Hi,

  I have very simple code:

  class XmlValue a where
      toValue :: a -> String

  instance XmlValue String where
      toValue _ = "lorem"

  It gives following error:

  Illegal instance declaration for `XmlValue String'
  (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.)

  So, maybe I'll remove that synonym:

  instance XmlValue [Char] where
       toValue _ = "lorem"

  This gives following error:

  Illegal instance declaration for `XmlValue [Char]'
   (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.)

  So, maybe I'll try to use that (T a1 ... an) form:

  instance XmlValue ([] String) where
      toValue _ = "lorem"

  Illegal instance declaration for `XmlValue [String]'
   (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.)

  Ok. Maybe I'll try to enable FlexibleInstances with  {-# LANGUAGE FlexibleInstances #-}

  Ok, this works, but:

  http://stackoverflow.com/questions/15285822/cant-make-string-an-instance-of-a-class-in-haskell (second answer):

  "
  However, Haskell98 forbids this type of typeclass in order to keep things simple and to make it harder for people to write overlapping instances like 
  instance Slang [a] where
      -- Strings would also fit this definition.
      slangify list = "some list"
  "

  Ok, that's bad. But I want only [Char], [a] shouldn't work.

  https://ghc.haskell.org/trac/haskell-prime/wiki/FlexibleInstances says, that I'll get all or nothing.

  I want to enable "instance [Char] where..." but disable "instance [a] where...". Is it possible?

Thanks,
Emanuel