deepseq-generics- GHC.Generics-based Control.DeepSeq.rnf implementation

Copyright(c) 2012, Herbert Valerio Riedel
LicenseBSD-style (see the LICENSE file)
MaintainerHerbert Valerio Riedel <>
Safe HaskellSafe



NOTE: Starting with deepseq-, NFData gained support for generic derivation via DefaultSignatures. The new default rnf method implementation is then equivalent to

instance NFData MyType where
  rnf = genericRnfV1

See documentation of rnf for more details on how to use the new built-in Generic support.



genericRnf :: (Generic a, GNFData (Rep a)) => a -> () Source

GHC.Generics-based rnf implementation

This provides a generic rnf implementation for one type at a time. If the type of the value genericRnf is asked to reduce to NF contains values of other types, those types have to provide NFData instances. This also means that recursive types can only be used with genericRnf if a NFData instance has been defined as well (see examples below).

The typical usage for genericRnf is for reducing boilerplate code when defining NFData instances for ordinary algebraic datatypes. See the code below for some simple usage examples:

{-# LANGUAGE DeriveGeneric #-}

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

-- simple record
data Foo = Foo AccountId Name Address
         deriving Generic

type Address      = [String]
type Name         = String
newtype AccountId = AccountId Int

instance NFData AccountId
instance NFData Foo where rnf = genericRnf

-- recursive list-like type
data N = Z | S N deriving Generic

instance NFData N where rnf = genericRnf

-- parametric & recursive type
data Bar a = Bar0 | Bar1 a | Bar2 (Bar a)
           deriving Generic

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

NOTE: The GNFData type-class showing up in the type-signature is used internally and not exported.

genericRnfV1 :: (Generic a, GNFDataV1 (Rep a)) => a -> () Source

Variant of genericRnf which supports derivation for uninhabited types.

For instance, the type

data TagFoo deriving Generic

would cause a compile-time error with genericRnf, but with genericRnfV1 the error is deferred to run-time:

Prelude> genericRnf (undefined :: TagFoo)

    No instance for (GNFData V1) arising from a use of `genericRnf'
    Possible fix: add an instance declaration for (GNFData V1)
    In the expression: genericRnf (undefined :: TagFoo)
    In an equation for `it': it = genericRnf (undefined :: TagFoo)

Prelude> genericRnfV1 (undefined :: TagFoo)
*** Exception: Control.DeepSeq.Generics.genericRnfV1: uninhabited type

genericRnfV1 corresponds to deepseq-'s default rnf method implementation.

NOTE: The GNFDataV1 type-class showing up in the type-signature is used internally and not exported.