Suppose you want to validate some data from the user. Say, that the password adheres to some rules.
You might do something like this:
```haskell
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`.
```haskell
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:
```haskell
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:
```haskell
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`.)
