{-# LANGUAGE TypeSynonymInstances #-}
module Foo where

import Text.XHtml.Strict

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

instance HTML Int where
  toHtml = primHtml.show

instance HTML String where
  toHtml = primHtml

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

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

foo = gen unordList

{-
Without a type signature for gen, it gives this error:

Foo.hs:26:35:
    Couldn't match expected type `[Char]' against inferred type `Int'
      Expected type: [String] -> a
      Inferred type: [Int] -> a1
    In the second argument of `(<<)', namely `(listFunc ss)'
    In the second argument of `(+++)', namely `(p << (listFunc ss))'

-}