List of checkboxes with digestive-functors

Question

How do I use digestive functors to create a form that has a programmatically generated list of checkboxes, which would return a list. For example:

[x] Milk
[ ] Cereals
[x] Ground meat

would return ["Milk", "Ground meat"].

I'm expecting the type would be something like:

form :: (Functor m, Monad m) => [String] -> HappstackForm m Html BlazeFormHtml [String]

Answer 1

There is no standard way to do so, but digestive-functors is highly composable using the Applicative interface, so you can easily create what you want.

You can define a checkBox which returns a Maybe String, i.e. the name of the element if it was checked.

checkBox :: (Functor m, Monad m)
         => String -> HappstackForm m Html BlazeFormHtml (Maybe String)
checkBox str = fmap maybeStr (inputCheckBox False) <++ label str
  where
    maybeStr True  = Just str
    maybeStr False = Nothing

You can then loop over a list of strings to create a checkbox like this for each element in the list:

listForm' :: (Functor m, Monad m)
          => [String]
          -> HappstackForm m Html BlazeFormHtml [Maybe String]
listForm' = foldr (\x xs -> fmap (:) x <*> xs) (pure []) . map checkBox

The catMaybes :: [Maybe a] -> [a] helps you to reduce the result further:

listForm :: (Functor m, Monad m)
         => [String]
         -> HappstackForm m Html BlazeFormHtml [String]
listForm = fmap catMaybes . listForm'

And finally, we can instantiate the actual form:

food :: [String]
food = ["Milk", "Cereals", "Ground meat"]

foodForm :: (Functor m, Monad m)
         => HappstackForm m Html BlazeFormHtml [String]
foodForm = listForm food