src/Ganeti/Utils/Validate.hs

{-# LANGUAGE FlexibleInstances, GeneralizedNewtypeDeriving #-}

{-| A validation monad and corresponding utilities

The monad allows code to emit errors during checking.

-}

{-

Copyright (C) 2014 Google Inc.
All rights reserved.

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2. Redistributions in binary form must reproduce the above copyright
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
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-}

module Ganeti.Utils.Validate
  ( ValidationMonadT()
  , ValidationMonad
  , report
  , reportIf
  , runValidate
  , runValidateT
  , execValidateT
  , execValidate
  , evalValidateT
  , evalValidate
  , Validatable(..)
  , validate'
  ) where

import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.Error
import Control.Monad.Writer
import qualified Data.Foldable as F
import Data.Functor.Identity
import Data.List (intercalate)
import Data.Sequence

-- | Monad for running validation checks.
newtype ValidationMonadT m a =
  ValidationMonad { runValidationMonad :: WriterT (Seq String) m a }
  deriving (Functor, Applicative, Monad)

type ValidationMonad = ValidationMonadT Identity

-- | An utility function that emits a single message into a validation monad.
report :: (Monad m) => String -> ValidationMonadT m ()
report = ValidationMonad . tell . singleton

-- | An utility function that conditionally emits a message into
-- a validation monad.
-- It's a combination of 'when' and 'report'.
reportIf :: (Monad m) => Bool -> String -> ValidationMonadT m ()
reportIf b = when b . report

-- | An utility function that runs a monadic validation action
-- and returns the list of errors.
runValidateT :: (Monad m) => ValidationMonadT m a -> m (a, [String])
runValidateT = liftM (second F.toList) . runWriterT . runValidationMonad

-- | An utility function that runs a monadic validation action
-- and returns the list of errors.
runValidate :: ValidationMonad a -> (a, [String])
runValidate = runIdentity . runValidateT

-- | An utility function that runs a monadic validation action
-- and returns the list of errors.
execValidateT :: (Monad m) => ValidationMonadT m () -> m [String]
execValidateT = liftM F.toList . execWriterT . runValidationMonad

-- | An utility function that runs a validation action
-- and returns the list of errors.
execValidate :: ValidationMonad () -> [String]
execValidate = runIdentity . execValidateT

-- | A helper function for throwing an exception if a list of errors
-- is non-empty.
throwIfErrors :: (MonadError e m, Error e) => (a, [String]) -> m a
throwIfErrors (x, []) = return x
throwIfErrors (_, es) = throwError (strMsg $ "Validation errors: "
                                             ++ intercalate "; " es)

-- | Runs a validation action and if there are errors, combine them
-- into an exception.
evalValidate :: (MonadError e m, Error e) => ValidationMonad a -> m a
evalValidate = throwIfErrors . runValidate

-- | Runs a validation action and if there are errors, combine them
-- into an exception.
evalValidateT :: (MonadError e m, Error e) => ValidationMonadT m a -> m a
evalValidateT k = runValidateT k >>= throwIfErrors

-- | A typeclass for objects that can be validated.
-- That is, they can perform an internal check and emit any
-- errors encountered.
-- Emiting no errors means the object is valid.
class Validatable a where
  validate :: a -> ValidationMonad ()

-- | Run validation and return the original value as well.
-- the original value.
validate' :: (Validatable a) => a -> ValidationMonad a
validate' x = x <$ validate x