Simple Haskell function to handle HTML attributes that may or may not have a value specified.

html_attributes.hs

-- Simple function to handle HTML attributes that may or may not have a value specified.

import qualified Data.Map as Map
import Data.Map (Map)

-- HTML Property Key | Value
type Prop k v = Either k v

key = Left
val = Right

htmlProps :: Ord k => [Prop k v] -> Maybe (Map k (Maybe v))
htmlProps = fmap Map.fromList . go
  where go :: [Prop k v] -> Maybe [(k, Maybe v)]
        go [] = Just []
        go [Left k] = Just [(k, Nothing)] 
        go (Left k : rest@(Left _ : _)) = ((k, Nothing) :) <$> go rest
        go (Left k : Right v : rest) = ((k, Just v) :) <$>  go rest
        go (Right _ : _) = Nothing

-- Example Usage
-- This prints output: Just (fromList [("class",Just "foo bar baz"),("hidden",Nothing),("id",Just "pass"),("required",Nothing)])
main = print $ htmlProps [
        key "id", val "pass", 
        key "required", 
        key "hidden", 
        key "class", val "foo bar baz"
      ]