ValidationT
A simple data validation library. The main idea is to provide an easy way to
validate web form data by aggregating errors for each field.
Usage
Suppose you want to validate some data from the user. Say, that the password adheres to some rules.
You might do something like this:
validatePassword :: Monad m => String -> ValidationT [String] m ()
validatePassword password = do
vLength password
vAlpha password
vNum password
where
vLength p = when (length p < 8)
$ vWarning ["The password should be at least 8 characters long."]
vAlpha p = unless (any isAlpha p)
$ vWarning ["The password should contain at least one alphabetic character."]
vNum p = unless (any isDigit p)
$ vWarning ["The password should contain at least one numeric character."]
ValidationT e m a
essentially just gathers the errors thrown by vWarning
.
vWarning :: (Monad m, Monoid e) => e -> ValidationT e m ()
vWarning
mappend
s the given e
to the already collected errors. This is why the warnings are contained inside a list.
There is also vError
. The only difference between vWarning
and vError
is that vError
stops further execution (and further collection of errors and warnings).
You would use the validation like this:
main :: IO ()
main = do
password <- getLine
result <- runValidationTEither . validatePassword $ password
putStrLn $ case result of
Left errs -> unlines err
Right () -> "You are fine."
You could, of course, do more complicated things like use vWarningL
and vErrorL
to add an error to a mempty
structure, which gets mappend
ed with other errors.
The library comes with a MonoidMap
newtype
wrapper around Map
, which mappend
s the values themselves on conflict. This can be useful if you have a multiple points of failure and you want to distinguich between them – validating a username and a password for example:
data Piece
= Password
| UserName
deriving (Eq, Show, Ord)
validatePassword :: Monad m => String -> ValidationT (MonoidMap Piece [String]) m ()
validatePassword password = do
vLength password
vAlpha password
vNum password
where
warning = mmSingleton UserName
vLength p = when (length p < 8)
$ warning ["The password should be at least 8 characters long."]
vAlpha p = unless (any isAlpha p)
$ warning ["The password should contain at least one alphabetic character."]
vNum p = unless (any isDigit p)
$ warning ["The password should contain at least one numeric character."]
(mmSingleton
is a covenience initializer for MonoidMap
.)