diff --git a/CHANGES.txt b/CHANGES.txt
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,5 +1,12 @@
 Changelog for NSIS
 
+0.3
+    Make strCheck take a message
+    Add a plugin for helping with radio button
+    Make constant force an evaluation
+    Make section return the section id
+    Generalise the type of nsis
+    Add a radio button example
 0.2.5
     Support OName on file
 0.2.4
diff --git a/Development/NSIS.hs b/Development/NSIS.hs
--- a/Development/NSIS.hs
+++ b/Development/NSIS.hs
@@ -84,6 +84,7 @@
     FileMode(..), SectionFlag(..), ShowWindow(..), FinishOptions(..), DetailsPrint(..)
     ) where
 
+import Control.Monad
 import Development.NSIS.Sugar
 import Development.NSIS.Show
 import Development.NSIS.Optimise
@@ -91,12 +92,12 @@
 
 
 -- | Create the contents of an NSIS script from an installer specification.
-nsis :: Action () -> String
-nsis = unlines . showNSIS . optimise . runAction
+nsis :: Action a -> String
+nsis = unlines . showNSIS . optimise . runAction . void
 
 
 -- | Like 'nsis', but don't try and optimise the resulting NSIS script. Useful
 --   to figure out how the underlying installer works, or if you believe the
 --   optimisations are introducing bugs (but please do report any such bugs!).
-nsisNoOptimise :: Action () -> String
-nsisNoOptimise = unlines . showNSIS . runAction
+nsisNoOptimise :: Action a -> String
+nsisNoOptimise = unlines . showNSIS . runAction . void
diff --git a/Development/NSIS/Library.hs b/Development/NSIS/Library.hs
--- a/Development/NSIS/Library.hs
+++ b/Development/NSIS/Library.hs
@@ -28,12 +28,19 @@
                     rest @= strDrop 1 rest)
         res
 
--- | Attempt to check that you haven't reached the string limit
-strCheck :: Exp String -> Exp String
-strCheck x = share x $ \x -> do
+-- | NSIS (the underlying installer, not this library) uses fixed length string buffers,
+--   defaulting to 1024 bytes. Any strings longer than
+--   the limit may cause truncation or segfaults. You can get builds supporting longer strings
+--   from <http://nsis.sourceforge.net/Special_Builds>.
+--
+--   Given @strCheck msg val@, if @val@ exceeds the limit it will 'abort' with @msg@, otherwise
+--   it will return 'val'.
+strCheck :: Exp String -> Exp String -> Exp String
+strCheck msg x = share x $ \x -> do
     let special = "@!!_NSIS"
-    iff_ (not_ $ special `strIsSuffixOf` (x & special)) $
-        abort "String limit exceeded"
+    iff_ (not_ $ special `strIsSuffixOf` (x & special)) $ do
+        void $ messageBox [MB_ICONSTOP] $ "ERROR: String limit exceeded,\n" & msg
+        abort $ "ERROR: String limit exceeded, " & msg
     x
 
 
diff --git a/Development/NSIS/Plugins/EnvVarUpdate.hs b/Development/NSIS/Plugins/EnvVarUpdate.hs
--- a/Development/NSIS/Plugins/EnvVarUpdate.hs
+++ b/Development/NSIS/Plugins/EnvVarUpdate.hs
@@ -36,14 +36,14 @@
 -- | Set an environment variable by writing to the registry.
 setEnvVar :: HKEY -> Exp String -> Exp String -> Action ()
 setEnvVar h key val = do
-    writeRegExpandStr h (fromString $ resolve h) key (strCheck val)
+    writeRegExpandStr h (fromString $ resolve h) key (strCheck ("setting environment variable %" & key & "%") val)
     void $ sendMessage [Timeout 5000] hwnd_BROADCAST wm_WININICHANGE (0 :: Exp Int) ("STR:Environment" :: Exp String)
 
 
 -- | Read a variable from the registry. If you are not modifying the variable
 --   you should use 'envVar' instead.
 getEnvVar :: HKEY -> Exp String -> Exp String
-getEnvVar h key = strCheck $ readRegStr h (fromString $ resolve h) key
+getEnvVar h key = strCheck ("reading environment variable %" & key & "%") $ readRegStr h (fromString $ resolve h) key
 
 
 -- | Delete the environment variable in the registry.
diff --git a/Development/NSIS/Plugins/Sections.hs b/Development/NSIS/Plugins/Sections.hs
new file mode 100644
--- /dev/null
+++ b/Development/NSIS/Plugins/Sections.hs
@@ -0,0 +1,47 @@
+
+-- | Plugin to change how many sections can be selected at once.
+module Development.NSIS.Plugins.Sections(atMostOneSection, exactlyOneSection) where
+
+import Control.Monad
+import Development.NSIS
+
+
+atMostOneSection :: [SectionId] -> Action ()
+atMostOneSection xs = do
+    selected <- mutableInt_ (-1)
+    let ensure = do
+            -- find the lowest selected, which isn't already equal to selected
+            prev <- constant_ selected
+            forM_ (reverse $ zip [0..] xs) $ \(i,x) -> do
+                iff_ (prev %/= int i %&& sectionGet x SF_Selected) $ do
+                    selected @= int i
+
+            -- ensure only (at most) selected is actually selected
+            forM_ (zip [0..] xs) $ \(i,x) -> do
+                iff_ (selected %/= int i %&& sectionGet x SF_Selected) $
+                    sectionSet x SF_Selected false
+    ensure
+    onSelChange ensure
+
+exactlyOneSection :: [SectionId] -> Action ()
+exactlyOneSection xs = do
+    selected <- mutableInt_ (-1)
+
+    let ensure = do
+            -- find the lowest selected, which isn't already equal to selected
+            prev <- constant_ selected
+            forM_ (reverse $ zip [0..] xs) $ \(i,x) ->
+                iff_ (prev %/= int i %&& sectionGet x SF_Selected) $
+                    selected @= int i
+            iff_ (selected %== (-1)) $
+                selected @= 0
+
+            -- ensure only selected is actually selected
+            forM_ (zip [0..] xs) $ \(i,x) -> do
+                let b = selected %== int i
+                iff_ (b %/= sectionGet x SF_Selected) $
+                    sectionSet x SF_Selected b
+
+    ensure
+    onSelChange ensure
+
diff --git a/Development/NSIS/Sugar.hs b/Development/NSIS/Sugar.hs
--- a/Development/NSIS/Sugar.hs
+++ b/Development/NSIS/Sugar.hs
@@ -343,8 +343,21 @@
 -- | Create a constant with no name, ensuring the expression is shared.
 --   Equivalent to @'share' 'return'@.
 constant_ :: Exp t -> Action (Exp t)
-constant_ x = do x <- x; return $ return x
+constant_ x = do
+    -- either the expression is entirely constant, or has mutable variables inside it
+    Value x <- x
+    if null [() | Var_{} <- x] then
+        -- if it's totally constant, we want to leave it that way so it works in non-eval settings (e.g. file)
+        return $ return $ Value x
+     else do
+        -- if it's mutable, we want to share the value, but also snapshot it so that the mutable variables
+        -- don't change afterwards
+        v <- var
+        return (Value $ val v) @= return (Value x)
+        -- add the Literal so that assignment throws an error in future
+        return $ return $ Value [Var_ v, Literal ""]
 
+
 -- | The 'Exp' language is call-by-name, meaning you must use share to avoid evaluating an exression
 --   multiple times. Using 'share', if the expression has any side effects
 --   they will be run immediately, but not on subsequent uses. When defining functions operating on
@@ -963,13 +976,14 @@
         f c (OName x) = do Value x <- x; return c{fileOName=Just x}
         f c x = error $ "Invalid attribute to file: " ++ show x
 
-section :: Exp String -> [Attrib] -> Action () -> Action ()
+section :: Exp String -> [Attrib] -> Action () -> Action SectionId
 section name as act = do
     sec <- newSectionId
     Value name <- name
     (xs, _) <- capture $ scope act
     x <- foldM f def{secId=sec, secName=name} as
     emit $ Section x xs
+    return $ secId x
     where
         f c Unselected = return c{secUnselected=True}
         f c Required = return c{secRequired=True}
@@ -977,13 +991,14 @@
         f c (Id x) = return c{secId=x}
         f c x = error $ "Invalid attribute to section: " ++ show x
 
-sectionGroup :: Exp String -> [Attrib] -> Action () -> Action ()
+sectionGroup :: Exp String -> [Attrib] -> Action () -> Action SectionId
 sectionGroup name as act = do
     sec <- newSectionId
     Value name <- name
     (xs, _) <- capture $ scope act
     x <- foldM f def{secgId=sec, secgName=name} as
     emit $ SectionGroup x xs
+    return $ secgId x
     where
         f c Expanded = return c{secgExpanded=True}
         f c (Description x) = do Value x <- x; return c{secgDescription=x}
@@ -991,7 +1006,7 @@
         f c x = error $ "Invalid attribute to sectionGroup: " ++ show x
 
 uninstall :: Action () -> Action ()
-uninstall = section "Uninstall" []
+uninstall = void . section "Uninstall" []
 
 -- | Delete file (which can be a file or wildcard, but should be specified with a full path) from the target system.
 --   If 'RebootOK' is specified and the file cannot be deleted then the file is deleted when the system reboots --
diff --git a/Examples/EnvVarUpdate.hs b/Examples/EnvVarUpdate.hs
--- a/Examples/EnvVarUpdate.hs
+++ b/Examples/EnvVarUpdate.hs
@@ -39,3 +39,13 @@
 
         alert $ "USER $$PATH = " & getEnvVar HKCU "PATH"
         alert $ "MACHINE $$PATH = " & getEnvVar HKLM "PATH"
+
+        alert "Expecting to abort with a String limit error"
+        deleteEnvVar HKCU "NSIS_TEST"
+        val <- mutable_ "This is a test that will overflow at some point"
+        i <- mutable_ 0
+        while (i %< 100) $ do
+           i @= i + 1
+           val @= val & val
+           setEnvVar HKCU "NSIS_TEST" val
+        alert "Failed test, should have got a string limit error"
diff --git a/Examples/Example2.hs b/Examples/Example2.hs
--- a/Examples/Example2.hs
+++ b/Examples/Example2.hs
@@ -15,10 +15,12 @@
 
 example2 = do
     -- The name of the installer
-    name "Example2"
+    _ <- constantStr "Ex" "2"
 
+    name "Example$Ex"
+
     -- The file to write
-    outFile "example2.exe"
+    outFile "example$Ex.exe"
 
     -- The default installation directory
     installDir "$PROGRAMFILES/Example2"
@@ -50,7 +52,7 @@
         setOutPath "$INSTDIR"
 
         -- Put file there
-        file [] "Examples/Example2.hs"
+        file [] "Examples/Example$Ex.hs"
 
         -- Write the installation path into the registry
         writeRegStr HKLM "SOFTWARE/NSIS_Example2" "Install_Dir" "$INSTDIR"
diff --git a/Examples/Radio.hs b/Examples/Radio.hs
new file mode 100644
--- /dev/null
+++ b/Examples/Radio.hs
@@ -0,0 +1,33 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Examples.Radio(radio) where
+
+import Development.NSIS
+import Development.NSIS.Plugins.EnvVarUpdate
+import Development.NSIS.Plugins.Sections
+
+
+radio = do
+    name "Radio"
+    outFile "radio.exe"
+    installDir "$DESKTOP/Radio"
+    requestExecutionLevel User
+
+    -- page Directory
+    page Components
+    page InstFiles
+
+    section "Core files" [Required] $ do
+        setOutPath "$INSTDIR"
+        file [] "Examples/Radio.hs"
+
+    local <- section "Add to user %PATH%" [] $ do
+        setEnvVarPrepend HKCU "PATH" "$INSTDIR"
+    global <- section "Add to system %PATH%" [] $ do
+        setEnvVarPrepend HKLM "PATH" "$INSTDIR"
+    atMostOneSection [local,global]
+
+    a <- section "I like Marmite" [] $ return ()
+    b <- section "I hate Marmite" [] $ return ()
+    c <- section "I don't care" [] $ return ()
+    exactlyOneSection [a,b,c]
diff --git a/Main.hs b/Main.hs
--- a/Main.hs
+++ b/Main.hs
@@ -15,15 +15,16 @@
 import Examples.Example2
 import Examples.Finish
 import Examples.Primes
+import Examples.Radio
 import Examples.Taskbar
 import Examples.WinMessages
 import Examples.EnvVarUpdate
 
 
 examples = let (*) = (,) in
-    ["example1" * example1, "example2" * example2, "base64" * base64
-    ,"finish" * finish, "primes" * primes, "taskbar" * taskbar
-    ,"winmessages" * winmessages, "envvarupdate" * envvarupdate]
+    ["example1" * void example1, "example2" * void example2, "base64" * void base64
+    ,"finish" * void finish, "primes" * void primes, "radio" * void radio, "taskbar" * void taskbar
+    ,"winmessages" * void winmessages, "envvarupdate" * void envvarupdate]
 
 
 main = do
diff --git a/nsis.cabal b/nsis.cabal
--- a/nsis.cabal
+++ b/nsis.cabal
@@ -1,7 +1,7 @@
 cabal-version:      >= 1.10
 build-type:         Simple
 name:               nsis
-version:            0.2.5
+version:            0.3
 license:            BSD3
 license-file:       LICENSE
 category:           Development
@@ -39,6 +39,7 @@
         Development.NSIS.Plugins.Base64
         Development.NSIS.Plugins.EnvVarUpdate
         Development.NSIS.Plugins.Taskbar
+        Development.NSIS.Plugins.Sections
         Development.NSIS.Plugins.WinMessages
     other-modules:
         Development.NSIS.Library
@@ -65,5 +66,6 @@
         Examples.Example2
         Examples.Finish
         Examples.Primes
+        Examples.Radio
         Examples.Taskbar
         Examples.WinMessages