Discrepancies between derived Show instances and structuralShowsPrec

[RyanGlScott]

In certain cases, the output of structuralShowsPrec will differ from what deriving Show produces. Here is a test case which illustrates this:

{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Data.Parameterized.TH.GADT

data T
  = MkRec { unRec :: Int }
  | Int :@@: Int
  deriving Show

$(return [])

puShowT :: T -> String
puShowT t = $(structuralShowsPrec [t| T |]) 0 t ""

main :: IO ()
main = do
  let r   = MkRec 42
      inf = 42 :@@: 42
  putStrLn $ "MkRec (derived):\t\t" ++ show r
  putStrLn $ "MkRec (parameterized-utils):\t" ++ puShowT r
  putStrLn $ "(:@@:) (derived):\t\t" ++ show inf
  putStrLn $ "(:@@:) (parameterized-utils):\t" ++ puShowT inf

This will output:

MkRec (derived):                MkRec {unRec = 42}
MkRec (parameterized-utils):    MkRec 42
(:@@:) (derived):               42 :@@: 42
(:@@:) (parameterized-utils):   :@@: 42 42

This shows that structuralShowsPrec ignores record and infix data constructors, which deriving Show is aware of.