deepseq-generics-0.1.1.2: GHC.Generics-based Control.DeepSeq.rnf implementation

Copyright | (c) 2012, Herbert Valerio Riedel
License | BSD-style (see the LICENSE file)
Maintainer | Herbert Valerio Riedel <hvr@gnu.org>
Stability | stable
Portability | GHC
Safe Haskell | Safe
Language | Haskell2010

Control.DeepSeq.Generics

Contents

- Control.DeepSeq re-exports

Description

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)

NOTE: Starting with `deepseq-1.4.0.0`, `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.

Synopsis

- genericRnf :: (Generic a, GNFData (Rep a)) => a -> ()
- genericRnfV1 :: (Generic a, GNFDataV1 (Rep a)) => a -> ()
- deepseq :: NFData a => a -> b -> b
- force :: NFData a => a -> a
- class NFData a where
  - rnf :: a -> ()
- ($!!) :: NFData a => (a -> b) -> a -> b

# Documentation

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

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 -> ()

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)

    <interactive>:1:1:
        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

Since: 0.1.1.0

# 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.

Since: 1.1.0.0

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.

`force` can be conveniently used in combination with `ViewPatterns`:

    {-# LANGUAGE BangPatterns, ViewPatterns #-}
    import Control.DeepSeq

    someFun :: ComplexData -> SomeResult
    someFun (force -> !arg) = {- 'arg' will be fully evaluated -}

Another useful application is to combine `force` with `evaluate` in order to force deep evaluation relative to other `IO` operations:

    import Control.Exception (evaluate)
    import Control.DeepSeq

    main = do
      result <- evaluate $ force $ pureComputation
      {- 'result' will be fully evaluated at this point -}
      return ()

Since: 1.2.0.0

class NFData a where

A class of types that can be fully evaluated.

Since: 1.1.0.0

Minimal complete definition

Nothing

Methods

rnf :: a -> ()

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

### `Generic` `NFData` deriving

Starting with GHC 7.2, you can automatically derive instances for types possessing a `Generic` instance.

    {-# LANGUAGE DeriveGeneric #-}

    import GHC.Generics (Generic)
    import Control.DeepSeq

    data Foo a = Foo a String
                 deriving (Eq, Generic)

    instance NFData a => NFData (Foo a)

    data Colour = Red | Green | Blue
                  deriving Generic

    instance NFData Colour

Starting with GHC 7.10, the example above can be written more concisely by enabling the new `DeriveAnyClass` extension:

    {-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}

    import GHC.Generics (Generic)
    import Control.DeepSeq

    data Foo a = Foo a String
                 deriving (Eq, Generic, NFData)

    data Colour = Red | Green | Blue
                  deriving (Generic, NFData)

### Compatibility with previous `deepseq` versions

Prior to version 1.4.0.0, the default implementation of the `rnf` method was defined as

    rnf a = seq a ()

However, starting with `deepseq-1.4.0.0`, the default implementation is based on `DefaultSignatures` allowing for more accurate auto-derived `NFData` instances. If you need the previously used exact default `rnf` method implementation semantics, use

    instance NFData Colour where rnf x = seq x ()

or alternatively

    {-# LANGUAGE BangPatterns #-}
    instance NFData Colour where rnf !_ = ()

Instances

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 TypeRep | NOTE: Only defined for `base-4.8.0.0` and later Since: 1.4.0.0
NFData () |
NFData Void | Defined as ``rnf` = `absurd``. Since: 1.4.0.0
NFData Unique | Since: 1.4.0.0
NFData Natural | Since: 1.4.0.0
NFData Version | Since: 1.3.0.0
NFData ThreadId | Since: 1.4.0.0
NFData CChar | Since: 1.4.0.0
NFData CSChar | Since: 1.4.0.0
NFData CUChar | Since: 1.4.0.0
NFData CShort | Since: 1.4.0.0
NFData CUShort | Since: 1.4.0.0
NFData CInt | Since: 1.4.0.0
NFData CUInt | Since: 1.4.0.0
NFData CLong | Since: 1.4.0.0
NFData CULong | Since: 1.4.0.0
NFData CLLong | Since: 1.4.0.0
NFData CULLong | Since: 1.4.0.0
NFData CFloat | Since: 1.4.0.0
NFData CDouble | Since: 1.4.0.0
NFData CPtrdiff | Since: 1.4.0.0
NFData CSize | Since: 1.4.0.0
NFData CWchar | Since: 1.4.0.0
NFData CSigAtomic | Since: 1.4.0.0
NFData CClock | Since: 1.4.0.0
NFData CTime | Since: 1.4.0.0
NFData CUSeconds | Since: 1.4.0.0
NFData CSUSeconds | Since: 1.4.0.0
NFData CFile | Since: 1.4.0.0
NFData CFpos | Since: 1.4.0.0
NFData CJmpBuf | Since: 1.4.0.0
NFData CIntPtr | Since: 1.4.0.0
NFData CUIntPtr | Since: 1.4.0.0
NFData CIntMax | Since: 1.4.0.0
NFData CUIntMax | Since: 1.4.0.0
NFData All | Since: 1.4.0.0
NFData Any | Since: 1.4.0.0
NFData TyCon | NOTE: Only defined for `base-4.8.0.0` and later Since: 1.4.0.0
NFData Fingerprint | Since: 1.4.0.0
NFData a => NFData [a] |
(Integral a, NFData a) => NFData (Ratio a) |
NFData (StableName a) | Since: 1.4.0.0
NFData a => NFData (Identity a) | Since: 1.4.0.0
NFData (Fixed a) | Since: 1.3.0.0
NFData a => NFData (Complex a) |
NFData a => NFData (ZipList a) | Since: 1.4.0.0
NFData a => NFData (Dual a) | Since: 1.4.0.0
NFData a => NFData (Sum a) | Since: 1.4.0.0
NFData a => NFData (Product a) | Since: 1.4.0.0
NFData a => NFData (First a) | Since: 1.4.0.0
NFData a => NFData (Last a) | Since: 1.4.0.0
NFData a => NFData (Down a) | Since: 1.4.0.0
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. Since: 1.3.0.0
(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 (Const a b) | Since: 1.4.0.0
NFData (Proxy * a) | Since: 1.4.0.0
(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 infixr 0

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

Since: 1.2.0.0