X-Git-Url: https://code.grnet.gr/git/ganeti-local/blobdiff_plain/25779212dca36503765c2353da08cd659b0ca8de..b90974684da8e881ad8a54b9368896439a984a97:/htools/Ganeti/BasicTypes.hs

diff --git a/htools/Ganeti/BasicTypes.hs b/htools/Ganeti/BasicTypes.hs
index 61a7e56..149540f 100644
--- a/htools/Ganeti/BasicTypes.hs
+++ b/htools/Ganeti/BasicTypes.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE FlexibleInstances #-}
+
 {-
 
 Copyright (C) 2009, 2010, 2011, 2012 Google Inc.
@@ -20,7 +22,11 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 -}
 
 module Ganeti.BasicTypes
-  ( Result(..)
+  ( GenericResult(..)
+  , Result
+  , ResultT(..)
+  , resultT
+  , FromString(..)
   , isOk
   , isBad
   , eitherToResult
@@ -38,30 +44,40 @@ module Ganeti.BasicTypes
 
 import Control.Applicative
 import Control.Monad
+import Control.Monad.Trans
 import Data.Function
 import Data.List
 
--- | 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
+-- | Generic monad for our error handling mechanisms.
+data GenericResult a b
+  = Bad a
+  | Ok b
     deriving (Show, Read, Eq)
 
-instance Monad Result where
+-- | Type alias for a string Result.
+type Result = GenericResult String
+
+-- | Type class for things that can be built from strings.
+class FromString a where
+  mkFromString :: String -> a
+
+-- | Trivial 'String' instance; requires FlexibleInstances extension
+-- though.
+instance FromString [Char] where
+  mkFromString = id
+
+-- | 'Monad' instance for 'GenericResult'.
+instance (FromString a) => Monad (GenericResult a) where
   (>>=) (Bad x) _ = Bad x
   (>>=) (Ok x) fn = fn x
   return = Ok
-  fail = Bad
+  fail   = Bad . mkFromString
 
-instance Functor Result where
+instance Functor (GenericResult a) where
   fmap _ (Bad msg) = Bad msg
   fmap fn (Ok val) = Ok (fn val)
 
-instance MonadPlus Result where
+instance MonadPlus (GenericResult String) where
   mzero = Bad "zero Result when used as MonadPlus"
   -- for mplus, when we 'add' two Bad values, we concatenate their
   -- error descriptions
@@ -69,25 +85,48 @@ instance MonadPlus Result where
   (Bad _) `mplus` x = x
   x@(Ok _) `mplus` _ = x
 
-instance Applicative Result where
+instance Applicative (GenericResult a) where
   pure = Ok
   (Bad f) <*> _       = Bad f
   _       <*> (Bad x) = Bad x
   (Ok f)  <*> (Ok x)  = Ok $ f x
 
--- | Simple checker for whether a 'Result' is OK.
-isOk :: Result a -> Bool
+-- | This is a monad transformation for Result. It's implementation is
+-- based on the implementations of MaybeT and ErrorT.
+newtype ResultT a m b = ResultT {runResultT :: m (GenericResult a b)}
+
+instance (Monad m, FromString a) => Monad (ResultT a m) where
+  fail err = ResultT (return . Bad $ mkFromString err)
+  return   = lift . return
+  x >>= f  = ResultT $ do
+               a <- runResultT x
+               case a of
+                 Ok val -> runResultT $ f val
+                 Bad err -> return $ Bad err
+
+instance MonadTrans (ResultT a) where
+  lift x = ResultT (liftM Ok x)
+
+instance (MonadIO m, FromString a) => MonadIO (ResultT a m) where
+  liftIO = lift . liftIO
+
+-- | Lift a `Result` value to a `ResultT`.
+resultT :: Monad m => GenericResult a b -> ResultT a m b
+resultT = ResultT . return
+
+-- | Simple checker for whether a 'GenericResult' is OK.
+isOk :: GenericResult a b -> Bool
 isOk (Ok _) = True
-isOk _ = False
+isOk _      = False
 
--- | Simple checker for whether a 'Result' is a failure.
-isBad :: Result a  -> Bool
+-- | Simple checker for whether a 'GenericResult' is a failure.
+isBad :: GenericResult a b -> 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
+-- | Converter from Either to 'GenericResult'.
+eitherToResult :: Either a b -> GenericResult a b
+eitherToResult (Left  s) = Bad s
+eitherToResult (Right v) = Ok  v
 
 -- | Annotate a Result with an ownership information.
 annotateResult :: String -> Result a -> Result a