[GHC] #11628: Unexpected results with Read/Show

22 Feb 2016

      #11628: Unexpected results with Read/Show
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           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In the following simplified example, `Foo` and `U` correspond to GADTs
 that GHC will not derive `Read`/`Show` for. I attempted to work around
 that by using newtypes for each GADT constructor, and letting GHC derive
 the `Show`/`Read` instances for those instead. However, I get a runtime
 error (`Prelude.read: no parse`) on the second print statement in `main`:

 {{{
 {-# LANGUAGE FlexibleContexts, FlexibleInstances, GADTs,
 ScopedTypeVariables #-}

 import Text.Read (Read(readPrec))

 newtype Bar r = Bar r deriving (Show, Read)
 newtype Foo r = Foo (Bar r)
 -- use the GHC-derived Show/Read for Bar
 instance (Show r) => Show (Foo r) where
   show (Foo x) = show x
 instance (Read r) => Read (Foo r) where
   readPrec = Foo <$> readPrec

 data U t rep r where
   U1  :: t r -> U t Int r
   U2 :: t r -> U t Char r
 -- use the Read/Show instances for U1Wrap and U2Wrap
 newtype U1Wrap t r = U1Wrap {unU1Wrap :: t r} deriving (Show, Read)
 newtype U2Wrap t r = U2Wrap (t r) deriving (Show, Read)
 instance (Read (t r)) => Read (U t Int r) where
   readPrec = (U1 . unU1Wrap) <$> readPrec
 instance (Read (U2Wrap t r)) => Read (U t Char r) where
   readPrec = do
     x <- readPrec
     return $ case x of
       (U2Wrap y) -> U2 y
 instance (Show (t r)) => Show (U t Int r) where
   show (U1 x) = show $ U1Wrap x
 instance (Show (t r)) => Show (U t Char r) where
   show (U2 x) = show (U2Wrap x :: U2Wrap t r)

 main :: IO ()
 main = do
   let x = U1 $ Foo $ Bar 3
       y = U2 $ Foo $ Bar 3
   print $ show (read (show x) `asTypeOf` x)
   print $ show (read (show y) `asTypeOf` y)
 }}}

 Someone mentioned that I should define `showsPrec` rather than `show`, but
 these are listed as alternatives in
 [https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.8.2.0
 /Text-Show.html the docs].

 It's not clear to me if GHCs derived instances are invalid, or if I'm
 doing something illegal. In the latter case, the docs need some
 improvement.

 (Verified this behavior in 7.10.2 and HEAD.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11628
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