Resolve HTML Properties into a `Map`, while also dealing with missing / default property values

resolve_html_props.md

Description

In HTML, while defining elements, property values can be omitted. Here, the htmlProps function takes a list of property keys or values, and resolves it into a Map of keys to values by the following rules: - A Key followed by a Val associates the key with the val in result map. - A Key followed by another Key means the first key has value Nothing in result map. - A Val without a Key before it indicates an error.

Code

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 x] = Just [(x, 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

Input

htmlProps [
        key "id", val "pass",
        key "required",
        key "hidden",
        key "class", val "foo bar baz"
      ]

Output (Prettified)

Just (fromList [("class",Just "foo bar baz"),
                ("hidden",Nothing),
                ("id",Just "pass"),
                ("required",Nothing)])

TODO

• Instead of Maybe, htmlProps should return Either indicating position and value where error occured.