Haskell Html builder using polyvariadic functions

Html.hs

import Data.Monoid

class IsNodes ns where
    toNodes :: ns -> Nodes
    fromSeveral :: [ns] -> Nodes

instance IsNodes Nodes where
    toNodes = id
    fromSeveral = mconcat

instance IsNodes Node where
    toNodes n = Nodes [n]
    fromSeveral = Nodes

instance IsNodes Char where
    toNodes c = Nodes [Inner [c]]
    fromSeveral cs = Nodes [Inner cs]

instance IsNodes n => IsNodes [n] where
    toNodes = fromSeveral
    fromSeveral = fromSeveral

class HtmlBuilder a where
    build :: IsNodes nodes => nodes -> a

data Tag = Html | Head | Body | P | Div | Ul | Ol | Li deriving (Eq)
data Node
    = Open Tag
    | Close Tag
    | Inner String
    | Attribute String String
    deriving Show
newtype Nodes = Nodes [Node]

instance Show Tag where
    show Html = "html"
    show Head = "head"
    show Body = "body"
    show P = "p"
    show Div = "div"
    show Ul = "ul"
    show Ol = "ol"
    show Li = "li"

instance Show Nodes where
    show (Nodes (Open t:ns)) = "<"
        ++ show t
        ++ (when (' ':) (not . null) . unwords . fmap showAttribute . takeWhile isAttribute $ ns)
        ++ ">"
        ++ show (Nodes . dropWhile isAttribute $ ns)
    show (Nodes (Close t:ns)) = "</" ++ show t ++ ">" ++ (show . Nodes $ ns)
    show (Nodes (Inner s:ns)) = s ++ (show . Nodes $ ns)
    show (Nodes []) = ""
    show (Nodes (Attribute _ _:_)) = error "Attribute without opening tag"

when :: (a -> a) -> (a -> Bool) -> a -> a
when f p i = if p i then f i else i

showAttribute :: Node -> String
showAttribute (Attribute k v) = k ++ "=" ++ v
showAttribute n = error $ "Can't show attribute for node " ++ show n

isAttribute :: Node -> Bool
isAttribute (Attribute _ _) = True
isAttribute _ = False

instance Monoid Nodes where
    mempty = Nodes mempty
    mappend (Nodes xs) (Nodes ys) = Nodes (mappend xs ys)

instance (IsNodes h, HtmlBuilder b) => HtmlBuilder (h -> b) where
    build h x = build (toNodes h <> toNodes x)

instance HtmlBuilder Nodes where
    build = toNodes

instance HtmlBuilder Html where
    build = htmlFromNodes

data Html = Block Tag [(String, String)] [Html] deriving (Show, Eq)

htmlFromNodes :: IsNodes n => n -> Html
htmlFromNodes = (\(Nodes ns) -> htmlFromNodes' ns) . toNodes
    where
        htmlFromNodes' = undefined

html_ = Open Html
_html = Close Html
head_ = Open Head
_head = Close Head
body_ = Open Body
_body = Close Body
p_ = Open P
_p = Close P
div_ = Open Div
_div = Close Div
li_ = Open Li
_li = Close Li
ol_ = Open Ol
_ol = Close Ol
ul_ = Open Ul
_ul = Close Ul
inner = Inner
asNodes :: Nodes -> Nodes
asNodes = toNodes

id_ = Attribute "id"

example :: Nodes
example = build
    html_
        div_
            p_ "This is some text" _p
        _div
        div_ (id_ "main")
            p_ "This is some more text" _p
            p_ "This is the last paragraph" _p
        _div
    _html