#12874: Read/Show Incompatibility in base -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- 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: {{{#!hs 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. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12874 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler