Question

I have a form in which the user can either select from a pre-existing list of values, or create a new (Text) value.

What will be the most elegant way to code that?

My best (and IMO not so elegant) way of doing so was by defining:

data MyInput = MyInput {createNew :: Bool, newVal :: Maybe Text, existingVal :: Maybe Text}

myForm :: [(Text,Text)] -> Html -> MForm MySite MySite (FormResult MyInput,Widget)
myForm exisingVals = renderTable $ MyInput
    <$> areq boolField "Create new" (Just False)
    <*> aopt textField "New val" Nothing
    <*> aopt (selectField existingVals) "Existing values" Nothing

And once the form is received, pass the result through something like:

getMyValue :: MyInput -> Either ErrorMsg Text
getMyValue i = if createNew i
                   then if newVal i == Nothing || existingVal i /= Nothing
                            then Left "Missing new value or illegal input"
                            else Right . fromJust . newVal $ i
                   else if existingVal i == Nothing || newVal i /= Nothing
                            then Left "Missing selection or illegal input"
                            else Right . fromJust . existingVal $ i

And have the handler decide whether to re-send the form, or proceed according to the result.

Any better/shorter suggestions?
My real form has two such select/create fields, which makes the data structure and processing even more tedious.
Thanks,

Answer 1

You can factor out the common code, use pattern matching and guards, and generalize with a higher-order-function that accepts the accessor functions:

import Control.Arrow ((&&&))

getVal isNew newVal oldVal i | isNew i   = checkVal "new value" $ (newVal &&& oldVal) i
                             | otherwise = checkVal "selection" $ (oldVal &&& newVal) i
  where checkVal _ (Just val, Nothing) = Right val
        checkVal name _                = Left $ "Missing " ++ name ++ " or illegal input"

getMyVal = getVal createNew newVal existingVal