[Hs-Generics] (issue tracker,) deriving multiple instances

[lennart spitzner]

hi,

firstly, the "official" syb issue tracker is read-only.

My real question is if the code below is not supposed to work, or if
there exists a way to make it work. The direct cause of the error is
obvious, and I can think of an (ugly) workaround (define different instances
in separate modules), but I'd like to know if such usage of Data is considered
bad for some reason.

{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE FlexibleInstances #-}
import Data.Data ( Data )
data Foo f = Foo (f Bool) (f Int)
deriving instance Data (Foo [])
deriving instance Data (Foo Maybe)

ghc-7.10.3 error (ghc-8.0.1 shows the same general behaviour, though the

Multiple declarations of ‘$cr2C’
Declared at: Main.hs:44:1
Main.hs:45:1
Multiple declarations of ‘$tr2D’
Declared at: Main.hs:44:1
Main.hs:45:1
Duplicate type signatures for ‘$tr2D’
at Main.hs:44:1-31
Main.hs:45:1-34
Duplicate type signatures for ‘$cr2C’
at Main.hs:44:1-31
Main.hs:45:1-34
instance Data.Data.Data (Main.Foo []) where
Data.Data.gfoldl k_a45I z_a45J (Main.Foo a1_a45K a2_a45L)
= ((z_a45J Main.Foo `k_a45I` a1_a45K) `k_a45I` a2_a45L)
Data.Data.gunfold k_a45M z_a45N _
= k_a45M (k_a45M (z_a45N Main.Foo))
Data.Data.toConstr (Main.Foo _ _) = Main.$cr3ji
Data.Data.dataTypeOf _ = Main.$tr3gf
Main.$tr3gf :: Data.Data.DataType
Main.$cr3ji :: Data.Data.Constr
Main.$tr3gf = Data.Data.mkDataType "Foo" [Main.$cr3ji]
Main.$cr3ji
= Data.Data.mkConstr Main.$tr3gf "Foo" [] Data.Data.Prefix