diff --git a/butcher.cabal b/butcher.cabal
index bd6a3f1..79bc350 100644
--- a/butcher.cabal
+++ b/butcher.cabal
@@ -37,6 +37,7 @@ library
                        UI.Butcher.Monadic.Flag
                        UI.Butcher.Monadic.Pretty
                        UI.Butcher.Monadic.IO
+                       UI.Butcher.Monadic.Interactive
                        UI.Butcher.Monadic.BuiltinCommands
   other-modules:       UI.Butcher.Monadic.Internal.Types
                        UI.Butcher.Monadic.Internal.Core
diff --git a/src/UI/Butcher/Monadic/Interactive.hs b/src/UI/Butcher/Monadic/Interactive.hs
new file mode 100644
index 0000000..c25726a
--- /dev/null
+++ b/src/UI/Butcher/Monadic/Interactive.hs
@@ -0,0 +1,132 @@
+-- | Utilities when writing interactive programs that interpret commands,
+-- e.g. a REPL.
+module UI.Butcher.Monadic.Interactive
+  ( simpleCompletion
+  , interactiveHelpDoc
+  , partDescStrings
+  )
+where
+
+
+
+#include "prelude.inc"
+
+import qualified Text.PrettyPrint as PP
+
+import           UI.Butcher.Monadic.Internal.Types
+import           UI.Butcher.Monadic.Internal.Core
+import           UI.Butcher.Monadic.Pretty
+
+
+
+-- | Derives a potential completion from a given input string and a given
+-- 'CommandDesc'. Considers potential subcommands and where available the
+-- completion info present in 'PartDesc's.
+simpleCompletion
+  :: String         -- ^ input string
+  -> CommandDesc () -- ^ CommandDesc obtained on that input string
+  -> String         -- ^ "remaining" input after the last successfully parsed
+                    -- subcommand. See 'UI.Butcher.Monadic.runCmdParserExt'.
+  -> String         -- ^ completion, i.e. a string that might be appended
+                    -- to the current prompt when user presses tab.
+simpleCompletion line cdesc pcRest =
+  List.drop (List.length lastWord) $ case choices of
+    [] -> ""
+    (c1:cr) ->
+      case
+          filter (\s -> List.all (s`isPrefixOf`) cr) $ reverse $ List.inits c1
+        of
+          []    -> ""
+          (x:_) -> x
+ where
+  nameDesc = case _cmd_mParent cdesc of
+    Nothing                        -> cdesc
+    Just (_, parent) | null pcRest -> parent
+    Just{}                         -> cdesc
+  lastWord = reverse $ takeWhile (not . Char.isSpace) $ reverse $ line
+  choices :: [String]
+  choices = join
+    [ [ r
+      | (Just r, _) <- Data.Foldable.toList (_cmd_children nameDesc)
+      , lastWord `isPrefixOf` r
+      ]
+    , [ s
+      | s <- partDescStrings =<< _cmd_parts nameDesc
+      , lastWord `isPrefixOf` s
+      ]
+    ]
+
+
+-- | Produces a 'PP.Doc' as a hint for the user during interactive command
+-- input. Takes the current (incomplete) prompt line into account. For example
+-- when you have commands (among others) \'config set-email\' and
+-- \'config get-email\', then on empty prompt there will be an item \'config\';
+-- on the partial prompt \'config \' the help doc will contain the
+-- \'set-email\' and \'get-email\' items.
+interactiveHelpDoc
+  :: String         -- ^ input string
+  -> CommandDesc () -- ^ CommandDesc obtained on that input string
+  -> String         -- ^ "remaining" input after the last successfully parsed
+                    -- subcommand. See 'UI.Butcher.Monadic.runCmdParserExt'.
+  -> Int            -- ^ max length of help text
+  -> PP.Doc
+interactiveHelpDoc cmdline desc pcRest maxLines = if
+  | null cmdline             -> helpStrShort
+  | List.last cmdline == ' ' -> helpStrShort
+  | otherwise                -> helpStr
+ where
+  helpStr = if List.length optionLines > maxLines
+    then
+      PP.fcat $ List.intersperse (PP.text "|") $ PP.text . fst <$> optionLines
+    else PP.vcat $ optionLines <&> \case
+      (s, "") -> PP.text s
+      (s, h ) -> PP.text s PP.<> PP.text h
+   where
+    nameDesc = case _cmd_mParent desc of
+      Nothing                        -> desc
+      Just (_, parent) | null pcRest -> parent
+      Just{}                         -> desc
+
+    lastWord = reverse $ takeWhile (not . Char.isSpace) $ reverse $ cmdline
+    optionLines :: [(String, String)]
+    optionLines = -- a list of potential words that make sense, given
+                    -- the current input.
+                  join
+      [ [ (s, e)
+        | (Just s, c) <- Data.Foldable.toList (_cmd_children nameDesc)
+        , lastWord `isPrefixOf` s
+        , let e = join $ join
+                [ [ " ARGS" | not $ null $ _cmd_parts c ]
+                , [ " CMDS" | not $ null $ _cmd_children c ]
+                , [ ": " ++ show h | Just h <- [_cmd_help c] ]
+                ]
+        ]
+      , [ (s, "")
+        | s <- partDescStrings =<< _cmd_parts nameDesc
+        , lastWord `isPrefixOf` s
+        ]
+      ]
+  helpStrShort = ppUsageWithHelp desc
+
+
+-- | Obtains a list of "expected"/potential strings for a command part
+-- described in the 'PartDesc'. In constrast to the 'simpleCompletion'
+-- function this function does not take into account any current input, and
+-- consequently the output elements can in general not be appended to partial
+-- input to form valid input.
+partDescStrings :: PartDesc -> [String]
+partDescStrings = \case
+  PartLiteral  s      -> [s]
+  PartVariable _      -> []
+  -- TODO: we could handle seq of optional and such much better
+  PartOptional x      -> partDescStrings x
+  PartAlts     alts   -> alts >>= partDescStrings
+  PartSeq      []     -> []
+  PartSeq      (x:_)  -> partDescStrings x
+  PartDefault    _  x -> partDescStrings x
+  PartSuggestion ss x -> [ s | s <- ss ] ++ partDescStrings x
+  PartRedirect   _  x -> partDescStrings x
+  PartReorder xs      -> xs >>= partDescStrings
+  PartMany    x       -> partDescStrings x
+  PartWithHelp _h x   -> partDescStrings x -- TODO: handle help
+