add fieldInputChoices for multiple choices selection

[eryx67]

add forms for multiple choice selection and elements for it.
Unfortunately this use case couldn't be done without extending Form.Internal.Field

[cimmanon]

I've been testing this, but it seems like it doesn't properly set the values as selected when looking at the view prior to posting the form? I've tried poking at it myself and can't quite figure out how to debug where the problem is.

module Test () where

import Data.Text (Text)
import Control.Monad ((=<<))
import Text.Digestive

x :: [Text]
x = ["a", "b", "c"]

choiceTest :: Monad m => Formlet Text m [Text]
choiceTest = choices (zip x x)

--test :: [(Text, Text, Bool)]
test = fieldInputChoices "" =<< (getForm "x" $ choiceTest Nothing)

--test2 :: [(Text, Text, Bool)]
test2 = fieldInputChoices "" =<< (getForm "x" $ choiceTest (Just ["a", "b"]))

Output:

*Test> test
[("0","a",False),("1","b",False),("2","c",False)]

*Test> test2
[("0","a",False),("1","b",False),("2","c",False)]

Personally, I think it might be better if Choice was modified to have a type signature of [(Text, [(Text, (a, v))])] -> [Int] -> Field v [(a, Int)], rather than adding a Choices type. The choice collection of functions could easily be converted to working with lists while still returning a Maybe to keep them backwards compatible (see the file and fileMultiple functions).