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

MaintainerHerbert Valerio Riedel <>
Safe HaskellSafe-Inferred




Note: Beyond the primary scope of providing the genericRnf helper, this module also re-exports the definitions from Control.DeepSeq for convenience. If this poses any problems, just use qualified or explicit import statements (see code usage example in the genericRnf description)



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 on purpose currently.

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: NF not defined for uninhabited types


Control.DeepSeq re-exports

deepseq :: NFData a => a -> b -> b

deepseq: fully evaluates the first argument, before returning the second.

The name deepseq is used to illustrate the relationship to seq: where seq is shallow in the sense that it only evaluates the top level of its argument, deepseq traverses the entire data structure evaluating it completely.

deepseq can be useful for forcing pending exceptions, eradicating space leaks, or forcing lazy I/O to happen. It is also useful in conjunction with parallel Strategies (see the parallel package).

There is no guarantee about the ordering of evaluation. The implementation may evaluate the components of the structure in any order or in parallel. To impose an actual order on evaluation, use pseq from Control.Parallel in the parallel package.

force :: NFData a => a -> a

a variant of deepseq that is useful in some circumstances:

 force x = x `deepseq` x

force x fully evaluates x, and then returns it. Note that force x only performs evaluation when the value of force x itself is demanded, so essentially it turns shallow evaluation into deep evaluation.

class NFData a where

A class of types that can be fully evaluated.


rnf :: a -> ()

rnf should reduce its argument to normal form (that is, fully evaluate all sub-components), and then return '()'.

The default implementation of rnf is

 rnf a = a `seq` ()

which may be convenient when defining instances for data types with no unevaluated fields (e.g. enumerations).


NFData Bool 
NFData Char 
NFData Double 
NFData Float 
NFData Int 
NFData Int8 
NFData Int16 
NFData Int32 
NFData Int64 
NFData Integer 
NFData Word 
NFData Word8 
NFData Word16 
NFData Word32 
NFData Word64 
NFData () 
NFData Version 
NFData a => NFData [a] 
(Integral a, NFData a) => NFData (Ratio a) 
NFData (Fixed a) 
(RealFloat a, NFData a) => NFData (Complex a) 
NFData a => NFData (Maybe a) 
NFData (a -> b)

This instance is for convenience and consistency with seq. This assumes that WHNF is equivalent to NF for functions.

(NFData a, NFData b) => NFData (Either a b) 
(NFData a, NFData b) => NFData (a, b) 
(Ix a, NFData a, NFData b) => NFData (Array a b) 
(NFData a, NFData b, NFData c) => NFData (a, b, c) 
(NFData a, NFData b, NFData c, NFData d) => NFData (a, b, c, d) 
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) => NFData (a1, a2, a3, a4, a5) 
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) => NFData (a1, a2, a3, a4, a5, a6) 
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7) => NFData (a1, a2, a3, a4, a5, a6, a7) 
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7, NFData a8) => NFData (a1, a2, a3, a4, a5, a6, a7, a8) 
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7, NFData a8, NFData a9) => NFData (a1, a2, a3, a4, a5, a6, a7, a8, a9) 

($!!) :: NFData a => (a -> b) -> a -> b

the deep analogue of $!. In the expression f $!! x, x is fully evaluated before the function f is applied to it.