Make asDouble return an Either

src/Aeson.hs

@@ -37,21 +37,20 @@ asInt o          = error $ "Expected an integer but received: " <> show o
 -- | Try to extract a Double from Value
 --
 -- >>> asDouble (Number 10.3)
--- 10.3
+-- Right 10.3
 --
 -- >>> asDouble (String "10.5")
--- 10.5
+-- Right 10.5
 --
 -- >>> asDouble (String "foo")
--- *** Exception: Expected a double, but received: foo
--- ...
-asDouble :: Value -> Double
-asDouble (Number n) = S.toRealFloat n
+-- Left "Expected a double, but received: foo"
+asDouble :: Value -> Either String Double
+asDouble (Number n) = Right $ S.toRealFloat n
 asDouble (String s) =
   case T.double s of
-    (Right (n, _)) -> n
-    (Left _)       -> error $ "Expected a double, but received: " <> T.unpack s
-asDouble o          = error $ "Expected a double, but received: " <> show o
+    (Right (n, _)) -> Right n
+    (Left _)       -> Left $ "Expected a double, but received: " <> T.unpack s
+asDouble o          = Left $ "Expected a double, but received: " <> show o
 
 
 asArray :: Value -> V.Vector Value

src/Fake.hs

@@ -11,7 +11,7 @@ module Fake (
 
 import qualified Aeson                      as A
 import           Control.Monad              (forM, replicateM)
-import           Control.Monad.Except       (ExceptT)
+import           Control.Monad.Except       (ExceptT, MonadError)
 import qualified Control.Monad.Except       as Except
 import           Control.Monad.IO.Class     (MonadIO, liftIO)
 import           Control.Monad.State.Class  (MonadState)
@@ -48,6 +48,7 @@ newtype Fake a = Fake { runFake :: ExceptT String (StateT Env IO) a }
   , Monad
   , MonadState Env
   , MonadIO
+  , MonadError String
   )
 
 runFakeT :: Maybe Int -> Fake a -> IO a
@@ -102,8 +103,8 @@ randomInt lower upper = do
 -- Number 1.500000257527587
 randomDouble :: Expr -> Expr -> Fake Value
 randomDouble lower upper = do
-  lower' <- A.asDouble <$> eval lower
-  upper' <- A.asDouble <$> eval upper
+  lower' <- Except.liftEither =<< A.asDouble <$> eval lower
+  upper' <- Except.liftEither =<< A.asDouble <$> eval upper
   Number . S.fromFloatDigits <$> State.state (randomR (lower', upper'))