Added readme

README.md

@@ -0,0 +1,75 @@
+![Travis CI Badge](https://img.shields.io/travis/typeable/validationt)
+
+# ValidationT
+
+A very small validation data library.
+
+## 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:
+
+```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`.)