Generic RNF

Main.hs

{-# LANGUAGE DeriveGeneric #-}

module Main where

import Control.DeepSeq
import Control.DeepSeq.Generics (genericRnf)
import GHC.Generics

data List a = Nil | Cons a (List a) deriving (Generic)

instance NFData a => NFData (List a) where
  rnf = genericRnf

main :: IO ()
main = undefined

output.txt

Building D-0.1.0.0...
Preprocessing executable 'D' for D-0.1.0.0...
[1 of 1] Compiling Main             ( Main.hs, dist/build/D/D-tmp/Main.o )

==================== Derived instances ====================
Derived instances:
  instance Generic (List a_a4B7) where
    from Nil = M1 (L1 (M1 U1))
    from (Cons g1_a4B8 g2_a4B9)
      = M1 (R1 (M1 ((:*:) (M1 (K1 g1_a4B8)) (M1 (K1 g2_a4B9)))))
    to (M1 (L1 (M1 U1))) = Nil
    to (M1 (R1 (M1 ((:*:) (M1 (K1 g1_a4Ba)) (M1 (K1 g2_a4Bb))))))
      = Cons g1_a4Ba g2_a4Bb

  instance Datatype D1List where
    datatypeName _ = "List"
    moduleName _ = "Main"

  instance Constructor C1_0List where conName _ = "Nil"

  instance Constructor C1_1List where conName _ = "Cons"


Generic representation:

  Generated datatypes for meta-information:
    D1List
    C1_0List
    C1_1List
    S1_1_0List
    S1_1_1List

  Representation types:
    type Rep (List a_a4B6) = D1
                               D1List
                               (C1 C1_0List U1
                                :+: C1
                                      C1_1List
                                      (S1 NoSelector (Rec0 a_a4B6)
                                       :*: S1 NoSelector (Rec0 (List a_a4B6))))


[1 of 1] Compiling Main             ( Main.hs, dist/build/D/D-tmp/Main.o )

==================== Derived instances ====================
Derived instances:
  instance Generic (List a_a4B6) where
    from Nil = M1 (L1 (M1 U1))
    from (Cons g1_a4B7 g2_a4B8)
      = M1 (R1 (M1 ((:*:) (M1 (K1 g1_a4B7)) (M1 (K1 g2_a4B8)))))
    to (M1 (L1 (M1 U1))) = Nil
    to (M1 (R1 (M1 ((:*:) (M1 (K1 g1_a4B9)) (M1 (K1 g2_a4Ba))))))
      = Cons g1_a4B9 g2_a4Ba

  instance Datatype D1List where
    datatypeName _ = "List"
    moduleName _ = "Main"

  instance Constructor C1_0List where conName _ = "Nil"

  instance Constructor C1_1List where conName _ = "Cons"


Generic representation:

  Generated datatypes for meta-information:
    D1List
    C1_0List
    C1_1List
    S1_1_0List
    S1_1_1List

  Representation types:
    type Rep (List a_a4B5) = D1
                               D1List
                               (C1 C1_0List U1
                                :+: C1
                                      C1_1List
                                      (S1 NoSelector (Rec0 a_a4B5)
                                       :*: S1 NoSelector (Rec0 (List a_a4B5))))


Linking dist/build/D/D ...

Command was
cabal build --ghc-options='-fforce-recomp -ddump-deriv -dsuppress-all'

I think my problem is that it's not deriving the NFData instance, it's just incidental to what the genericRnf function does with the datatype representation provided by the Generic instance.

That makes sense, but if I run with -O0 and -ddump-simpl, I can actually see the NFData dictionary for List:

-- NFData dictionary for List
$fNFDataList
$fNFDataList = a_r4Fs `cast` ...

-- rnf for List passing NFData dictionary for element type
a_r4Fs
a_r4Fs = \ @ a1_a4wI $dNFData_a4wJ -> $crnf_r4Fr $dNFData_a4wJ

-- rnf implementation using generic representation of List
Rec {
$crnf_r4Fr
$crnf_r4Fr =
  \ @ a1_X4xm $dNFData_X4xo ->
    genericRnf
      ($fGenericList)
      (($fGNFDataM1
          ($fGNFData:+:
             ($fGNFDataM1 $fGNFDataU1)
             ($fGNFDataM1
                ($fGNFData:*:
                   ($fGNFDataM1 ($fGNFDataK1 $dNFData_X4xo))
                   ($fGNFDataM1
                      ($fGNFDataK1 (($crnf_r4Fr $dNFData_X4xo) `cast` ...)))))))
       `cast` ...)
end Rec }

Maybe it's a version difference? I'm using ghc 7.10.1