Return Either instead of Maybe for lookup

1 files changed, 12 insertions, 5 deletions

diff --git a/Data/ConfigManager.hs b/Data/ConfigManager.hs
index 038276e..aa92554 100644
--- a/Data/ConfigManager.hs
+++ b/Data/ConfigManager.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE OverloadedStrings #-}
+
 -- |
 -- Module:      Data.ConfigManager
 -- License:     GPL-3
@@ -30,12 +32,10 @@ module Data.ConfigManager
 
 import Prelude hiding (lookup)
 import Text.Read (readMaybe)
-import Control.Monad (join)
 
 import Data.Text (Text)
 import qualified Data.Text as T
 import qualified Data.HashMap.Strict as M
-import Data.Maybe (fromMaybe)
 
 import qualified Data.ConfigManager.Reader as R
 import Data.ConfigManager.Types
@@ -47,14 +47,21 @@ readConfig = R.readConfig Required
 
 -- | Lookup for the value associated to a name.
 
-lookup :: Read a => Name -> Config -> Maybe a
-lookup name config = join . fmap (readMaybe . T.unpack) $ M.lookup name (hashMap config)
+lookup :: Read a => Name -> Config -> Either Text a
+lookup name config =
+  case M.lookup name (hashMap config) of
+    Nothing ->
+      Left . T.concat $ ["Value not found for Key ", name]
+    Just value ->
+      case readMaybe . T.unpack $ value of
+        Nothing -> Left . T.concat $ ["Reading error for key ", name]
+        Just result -> Right result
 
 -- | Lookup for the value associated to a name and return the default value if
 -- no binding exists with the given name.
 
 lookupDefault :: Read a => a -> Name -> Config -> a
-lookupDefault defaultValue name config = fromMaybe defaultValue $ lookup name config
+lookupDefault defaultValue name config = foldl (flip const) defaultValue $ lookup name config
 
 -- $format
 --