Copyright | (C) 2015-2016 Ryan Scott |
---|---|
License | BSD-style (see the file LICENSE) |
Maintainer | Ryan Scott |
Portability | Template Haskell |
Safe Haskell | None |
Language | Haskell2010 |
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] Source #
Generates a Foldable
instance declaration for the given data type or data
family instance.
makeFold :: Name -> Q Exp Source #
Generates a lambda expression which behaves like fold
(without requiring a
Foldable
instance).
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 derivedFunctor
instance. - Type variables of kind
* -> *
are assumed to haveFoldable
constraints. If this is not desirable, usemakeFoldr
ormakeFoldMap
.