diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright Eric Conlon (c) 2019
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Eric Conlon nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/README.md b/README.md
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--- /dev/null
+++ b/README.md
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+# climb
+
+[![CircleCI](https://circleci.com/gh/ejconlon/climb/tree/master.svg?style=svg)](https://circleci.com/gh/ejconlon/climb/tree/master)
+
+Building blocks for a GHCI-like REPL with colon-commands. (Not production ready!) See the [demo app](https://github.com/ejconlon/climb/blob/master/app/Main.hs) for usage, or see the lower-level [haskell-linenoise](https://github.com/ejconlon/haskell-linenoise) library this depends on.
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/app/Main.hs b/app/Main.hs
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--- /dev/null
+++ b/app/Main.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module Main where
+
+import Control.Monad.Catch (MonadThrow)
+import Control.Monad.IO.Class (MonadIO)
+import Control.Monad.IO.Unlift (MonadUnliftIO (..), UnliftIO (..))
+import Climb
+import Linenoise
+
+newtype Repl a = Repl { unRepl :: ReplT () () IO a }
+  deriving (Functor, Applicative, Monad, MonadIO, MonadThrow)
+
+instance MonadUnliftIO Repl where
+  askUnliftIO = do
+    UnliftIO run <- Repl askUnliftIO
+    pure (UnliftIO (run . unRepl))
+
+runRepl :: Repl a -> IO a
+runRepl r = fmap fst (runReplT (unRepl r) () ())
+
+options :: OptionCommands Repl
+options = mempty
+
+exec :: Command Repl
+exec = const (pure ReplContinue)
+
+completion :: Completion Repl
+completion = const (pure [])
+
+replDef :: ReplDef Repl
+replDef = ReplDef
+  { _rdOnInterrupt = ReplContinue
+  , _rdGreeting = "Hello, REPL!"
+  , _rdPrompt = "> "
+  , _rdOptionCommands = options
+  , _rdExecCommand = exec
+  , _rdCompletion = completion
+  }
+
+main :: IO ()
+main = runRepl (runReplDef replDef)
diff --git a/climb.cabal b/climb.cabal
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--- /dev/null
+++ b/climb.cabal
@@ -0,0 +1,65 @@
+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.31.2.
+--
+-- see: https://github.com/sol/hpack
+--
+-- hash: f959e3804f6a76af277dfee45863cf2b2521ec7733ce877dac7398aa36e9438b
+
+name:           climb
+version:        0.3.1
+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
+homepage:       https://github.com/ejconlon/climb#readme
+bug-reports:    https://github.com/ejconlon/climb/issues
+author:         Eric Conlon
+maintainer:     ejconlon@gmail.com
+copyright:      (c) 2019 Eric Conlon
+license:        BSD3
+license-file:   LICENSE
+build-type:     Simple
+extra-source-files:
+    README.md
+
+source-repository head
+  type: git
+  location: https://github.com/ejconlon/climb
+
+library
+  exposed-modules:
+      Climb
+  other-modules:
+      Paths_climb
+  hs-source-dirs:
+      src
+  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
+    , bytestring >=0.10
+    , containers >=0.6
+    , exceptions >=0.10
+    , linenoise >=0.3.1
+    , mtl >=2.2
+    , text >=1.2
+    , unliftio-core >=0.1
+  default-language: Haskell2010
+
+executable climb-demo
+  main-is: Main.hs
+  other-modules:
+      Paths_climb
+  hs-source-dirs:
+      app
+  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
+    , bytestring >=0.10
+    , climb
+    , containers >=0.6
+    , exceptions >=0.10
+    , linenoise >=0.3.1
+    , mtl >=2.2
+    , text >=1.2
+    , unliftio-core >=0.1
+  default-language: Haskell2010
diff --git a/src/Climb.hs b/src/Climb.hs
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--- /dev/null
+++ b/src/Climb.hs
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+{-# LANGUAGE OverloadedStrings #-}
+
+-- | Building blocks for a GHCI-like REPL with colon-commands.
+module Climb
+  ( Command
+  , CommandExc (..)
+  , Completion
+  , OptionCommands
+  , ReplDef (..)
+  , ReplDirective (..)
+  , bareCommand
+  , noOptionCommands
+  , noCompletion
+  , runReplDef
+  ) where
+
+import Control.Exception (Exception)
+import Control.Monad (unless)
+import Control.Monad.Catch (MonadThrow (..))
+import Control.Monad.Fix (fix)
+import Control.Monad.IO.Class (MonadIO (..))
+import Control.Monad.IO.Unlift (MonadUnliftIO)
+import Data.Foldable (for_)
+import Data.Typeable (Typeable)
+import Data.Map.Strict (Map)
+import qualified Data.Map.Strict as Map
+import Data.Text (Text)
+import qualified Data.Text as Text
+import qualified Data.Text.IO as TIO
+import Linenoise.Repl (ReplDirective (..), replM)
+
+-- | A 'Command' takes some input, performs some effect, and returns a directive (continue or quit).
+type Command m = Text -> m ReplDirective
+
+-- | List of 'Command's by name with help text.
+type OptionCommands m = Map Text (Text, Command m)
+
+-- | A 'Completion' takes some input and returns potential matches.
+type Completion m = Text -> m [Text]
+
+-- | Sometimes things go wrong...
+data CommandExc
+  = ExpectedNoInputError
+  -- ^ An option 'Command' got input when it expected None
+  | MissingCommandError !Text
+  -- ^ An option 'Command' was not found by name.
+  deriving (Eq, Show, Typeable)
+
+instance Exception CommandExc
+
+-- | 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)
+    }
+
+noOptionCommands :: OptionCommands m
+noOptionCommands = Map.empty
+
+noCompletion :: Applicative m => Completion m
+noCompletion = const (pure [])
+
+assertEmpty :: MonadThrow m => Text -> m ()
+assertEmpty input = unless (Text.null input) (throwM ExpectedNoInputError)
+
+-- | Helps you define commands that expect no input.
+bareCommand :: MonadThrow m => m ReplDirective -> Command m
+bareCommand act input = assertEmpty input >> act
+
+quitCommand :: MonadThrow m => Command m
+quitCommand = bareCommand (pure ReplQuit)
+
+helpCommand :: (MonadThrow m, MonadIO m) => OptionCommands m -> Command m
+helpCommand opts = bareCommand $ do
+  liftIO (TIO.putStrLn "Available commands:")
+  for_ (Map.toList opts) $ \(name, (desc, _)) -> liftIO (TIO.putStrLn (":" <> name <> "\t" <> desc))
+  pure ReplContinue
+
+defaultOptions :: (MonadThrow m, MonadIO m) => OptionCommands m -> OptionCommands m
+defaultOptions opts = Map.fromList
+    [ ("quit", ("quit", quitCommand))
+    , ("help", ("describe all commands", helpCommand opts))
+    ]
+
+outerCommand :: MonadThrow m => OptionCommands m -> Command m -> Command m
+outerCommand opts exec = \input ->
+  case Text.uncons input of
+    Just (':', rest) -> do
+      let (name, subInput) = Text.break (==' ') rest
+      case Map.lookup name opts of
+        Nothing -> throwM (MissingCommandError name)
+        Just (_, command) -> command (Text.drop 1 subInput)
+    _ -> exec input
+
+-- | Runs a REPL as defined.
+runReplDef :: (MonadThrow 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
+  liftIO (TIO.putStrLn greeting)
+  liftIO (TIO.putStrLn "Enter `:quit` to exit or `:help` to see all commands.")
+  replM onInterrupt prompt action comp