Data.Foldable.Deriving

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.

Foldable

deriveFoldable :: Name -> Q [Dec]

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

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

Like deriveFoldable, but takes an FFTOptions argument.

makeFoldMap :: Name -> Q Exp

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

makeFoldMapOptions :: FFTOptions -> Name -> Q Exp

Like makeFoldMap, but takes an FFTOptions argument.

makeFoldr :: Name -> Q Exp

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

makeFoldrOptions :: FFTOptions -> Name -> Q Exp

Like makeFoldr, but takes an FFTOptions argument.

makeFold :: Name -> Q Exp

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

makeFoldOptions :: FFTOptions -> Name -> Q Exp

Like makeFold, but takes an FFTOptions argument.

makeFoldl :: Name -> Q Exp

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

makeFoldlOptions :: FFTOptions -> Name -> Q Exp

Like makeFoldl, but takes an FFTOptions argument.

makeNull :: Name -> Q Exp

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

makeNullOptions :: FFTOptions -> Name -> Q Exp

Like makeNull, but takes an FFTOptions argument.

FFTOptions

newtype FFTOptions

Options that further configure how the functions in Data.Functor.Deriving should behave. (FFT stands for FunctorFoldableTraversable.)

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

Defined in Data.Functor.Deriving.Internal

Methods

(==) :: FFTOptions -> FFTOptions -> Bool

(/=) :: FFTOptions -> FFTOptions -> Bool

Ord FFTOptions

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

Defined in Data.Functor.Deriving.Internal

Methods

readsPrec :: Int -> ReadS FFTOptions

readList :: ReadS [FFTOptions]

readPrec :: ReadPrec FFTOptions

readListPrec :: ReadPrec [FFTOptions]

Show FFTOptions

Defined in Data.Functor.Deriving.Internal

Methods

showsPrec :: Int -> FFTOptions -> ShowS

show :: FFTOptions -> String

showList :: [FFTOptions] -> ShowS

defaultFFTOptions :: FFTOptions

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.