FieldSection.hs

{-# LANGUAGE FlexibleContexts #-}

-- | A section: Parser and printer.
-- See <https://en.wikipedia.org/wiki/Section_(category_theory)>
--
-- It's like an isomorphism, except one side is not a complete mapping
-- i.e. the parsing side, but the printing side is complete (anything
-- that is printed can be parsed).

module Yesod.Form.FieldSection where

import Data.Text (Text)
import Yesod

-- | Class which forms a section.
class FieldSection a where
    parseField :: Text -> Either Text a
    renderField :: a -> Text

instance FieldSection Text where
    parseField = Right
    renderField = id

-- | Make a field from the section formed by the type @a@ and 'Text'.
fieldSection
    :: (MonadHandler m, RenderMessage (HandlerSite m) Text, FieldSection b)
    => Field m Text -> Field m b
fieldSection field = checkMMap (return . parseField) renderField field