diff --git a/.gitignore b/.gitignore
index 19114cd..19095cd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,3 +11,4 @@ node_modules
 mkday.py
 tmp
 min.js.gz
+src/Revision.newt
diff --git a/Makefile b/Makefile
index 5e098b7..59e38c9 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,12 @@ RUNJS=node
 
 all: newt.js
 
-newt.js: ${SRCS}
+
+REV=$(shell git rev-parse --short HEAD)
+src/Revision.newt: .PHONY
+	echo "module Revision\nimport Prelude\ngitRevision : String\ngitRevision = \"${REV}\"" > src/Revision.newt
+
+newt.js: ${SRCS} src/Revision.newt
 	-rm build/* >/dev/null
 	$(RUNJS) bootstrap/newt.js src/Main.newt -o newt.js
 
diff --git a/src/Lib/ReplParser.newt b/src/Lib/ReplParser.newt
new file mode 100644
index 0000000..5ac3fab
--- /dev/null
+++ b/src/Lib/ReplParser.newt
@@ -0,0 +1,86 @@
+module Lib.ReplParser
+
+import Prelude
+import Lib.Parser.Impl
+import Lib.Parser
+import Lib.Token
+import Lib.Common
+import Data.List1
+
+data ReplCommand
+    = Load String
+    | OutputJS String
+    | Verbose (Maybe Int)
+    | GetDoc String
+    | BrowseCmd QName
+    | HelpCmd
+
+kw : String → Parser String
+kw s = satisfy (\t => t.val.text == s) "Expected \{show s}"
+
+replString : Parser String
+replString = do
+    token StartQuote
+    s <- token StringKind
+    token EndQuote
+    pure s
+
+replInt : Parser Int
+replInt = do
+    t <- token Number
+    pure $ stringToInt t
+
+replQN : Parser QName
+replQN = do
+  ident <- uident
+  rest <- many $ token Projection
+  let name = joinBy "" (ident :: rest)
+  pure $ uncurry QN $ unsnoc $ split1 name "."
+
+data ArgType = ArgNone | ArgString | ArgIdent | ArgOptInt | ArgQName
+
+argCon : ArgType → U
+argCon ArgNone = ReplCommand
+argCon ArgOptInt = Maybe Int → ReplCommand
+argCon ArgIdent = String → ReplCommand
+argCon ArgString = String → ReplCommand
+argCon ArgQName = QName → ReplCommand
+
+data CmdDesc : U where
+    MkCmd : String → String → (arg : ArgType) → argCon arg → CmdDesc
+
+commands : List CmdDesc
+commands
+    = MkCmd ":h" "Show this help" ArgNone HelpCmd
+    :: MkCmd ":help" "Show this help" ArgNone HelpCmd
+    :: MkCmd ":l" "load file" ArgString Load
+    :: MkCmd ":load" "load file" ArgString Load
+    :: MkCmd ":o" "write javascript file" ArgString OutputJS
+    :: MkCmd ":out" "write javascript file" ArgString OutputJS
+    :: MkCmd ":v" "change verbosity" ArgOptInt Verbose
+    :: MkCmd ":d" "document function" ArgIdent GetDoc
+    :: MkCmd ":doc" "document function" ArgIdent GetDoc
+    :: MkCmd ":b" "browse namespace" ArgQName BrowseCmd
+    -- type at point
+    -- solve hole
+    -- search by prefix (for autocomplete - ideally include types at point, but we'd need recovery)
+    --   Ideally we could auto-import too
+    -- case split
+    :: Nil
+
+parseCommand : Parser ReplCommand
+parseCommand = do
+    key <- ident
+    let (Just cmd) = lookup key commands
+        | _ => fail "Unknown command"
+    the (Parser ReplCommand) $ case cmd of
+        MkCmd _ _ ArgNone cstr => pure cstr
+        MkCmd _ _ ArgOptInt cstr => cstr <$> optional replInt
+        MkCmd _ _ ArgIdent cstr => cstr <$> (ident <|> uident)
+        MkCmd _ _ ArgString cstr => cstr <$> replString
+        MkCmd _ _ ArgQName cstr => cstr <$> replQN
+    where
+        lookup : String → List CmdDesc → Maybe CmdDesc
+        lookup key (cmd@(MkCmd nm _ _ _) :: rest) =
+            if key == nm then Just cmd else lookup key rest
+        lookup key Nil = Nothing
diff --git a/src/Lib/TopContext.newt b/src/Lib/TopContext.newt
index 055136e..79df380 100644
--- a/src/Lib/TopContext.newt
+++ b/src/Lib/TopContext.newt
@@ -19,7 +19,9 @@ lookup qn@(QN ns nm) top =
       Just mod => lookupMap' qn mod.modDefs
       Nothing => Nothing
 
--- TODO - look at imported namespaces, and either have a map of imported names or search imported namespaces..
+lookupAll : String → TopContext -> List TopEntry
+lookupAll raw top =
+  mapMaybe (flip lookup top) $ (QN top.ns raw) :: map (flip QN raw) top.imported
 
 lookupRaw : String -> TopContext -> Maybe TopEntry
 lookupRaw raw top =
diff --git a/src/Main.newt b/src/Main.newt
index eed682e..48810d7 100644
--- a/src/Main.newt
+++ b/src/Main.newt
@@ -18,8 +18,10 @@ import Lib.Tokenizer
 import Lib.TopContext
 import Lib.Types
 import Lib.Syntax
+import Lib.ReplParser
 import Node
 import Serialize
+import Revision
 
 -- For editors, dump some information about the context (fc, name, type)
 jsonTopContext : M Json
@@ -259,10 +261,72 @@ cmdLine (fn :: args) = do
   (out, files) <- cmdLine args
   pure (out, fn :: files)
 
+browseTop : QName → M Unit
+browseTop qn@(QN ns x) = do
+  top <- getTop
+  let ns = snoc ns x
+  let (Just mod) = lookupMap' ns top.modules
+    | _ => putStrLn "module \{show qn} not loaded"
+  go $ listValues mod.modDefs
+  where
+    go : List TopEntry → M Unit
+    go Nil = pure MkUnit
+    go (e :: es) = do
+      putStrLn "\{show e.name} : \{rpprint Nil e.type}"
+      go es
+
+replHeader : M Unit
+replHeader = putStrLn "Newt REPL (\{show gitRevision})\n"
+
+getDoc : String → M Unit
+getDoc name = do
+  top <- getTop
+  for_ (lookupAll name top) $ \e =>
+      putStrLn "\{show e.name} : \{rpprint Nil e.type}"
+
+replHelp : M Unit
+replHelp =
+  for_ commands $ \cmd => do
+    let args = case cmd of
+      MkCmd _ _ ArgNone _ => ""
+      MkCmd _ _ ArgQName _ => " Lib.Types"
+      MkCmd _ _ ArgOptInt _ => " [ 1 ]"
+      MkCmd _ _ ArgIdent _ => " map"
+      MkCmd _ _ ArgString _ => " \"name\""
+    case cmd of
+      (MkCmd kw desc _ _) => putStrLn "\{kw}\{args} - \{desc}"
+
+runCommand : ReplCommand → M Unit
+runCommand (Load fn) = processFile fn
+runCommand (HelpCmd) = replHelp
+runCommand (BrowseCmd qn) = browseTop qn
+runCommand (GetDoc name) = getDoc name
+runCommand (Verbose Nothing) = modifyTop [ verbose $= _+_ 1 ]
+runCommand (Verbose (Just v)) = modifyTop [ verbose := v ]
+runCommand (OutputJS fn) = writeSource fn
+
+runRepl : M Unit
+runRepl = do
+  liftIO $ putStr "> "
+  Right line <- liftIO {M} $ readLine
+    | Left err => pure MkUnit
+  let (Right toks) = tokenise "<stdin>" line
+    | Left err => putStrLn (showError line err) >> runRepl
+  let (Right cmd) = parse "<stdin>" parseCommand toks
+    | Left err => putStrLn (showError line err) >> runRepl
+  catchError (runCommand cmd) (\ err => putStrLn $ showError line err)
+  runRepl
+
+-- TODO translate args into REPL commands?
 main' : M Unit
 main' = do
   (arg0 :: args) <- liftIO {M} $ getArgs
     | _ => error emptyFC "error reading args"
+  -- run REPL if there are no args
+  let (_ :: _) = args
+        | _ => do
+            replHeader
+            runRepl
   (out, files) <- cmdLine args
   traverse processFile files
 
@@ -274,7 +338,6 @@ main' = do
     Nothing => pure MkUnit
     Just name => writeSource name
 
-
 main : IO Unit
 main = do
   -- we'll need to reset for each file, etc.
diff --git a/src/Node.newt b/src/Node.newt
index 8f2d2df..a7f9485 100644
--- a/src/Node.newt
+++ b/src/Node.newt
@@ -34,3 +34,24 @@ pfunc exitFailure : ∀ a. String → a := `(_, msg) => {
   process.exit(1);
 }`
 
+pfunc putStr uses (MkIORes MkUnit): String → IO Unit := `(s) => (w) => {
+  let {writeSync} = require('fs');
+  let buf = new TextEncoder().encode(s);
+  writeSync(1, buf);
+  return Prelude_MkIORes(Prelude_MkUnit, w);
+}`
+
+pfunc readLine uses (MkIORes Left Right) : IO (Either String String) := `(w) => {
+    let {readSync} = require('fs');
+    let buf = Buffer.alloc(1024);
+    let p = 0
+    while (readSync(0, buf, p, 1, null)) {
+        if (buf[p++] == 10) return Prelude_MkIORes(Prelude_Right(new TextDecoder().decode(buf.slice(0,p))),w);
+        if (p + 10 > buf.length) {
+            let tmp = Buffer.alloc(buf.length * 1.5);
+            tmp.set(buf);
+            buf = tmp;
+        }
+    }
+    return Prelude_MkIORes(Prelude_Left("EOF"));
+}`