Add NonEmpty newtype

[basvandijk]

Hi Johan,

I used this at work and it seems useful in general.

I'm not married to the name at all. It could also be called:

newtype NonNull a = NonNull {nonNull :: a}

Or something similar...

Cheers,

Bas

[basvandijk]

Mmm after thinking about it a bit more I realize this type is maybe a bit to specific to my use-case to add to cassava after all. What do you think? Just close it if you think it shouldn't belong here.

[tibbe]

I'll keep it in mind if this issue comes up again (which would provide some more support for it being generally useful).

[basvandijk]

No problem. BTW here's another extensible (but possibly overkill) design:

Defined in the core of the library:

newtype FieldPredicate p a = FieldPredicate {unFieldPredicate :: a}
    deriving (Eq, Ord, Read, Show)

instance ToField a => ToField (FieldPredicate p a) where
    toField = toField . unFieldPredicate

instance (Predicate p, FromField a) => FromField (FieldPredicate p a) where
    parseField s
        | unTagged (predicate :: Tagged p (Field -> Bool)) s =
            FieldPredicate <$> parseField s
        | otherwise = fail "Predicate failed"

class Predicate p where
    predicate :: Tagged p (Field -> Bool)

-- or get the one from Edward Kmett's tagged package:
newtype Tagged s b = Tagged { unTagged :: b }

Defined in a utility module or defined by the user herself:

{-# LANGUAGE EmptyDataDecls #-}

data NonEmpty

instance Predicate NonEmpty where
    predicate = Tagged $ not . B.null

nonEmpty :: FieldPredicate NonEmpty a -> a
nonEmpty = unFieldPredicate

To be used in the same way as before:

instance FromRecord Person where
    parseRecord v
        | length v == 2 = Person <$>
                          (nonEmpty <$> v .! 0) <*>
                                        v .! 1
        | otherwise     = mzero