checkboxesFieldList and multiSelectField and any other multiple field lose information on error

[ricksanchez]

The Field has some strange design:
the fieldParse and the fieldView has something that looks like contradiction.
See the types:
fieldParse :: [Text] -> [FileInfo] -> m (Either (SomeMessage (HandlerSite m)) (Maybe a))
fieldView ::FieldViewFunc m a

FieldViewFunc sadly doesn't follow the design of fieldParse.

fieldParse takes [Text] (n inputs) but fieldView takes Left Text a (only one). This is a big limitation for implementing dynamic forms that has "add row" option with a common submit.

This design affects the checkboxesFieldList and multiSelectField (sabotage only one value, and the whole widget will be blank).

Would it be wise to change the type of FieldViewFunc to

type FieldViewFunc m a
    = Text -- ^ ID
   -> Text -- ^ Name
   -> [(Text, Text)] -- ^ Attributes
   -> [Either Text a] -- ^ A list of Either (invalid text) or (legitimate result)
   -> Bool -- ^ Required?
   -> WidgetT (HandlerSite m) IO ()

?

[snoyberg]

It's probably a worthwhile change, and may not have a massive backwards compatibility hit (since I'm guessing most people are using built-in fields). I haven't thought through the impacts of this yet, and it may have negative results that prevent it from being possible.

The best way to move forward would likely be a PR making the change, though anyone who works on it needs to be aware that there's a real possibility that the PR wouldn't be accepted. Just don't want to set any false expectations.

[nmk]

This is an actual bug though. As @ricksanchez points out, a checkboxesField (and I assume multiSelectField as well) which fails validation loses the checked state of the checkboxes. Here is a demonstration: https://gist.github.com/adbd511ecd1144ed64dd75f9bcccbd4e

I ran into this while looking around the code for #1541. Fixing the problem there is easy, as a select field only has one submitted value, a checkboxesField has multiple, yet in the Left case in https://github.com/yesodweb/yesod/blob/master/yesod-form/Yesod/Form/Fields.hs#L501 only the first submitted value is to be found, there is no way to salvage the other ones.

Does anyone have a pointer on where to start looking in order to fix this?