Synnefo » snf-ganeti » ganeti-local

{-

Copyright (C) 2009, 2010, 2011, 2012 Google Inc.

This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or

(at your option) any later version.

This program is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with this program; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

02110-1301, USA.

-}

module Ganeti.BasicTypes

  ( Result(..)

  , isOk

  , isBad

  , eitherToResult

  , annotateResult

  , annotateIOError

  ) where

import Control.Monad

-- | This is similar to the JSON library Result type - /very/ similar,

-- but we want to use it in multiple places, so we abstract it into a

-- mini-library here.

--

-- The failure value for this monad is simply a string.

data Result a

    = Bad String

    | Ok a

    deriving (Show, Read, Eq)

instance Monad Result where

  (>>=) (Bad x) _ = Bad x

  (>>=) (Ok x) fn = fn x

  return = Ok

  fail = Bad

instance MonadPlus Result where

  mzero = Bad "zero Result when used as MonadPlus"

  -- for mplus, when we 'add' two Bad values, we concatenate their

  -- error descriptions

  (Bad x) `mplus` (Bad y) = Bad (x ++ "; " ++ y)

  (Bad _) `mplus` x = x

  x@(Ok _) `mplus` _ = x

-- | Simple checker for whether a 'Result' is OK.

isOk :: Result a -> Bool

isOk (Ok _) = True

isOk _ = False

-- | Simple checker for whether a 'Result' is a failure.

isBad :: Result a  -> Bool

isBad = not . isOk

-- | Converter from Either String to 'Result'.

eitherToResult :: Either String a -> Result a

eitherToResult (Left s) = Bad s

eitherToResult (Right v) = Ok v

-- | Annotate a Result with an ownership information.

annotateResult :: String -> Result a -> Result a

annotateResult owner (Bad s) = Bad $ owner ++ ": " ++ s

annotateResult _ v = v

-- | Annotates and transforms IOErrors into a Result type. This can be

-- used in the error handler argument to 'catch', for example.

annotateIOError :: String -> IOError -> IO (Result a)

annotateIOError description exc =

  return . Bad $ description ++ ": " ++ show exc