{-# LANGUAGE TypeSynonymInstances #-}
module Foo where

import Text.XHtml.Strict

data MyData =
  Foo {nums :: [Int]} |
  Bar {strs :: [Double]}

instance HTML Int where
  toHtml = primHtml.show

instance HTML Double where
  toHtml = primHtml.show

instance HTML MyData where
  toHtml (Foo ns) = primHtml ("Integers "++(show ns))
  toHtml (Bar ss) = primHtml ("Doubles "++(show ss))

gen (Foo ns) 
  | length ns == 0 = p << "no integers"
  | length ns == 1 = p << "one integer" +++ (p << (unordList ns))
  | length ns == 2 = (h1 << "several integers") +++ (p << (unordList ns))
gen (Bar ss) =
  (h1 << "Bar Heading") +++ (p << (unordList ss))

{-
  This one compiles fine.
-}