#7742: StandaloneDeriving on Read fails for GADTs --------------------------------------+------------------------------------- Reporter: hoffstaetter | Owner: Type: bug | Status: new Priority: normal | Component: Compiler Version: 7.6.2 | Keywords: Os: MacOS X | Architecture: Unknown/Multiple Failure: GHC rejects valid program | Blockedby: Blocking: | Related: --------------------------------------+------------------------------------- Comment(by kosmikus): I think GHC is correct here. Ignoring `readsPrec` for simplicity and just looking at `read`, you are trying to derive a function of `String -> Foo a` here. But in order to produce a `Foo a`, we have to ''construct'' a `Read a` dictionary. And there's no way to come up with one for an arbitrary `a`. That's why you need an external `Read a` constraint. The situation is quite different for `Show`, because then you're provided with a `Foo a`, can extract the `Show` dictionary hidden within and use that to produce a textual representation of the `a`. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7742#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler