances - this brings wai and other server-side packages into the dependency tree of your frontend. Last time I checked, ghcjs fails to compile in this case (but maybe that's changed recently?). We would have to either overrive servant-multipart and stub out the server-side parts, or get servant-multipart maintainers to split their package into servant-multipart-types and servant-multipart, so that we could depend on the former cleanly.

2. I'm not sure what servant-reflex should provide as an interface for MultipartForm. Looking at the source for servant-multipart, I think we would want to use their ToMultipart class and the functions that turn multipart data into a RequestBody... however RequestBody type comes from servant-client-core, which servant-reflex doesn't integrate with yet, but we could.

Are you interested in working on these?

[rsoeldner]

This looks not so easy. I have seperated the servant-multipart package see here: https://github.com/rsoeldner/servant-multipart

I tried adding the following instance to servant-reflex as a starting point:

  instance  {-# OVERLAPPABLE #-}
    (SupportsServantReflex t m, HasClient t m sublayout tag 
    ) => HasClient t m ((MultipartForm tt (MultipartData tt)) :> sublayout ) tag where
  
    type Client t m ((MultipartForm tt (MultipartData tt)) :> sublayout) tag =
      Event t (MultipartData tt) -> Client t m sublayout tag 
  
    clientWithRouteAndResultHandler Proxy q t req baseurl opts wrap trigs =
      clientWithRouteAndResultHandler (Proxy :: Proxy sublayout) q t req baseurl opts wrap

I tried to convert MultipartData using multipartToBody boundry $ toMultipart but
multipartToBody :: forall tag. MultipartBackend tag => LBS.ByteString -> MultipartData tag -> RequestBody requiresMultipartBackend.

Additionally I'm not sure how to fill the reqBody https://hackage.haskell.org/package/servant-reflex-0.3.4/docs/src/Servant.Common.Req.html#reqBody

Can you provide guidance ?

[imalsogreg]

Ooh, that looks tough to get around. I expect that the dependency on wai comes from the need to be able to stream bytes into the http request. I see two paths forward.

1. Give up on any POST requests that can't fit in memory; in this case toRequestBody could be written as a pure ByteString manipulation function.
2. Replace the request body handling in servant-reflex so that it's compatible with the javascript FormData object, which I'm only now becoming aware of now that I read about multipart-form at https://stackoverflow.com/questions/35192841/fetch-post-with-multipart-form-data

[imalsogreg]

Oh also, I forgot to explain reqBody. It's type Maybe (Dynamic t (Either Text (BL.ByteString, Text))) isn't document well enough.

• The Maybe allows a req body to optionally have no request body (for non-POST, non-PUT endpoints).
• The Dynamic is similar to all other dynamics in the request - it gets updated along with the widgets whose values are being serialized into your request body.
• The Either Text is used to indicate that something in the widgets was not acceptable as input in your form
• The (BL.ByteString, Text) store the request body bytestring, and the content-type respectively

Happy to elaborate on any of those - I know the Dynamic (Either ... pattern tends to be really counter-intuitive.