Synnefo » snf-ganeti » ganeti-local

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