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I just stumbled across a situation where the deriving machinery, in combination with types provided by base, produces Show/Read instances that are incompatible. Here is a minimal example:

import Data.Proxy

main :: IO ()
main = do
  x <- readLn
  print (x :: Thing (Proxy Int))

data Thing a = Thing a
  deriving (Read,Show)

If you run this program and type "Thing Proxy" into standard in, you get a parse error. If you type in "Thing (Proxy)", it works fine. Calling show on Thing Proxy gives the string "Thing Proxy" though. So, read . show =/= id in this case. This might just be an issue with Proxy's Read instance, but I'm not really sure.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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