Doc 'Eq' instance fails Substitutivity

[BenjaminCosman]

Consider the following code:

module Main (main) where
import Prelude hiding ((<>))
import Text.PrettyPrint

main :: IO ()
main = do
  let s1 = text "if:" $+$ nest 4 (text "pass")
  let s2 = text "if:\n    pass"
  let f = (text "x" <>)

  --not following Eq's Substitutivity 'law':
  --first is True but second is False
  print $   s1 ==   s2
  print $ f s1 == f s2

  --examine the values visually:
  print $   s1
  print $   s2
  print $ f s1
  print $ f s2
  --outputs the following (note extra space in the 3rd):
  -- if:
  --     pass
  -- if:
  --     pass
  -- xif:
  --      pass
  -- xif:
  --     pass

If this is not the intended behavior of the various functions involved (<>, $+$, nest, ==), then there is a bug that should be fixed. If this is the intended behavior, then the documentation on Eq should alert the reader to the fact that the Substitutivity Law is not followed.

[phadej]

It could be documented that Eq instance witnesses equivalence (==) `on` render, thus for most combinators in the library substitutivity indeed doesn't hold.

pr welcome