Improve TemplateHaskell code to support empty objects

[ganeti-local] / src / Ganeti / Utils.hs

diff --git a/src/Ganeti/Utils.hs b/src/Ganeti/Utils.hs
index 0ff1984..b4a8c3c 100644 (file)
--- a/src/Ganeti/Utils.hs
+++ b/src/Ganeti/Utils.hs
@@ -53,11 +53,14 @@ module Ganeti.Utils
   , trim
   , defaultHead
   , exitIfEmpty
+  , splitEithers
+  , recombineEithers
   ) where
 
 import Data.Char (toUpper, isAlphaNum, isDigit, isSpace)
 import Data.Function (on)
 import Data.List
+import Control.Monad (foldM)
 
 import Debug.Trace
 
@@ -382,3 +385,44 @@ defaultHead _   (x:_) = x
 exitIfEmpty :: String -> [a] -> IO a
 exitIfEmpty _ (x:_) = return x
 exitIfEmpty s []    = exitErr s
+
+-- | Split an 'Either' list into two separate lists (containing the
+-- 'Left' and 'Right' elements, plus a \"trail\" list that allows
+-- recombination later.
+--
+-- This is splitter; for recombination, look at 'recombineEithers'.
+-- The sum of \"left\" and \"right\" lists should be equal to the
+-- original list length, and the trail list should be the same length
+-- as well. The entries in the resulting lists are reversed in
+-- comparison with the original list.
+splitEithers :: [Either a b] -> ([a], [b], [Bool])
+splitEithers = foldl' splitter ([], [], [])
+  where splitter (l, r, t) e =
+          case e of
+            Left  v -> (v:l, r, False:t)
+            Right v -> (l, v:r, True:t)
+
+-- | Recombines two \"left\" and \"right\" lists using a \"trail\"
+-- list into a single 'Either' list.
+--
+-- This is the counterpart to 'splitEithers'. It does the opposite
+-- transformation, and the output list will be the reverse of the
+-- input lists. Since 'splitEithers' also reverses the lists, calling
+-- these together will result in the original list.
+--
+-- Mismatches in the structure of the lists (e.g. inconsistent
+-- lengths) are represented via 'Bad'; normally this function should
+-- not fail, if lists are passed as generated by 'splitEithers'.
+recombineEithers :: (Show a, Show b) =>
+                    [a] -> [b] -> [Bool] -> Result [Either a b]
+recombineEithers lefts rights trail =
+  foldM recombiner ([], lefts, rights) trail >>= checker
+    where checker (eithers, [], []) = Ok eithers
+          checker (_, lefts', rights') =
+            Bad $ "Inconsistent results after recombination, l'=" ++
+                show lefts' ++ ", r'=" ++ show rights'
+          recombiner (es, l:ls, rs) False = Ok (Left l:es,  ls, rs)
+          recombiner (es, ls, r:rs) True  = Ok (Right r:es, ls, rs)
+          recombiner (_,  ls, rs) t = Bad $ "Inconsistent trail log: l=" ++
+                                      show ls ++ ", r=" ++ show rs ++ ",t=" ++
+                                      show t