Handling Maybe values with digestive functors ?

[Simon Gardner]

Hi,

Can I ask, what is the correct way to handle the following situation?

I have a data type

1. data Customer = Customer {
2.   cid :: Maybe T.Text,
3.   name :: Maybe T.Text,
4.   phone :: Maybe T.Text,
5.   web_address :: Maybe T.Text,
6.   updated :: Maybe T.Text
7. } deriving (Show)

with which I'd like to create /render a form.

1. customerForm :: Monad m => Form Html m Customer
2. customerForm = Customer
3.   <$> "id" .: text Nothing
4.   <*> "name" .: text Nothing
5.   <*> "phone" .: text Nothing
6.   <*> "web_address" .: text Nothing
7.   <*> "updated" .: text Nothing

compilation fails with:

1.   Couldn't match type `T.Text' with `Maybe T.Text'
2.    Expected type: Form Html m (Maybe T.Text)
3.      Actual type: Form Html m T.Text
4.    In the return type of a call of `text'
5.     In the second argument of `(.:)', namely `text Nothing'
6.     In the second argument of `(<$>)', namely `"id" .: text Nothing'

[Daniel Patterson]

Hi Simon,

For your exact case, you want to use 'optionalText' instead of 'text':

http://hackage.haskell.org/package/digestive-functors-0.7.1.1/docs/Text-Digestive-Form.html#v:optionalText

But, more generally, look into the 'validate' and 'validateM'
functions. They allow you to transform values, potentially failing
because they are invalid. For example, 'optionalText' is defined with
'validate':

optionalText :: Monad m => Maybe Text -> Form v m (Maybe Text)
optionalText def = validate opt (text def)
where
opt t
| T.null t = return Nothing
| otherwise = return $ Just t

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[Simon Gardner]

brilliant, thanks Daniel !