add MonadError tryRead (#42)

Author: d1skort

MonadTransformers/4.5/MonadReadError.hs

@@ -0,0 +1,14 @@
+{-# LANGUAGE FlexibleContexts #-}
+module MonadReadError where
+
+import Control.Monad.Except
+
+
+data ReadError = EmptyInput | NoParse String deriving Show
+
+tryRead :: (Read a, MonadError ReadError m) => String -> m a
+tryRead [] = throwError EmptyInput
+tryRead s  = f $ reads s where
+  f [(n, "")] = return n
+  f [(n, xs)] = throwError $ NoParse s
+  f []        = throwError $ NoParse s