Copyright	(C) 2015-2017 Ryan Scott
License	BSD-style (see the file LICENSE)
Maintainer	Ryan Scott
Portability	Template Haskell
Safe Haskell	None
Language	Haskell2010

Data.Foldable.Deriving

Contents

Foldable
FFTOptions
deriveFoldable limitations

Description

Exports functions to mechanically derive Foldable instances in a way that mimics how the -XDeriveFoldable extension works since GHC 8.0.

These changes make it possible to derive Foldable instances for data types with existential constraints, e.g.,

data WrappedSet a where
    WrapSet :: Ord a => a -> WrappedSet a

deriving instance Foldable WrappedSet -- On GHC 8.0  on later
$(deriveFoldable ''WrappedSet)        -- On GHC 7.10 and earlier

In addition, derived Foldable instances from this module do not generate superfluous mempty expressions in its implementation of foldMap. One can verify this by compiling a module that uses deriveFoldable with the -ddump-splices GHC flag.

For more info on these changes, see this GHC wiki page.

Synopsis

deriveFoldable :: Name -> Q [Dec]
deriveFoldableOptions :: FFTOptions -> Name -> Q [Dec]
makeFoldMap :: Name -> Q Exp
makeFoldMapOptions :: FFTOptions -> Name -> Q Exp
makeFoldr :: Name -> Q Exp
makeFoldrOptions :: FFTOptions -> Name -> Q Exp
makeFold :: Name -> Q Exp
makeFoldOptions :: FFTOptions -> Name -> Q Exp
makeFoldl :: Name -> Q Exp
makeFoldlOptions :: FFTOptions -> Name -> Q Exp
newtype FFTOptions = FFTOptions {
- fftEmptyCaseBehavior :: Bool
}
defaultFFTOptions :: FFTOptions

`Foldable`

deriveFoldable :: Name -> Q [Dec] Source #

Generates a Foldable instance declaration for the given data type or data family instance.

deriveFoldableOptions :: FFTOptions -> Name -> Q [Dec] Source #

Like deriveFoldable, but takes an FFTOptions argument.

makeFoldMap :: Name -> Q Exp Source #

Generates a lambda expression which behaves like foldMap (without requiring a Foldable instance).

makeFoldMapOptions :: FFTOptions -> Name -> Q Exp Source #

Like makeFoldMap, but takes an FFTOptions argument.

makeFoldr :: Name -> Q Exp Source #

Generates a lambda expression which behaves like foldr (without requiring a Foldable instance).

makeFoldrOptions :: FFTOptions -> Name -> Q Exp Source #

Like makeFoldr, but takes an FFTOptions argument.

makeFold :: Name -> Q Exp Source #

Generates a lambda expression which behaves like fold (without requiring a Foldable instance).

makeFoldOptions :: FFTOptions -> Name -> Q Exp Source #

Like makeFold, but takes an FFTOptions argument.

makeFoldl :: Name -> Q Exp Source #

Generates a lambda expression which behaves like foldl (without requiring a Foldable instance).

makeFoldlOptions :: FFTOptions -> Name -> Q Exp Source #

Like makeFoldl, but takes an FFTOptions argument.

`FFTOptions`

newtype FFTOptions Source #

Options that further configure how the functions in Data.Functor.Deriving should behave. (FFT stands for 'Functor'/'Foldable'/'Traversable'.)

Constructors

FFTOptions
Fields fftEmptyCaseBehavior :: Bool If `True`, derived instances for empty data types (i.e., ones with no data constructors) will use the `EmptyCase` language extension. If `False`, derived instances will simply use `seq` instead. (This has no effect on GHCs before 7.8, since `EmptyCase` is only available in 7.8 or later.)

Instances

Eq FFTOptions Source #
Instance details Defined in Data.Functor.Deriving.Internal Methods (==) :: FFTOptions -> FFTOptions -> Bool # (/=) :: FFTOptions -> FFTOptions -> Bool #
Ord FFTOptions Source #
Instance details Defined in Data.Functor.Deriving.Internal Methods compare :: FFTOptions -> FFTOptions -> Ordering # (<) :: FFTOptions -> FFTOptions -> Bool # (<=) :: FFTOptions -> FFTOptions -> Bool # (>) :: FFTOptions -> FFTOptions -> Bool # (>=) :: FFTOptions -> FFTOptions -> Bool # max :: FFTOptions -> FFTOptions -> FFTOptions # min :: FFTOptions -> FFTOptions -> FFTOptions #
Read FFTOptions Source #
Instance details Defined in Data.Functor.Deriving.Internal Methods readsPrec :: Int -> ReadS FFTOptions # readList :: ReadS [FFTOptions] # readPrec :: ReadPrec FFTOptions # readListPrec :: ReadPrec [FFTOptions] #
Show FFTOptions Source #
Instance details Defined in Data.Functor.Deriving.Internal Methods showsPrec :: Int -> FFTOptions -> ShowS # show :: FFTOptions -> String # showList :: [FFTOptions] -> ShowS #

defaultFFTOptions :: FFTOptions Source #

Conservative FFTOptions that doesn't attempt to use EmptyCase (to prevent users from having to enable that extension at use sites.)

`deriveFoldable` limitations

Be aware of the following potential gotchas:

If you are using the -XGADTs or -XExistentialQuantification extensions, an existential constraint cannot mention the last type variable. For example, data Illegal a = forall a. Show a => Illegal a cannot have a derived Functor instance.
Type variables of kind * -> * are assumed to have Foldable constraints. If this is not desirable, use makeFoldr or makeFoldMap.

Foldable

FFTOptions

deriveFoldable limitations

`Foldable`

`FFTOptions`

`deriveFoldable` limitations