diff --git a/lib/Quox/CharExtra.idr b/lib/Quox/CharExtra.idr
index c9f718b..52b9fa0 100644
--- a/lib/Quox/CharExtra.idr
+++ b/lib/Quox/CharExtra.idr
@@ -96,17 +96,13 @@ genCat ch = assert_total $
        "Cn" => Other       NotAssigned
 
 
-export
-isPrintable : Char -> Bool
-isPrintable ch = case genCat ch of Other _ => False; _ => True
-
 export
 isIdStart : Char -> Bool
 isIdStart ch =
   case genCat ch of
-       Letter _              => True
-       Punctuation Connector => True -- _, tie bars, etc
-       _                     => False
+       Letter _ => True
+       Number _ => not ('0' <= ch && ch <= '9')
+       _        => False
 
 export
 isIdCont : Char -> Bool
@@ -117,14 +113,23 @@ isIdCont ch =
        Number _ => True
        _        => False
 
+export
+isIdConnector : Char -> Bool
+isIdConnector ch =
+  case genCat ch of Punctuation Connector => True; _ => False
+
+
 export
 isSymChar : Char -> Bool
 isSymChar ch =
   case genCat ch of
        Symbol      _     => True
+       Punctuation Dash  => True
        Punctuation Other => True
        _                 => False
 
 export
 isWhitespace : Char -> Bool
-isWhitespace ch = case genCat ch of Separator _ => True; _ => False
+isWhitespace ch =
+  ch == '\t' || ch == '\r' || ch == '\n' ||
+  case genCat ch of Separator _ => True; _ => False
diff --git a/lib/Quox/NatExtra.idr b/lib/Quox/NatExtra.idr
index 9e7d5de..42f627e 100644
--- a/lib/Quox/NatExtra.idr
+++ b/lib/Quox/NatExtra.idr
@@ -1,6 +1,9 @@
 module Quox.NatExtra
 
 import public Data.Nat
+import Data.Nat.Division
+import Data.SnocList
+import Data.Vect
 
 %default total
 
@@ -30,3 +33,25 @@ public export
 toLte : {n : Nat} -> m `LTE'` n -> m `LTE` n
 toLte LTERefl      = reflexive
 toLte (LTESuccR p) = lteSuccRight (toLte p)
+
+
+private
+0 baseNZ : n `GTE` 2 => NonZero n
+baseNZ @{LTESucc _} = SIsNonZero
+
+parameters {base : Nat} {auto 0 _ : base `GTE` 2} (chars : Vect base Char)
+  private
+  showAtBase' : List Char -> Nat -> List Char
+  showAtBase' acc 0 = acc
+  showAtBase' acc k =
+    let dig = natToFinLT (modNatNZ k base baseNZ) @{boundModNatNZ {}} in
+    showAtBase' (index dig chars :: acc)
+                (assert_smaller k $ divNatNZ k base baseNZ)
+
+  export
+  showAtBase : Nat -> String
+  showAtBase = pack . showAtBase' []
+
+export
+showHex : Nat -> String
+showHex = showAtBase $ fromList $ unpack "0123456789ABCDEF"
diff --git a/lib/quox-lib.ipkg b/lib/quox-lib.ipkg
index 5334aff..dd5a02a 100644
--- a/lib/quox-lib.ipkg
+++ b/lib/quox-lib.ipkg
@@ -9,6 +9,7 @@ depends = base, contrib, elab-util, sop, snocvect
 
 modules =
   Quox.NatExtra,
+  Quox.CharExtra,
   Quox.OPE,
   Quox.Pretty,
   Quox.Syntax,
diff --git a/tests/Tests.idr b/tests/Tests.idr
index a1fe8c6..b6a7462 100644
--- a/tests/Tests.idr
+++ b/tests/Tests.idr
@@ -2,6 +2,7 @@ module Tests
 
 import Options
 import TAP
+import Tests.CharExtra
 import Tests.Lexer
 import Tests.Parser
 import Tests.Equal
@@ -9,6 +10,7 @@ import System
 
 
 allTests = [
+  CharExtra.tests,
   Lexer.tests,
   Parser.tests,
   Equal.tests
diff --git a/tests/Tests/CharExtra.idr b/tests/Tests/CharExtra.idr
new file mode 100644
index 0000000..594c25a
--- /dev/null
+++ b/tests/Tests/CharExtra.idr
@@ -0,0 +1,56 @@
+module Tests.CharExtra
+
+import Quox.NatExtra
+import Quox.CharExtra
+import Data.List
+import Data.String
+import Data.Maybe
+import TAP
+
+
+escape : Char -> Maybe String
+escape '\'' = Nothing
+escape c =
+  if ord c > 0xFF then Nothing else
+  case unpack $ show c of
+       '\'' :: '\\' :: cs => pack . ('\\' ::) <$> init' cs
+       _                  => Nothing
+
+display : Char -> String
+display c =
+  let ord = cast c {to = Nat} in
+  let c'  = fromMaybe (singleton c) $ escape c in
+  if 0x20 <= ord && ord <= 0xFF then
+    "｢\{c'}｣"
+  else
+    let codepoint = padLeft 4 '0' $ showHex ord in
+    "｢\{c'}｣ (U+\{codepoint})"
+
+testOne : (Char -> Bool) -> Char -> Test
+testOne pred c = test (display c) $ unless (pred c) $ Left ()
+
+testAll : String -> (Char -> Bool) -> List Char -> Test
+testAll label pred chars = label :- map (testOne pred) chars
+
+
+tests = "char extras" :- [
+  testAll "id starts" isIdStart
+    ['a', 'á', '𝕕', '개', 'ʨ', '𒁙', '𝟙'],
+  testAll "not id starts" (not . isIdStart)
+    ['0', '_', '-', '‿', ' ', '[', ',', '.', '\1'],
+  testAll "id continuations" isIdCont
+    ['a', 'á', '𝕕', '개', 'ʨ', '𒁙', '0', '\''],
+  testAll "not id continuations" (not . isIdCont)
+    ['_', '‿', ' ', '[', ',', '.', '\1'],
+  testAll "id connectors" isIdConnector
+    ['_', '‿'],
+  testAll "not id connectors" (not . isIdConnector)
+    ['a', ' ', ',', '-'],
+  testAll "white space" isWhitespace
+    [' ', '\t', '\r', '\n',
+     '\x2028', -- line separator
+     '\x2029'  -- paragraph separator
+    ],
+  testAll "not white space" (not . isWhitespace)
+    ['a', '-', '_', '\1']
+]