diff --git a/app/Main.hs b/app/Main.hs
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -1,41 +1,62 @@
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE OverloadedStrings #-}
 
 module Main where
 
 import Climb
-import Control.Monad.Catch (MonadThrow)
-import Control.Monad.IO.Class (MonadIO)
-import Control.Monad.IO.Unlift (MonadUnliftIO (..), wrappedWithRunInIO)
-import Linenoise
+import Control.Exception (Exception)
+import Control.Monad.Catch (MonadCatch, MonadThrow (..))
+import Control.Monad.IO.Class (MonadIO (..))
+import Control.Monad.IO.Unlift (MonadUnliftIO (..))
+import Control.Monad.Reader (MonadReader (..), ReaderT (..), asks)
+import qualified Data.Map.Strict as Map
+import qualified Data.Text as T
+import qualified Data.Text.IO as TIO
+import Text.Read (readMaybe)
 
-newtype Repl a = Repl { unRepl :: ReplT () () IO a }
-  deriving (Functor, Applicative, Monad, MonadIO, MonadThrow)
+newtype BadGuessErr = BadGuessErr (Maybe Int)
+  deriving stock (Eq, Show)
 
-instance MonadUnliftIO Repl where
-  withRunInIO = wrappedWithRunInIO Repl unRepl
+instance Exception BadGuessErr
 
-runRepl :: Repl a -> IO a
-runRepl r = fmap fst (runReplT (unRepl r) () ())
+data ReplEnv = ReplEnv
+  { reMagicNumber :: !Int
+  } deriving stock (Eq, Show)
 
-options :: OptionCommands Repl
-options = mempty
+newtype ReplM a = ReplM { unReplM :: ReaderT ReplEnv IO a }
+  deriving newtype (Functor, Applicative, Monad, MonadReader ReplEnv, MonadIO, MonadUnliftIO, MonadThrow, MonadCatch)
 
-exec :: Command Repl
-exec = const (pure ReplContinue)
+runReplM :: ReplM a -> ReplEnv -> IO a
+runReplM = runReaderT . unReplM
 
-completion :: Completion Repl
-completion = const (pure [])
+guessCommand :: Command ReplM
+guessCommand input = do
+  num <- asks reMagicNumber
+  let mayGuess = readMaybe (T.unpack input)
+  if readMaybe (T.unpack input) == Just num
+    then ReplContinue <$ liftIO (putStrLn "You guessed it!")
+    else throwM (BadGuessErr mayGuess)
 
-replDef :: ReplDef Repl
+options :: OptionCommands ReplM
+options = Map.fromList [("guess", ("guess a number", guessCommand))]
+
+exec :: Command ReplM
+exec input = liftIO $ do
+  putStr "You said: "
+  TIO.putStrLn input
+  pure ReplContinue
+
+completion :: Completion ReplM
+completion _ = pure []
+
+replDef :: ReplDef ReplM
 replDef = ReplDef
-  { _rdOnInterrupt = ReplContinue
-  , _rdGreeting = "Hello, REPL!"
-  , _rdPrompt = "> "
-  , _rdOptionCommands = options
-  , _rdExecCommand = exec
-  , _rdCompletion = completion
+  { rdOnInterrupt = ReplContinue
+  , rdGreeting = "Hello, REPL!"
+  , rdPrompt = "> "
+  , rdOptionCommands = options
+  , rdExecCommand = exec
+  , rdCompletion = completion
   }
 
 main :: IO ()
-main = runRepl (runReplDef replDef)
+main = runReplM (runReplDef replDef) (ReplEnv 42)
diff --git a/climb.cabal b/climb.cabal
--- a/climb.cabal
+++ b/climb.cabal
@@ -1,13 +1,13 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.33.0.
+-- This file has been generated from package.yaml by hpack version 0.34.4.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: ef9b6c6a8b9065c4429bdbdaf4fb5f06470af3787a069de51b48eb6026c5631f
+-- hash: 13958ca7b14089cca51ac048a8cea2d64d6e8e460fa1cddae5a3d4f5bd15ec68
 
 name:           climb
-version:        0.3.3
+version:        0.4.0
 synopsis:       Building blocks for a GHCi-like REPL with colon-commands
 description:    Please see the README on GitHub at <https://github.com/ejconlon/climb#readme>
 category:       User Interfaces
@@ -33,6 +33,9 @@
       Paths_climb
   hs-source-dirs:
       src
+  default-extensions:
+      DerivingStrategies
+      GeneralizedNewtypeDeriving
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wpartial-fields -Wredundant-constraints -fno-warn-unused-top-binds
   build-depends:
       base >=4.12 && <5
@@ -51,6 +54,9 @@
       Paths_climb
   hs-source-dirs:
       app
+  default-extensions:
+      DerivingStrategies
+      GeneralizedNewtypeDeriving
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wpartial-fields -Wredundant-constraints -fno-warn-unused-top-binds -threaded -rtsopts -with-rtsopts=-N
   build-depends:
       base >=4.12 && <5
diff --git a/src/Climb.hs b/src/Climb.hs
--- a/src/Climb.hs
+++ b/src/Climb.hs
@@ -3,7 +3,7 @@
 -- | Building blocks for a GHCI-like REPL with colon-commands.
 module Climb
   ( Command
-  , CommandExc (..)
+  , CommandErr (..)
   , Completion
   , OptionCommands
   , ReplDef (..)
@@ -14,9 +14,9 @@
   , runReplDef
   ) where
 
-import Control.Exception (Exception)
+import Control.Exception (Exception (..), SomeAsyncException (..), SomeException)
 import Control.Monad (unless)
-import Control.Monad.Catch (MonadThrow (..))
+import Control.Monad.Catch (MonadCatch, MonadThrow (..), catchIf)
 import Control.Monad.Fix (fix)
 import Control.Monad.IO.Class (MonadIO (..))
 import Control.Monad.IO.Unlift (MonadUnliftIO)
@@ -26,7 +26,6 @@
 import Data.Text (Text)
 import qualified Data.Text as Text
 import qualified Data.Text.IO as TIO
-import Data.Typeable (Typeable)
 import Linenoise.Repl (ReplDirective (..), replM)
 
 -- | A 'Command' takes some input, performs some effect, and returns a directive (continue or quit).
@@ -39,24 +38,24 @@
 type Completion m = Text -> m [Text]
 
 -- | Sometimes things go wrong...
-data CommandExc
-  = ExpectedNoInputError
+data CommandErr
+  = CommandErrExpectedNoInput
   -- ^ An option 'Command' got input when it expected None
-  | MissingCommandError !Text
+  | CommandErrUnknownCommand !Text
   -- ^ An option 'Command' was not found by name.
-  deriving (Eq, Show, Typeable)
+  deriving stock (Eq, Show)
 
-instance Exception CommandExc
+instance Exception CommandErr
 
 -- | Defines a REPL with commands, options, and completion.
 data ReplDef m =
   ReplDef
-    { _rdOnInterrupt :: !ReplDirective
-    , _rdGreeting :: !Text
-    , _rdPrompt :: !Text
-    , _rdOptionCommands :: !(OptionCommands m)
-    , _rdExecCommand :: !(Command m)
-    , _rdCompletion :: !(Completion m)
+    { rdOnInterrupt :: !ReplDirective
+    , rdGreeting :: !Text
+    , rdPrompt :: !Text
+    , rdOptionCommands :: !(OptionCommands m)
+    , rdExecCommand :: !(Command m)
+    , rdCompletion :: !(Completion m)
     }
 
 noOptionCommands :: OptionCommands m
@@ -66,7 +65,7 @@
 noCompletion = const (pure [])
 
 assertEmpty :: MonadThrow m => Text -> m ()
-assertEmpty input = unless (Text.null input) (throwM ExpectedNoInputError)
+assertEmpty input = unless (Text.null input) (throwM CommandErrExpectedNoInput)
 
 -- | Helps you define commands that expect no input.
 bareCommand :: MonadThrow m => m ReplDirective -> Command m
@@ -93,15 +92,31 @@
     Just (':', rest) -> do
       let (name, subInput) = Text.break (==' ') rest
       case Map.lookup name opts of
-        Nothing -> throwM (MissingCommandError name)
+        Nothing -> throwM (CommandErrUnknownCommand name)
         Just (_, command) -> command (Text.drop 1 subInput)
     _ -> exec input
 
+isUserErr :: SomeException -> Bool
+isUserErr x =
+  case fromException x of
+    Just (SomeAsyncException _) -> False
+    _ -> True
+
+catchUserErr :: MonadCatch m => m a -> (SomeException -> m a) -> m a
+catchUserErr = catchIf isUserErr
+
+handleUserErr :: (MonadCatch m, MonadIO m) => Command m -> Command m
+handleUserErr action input = catchUserErr (action input) $ \err -> do
+  liftIO (TIO.putStr "Caught error: ")
+  liftIO (print err)
+  pure ReplContinue
+
 -- | Runs a REPL as defined.
-runReplDef :: (MonadThrow m, MonadUnliftIO m) => ReplDef m -> m ()
+runReplDef :: (MonadCatch m, MonadUnliftIO m) => ReplDef m -> m ()
 runReplDef (ReplDef onInterrupt greeting prompt opts exec comp) = do
   let allOpts = fix (\c -> defaultOptions c <> opts)
       action = outerCommand allOpts exec
+      handledAction = handleUserErr action
   liftIO (TIO.putStrLn greeting)
   liftIO (TIO.putStrLn "Enter `:quit` to exit or `:help` to see all commands.")
-  replM onInterrupt prompt action comp
+  replM onInterrupt prompt handledAction comp