Data.Traversable.Deriving

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

Derived Traversable instances from this module do not generate superfluous pure expressions in its implementation of traverse. One can verify this by compiling a module that uses deriveTraversable with the -ddump-splices GHC flag.

These changes make it possible to derive Traversable instances for data types with unlifted argument types, e.g.,

data IntHash a = IntHash Int# a

deriving instance Traversable IntHash -- On GHC 8.0  on later
$(deriveTraversable ''IntHash)        -- On GHC 7.10 and earlier

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

Traversable

deriveTraversable :: Name -> Q [Dec]

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

makeTraverse :: Name -> Q Exp

Generates a lambda expression which behaves like traverse (without requiring a Traversable instance).

makeSequenceA :: Name -> Q Exp

Generates a lambda expression which behaves like sequenceA (without requiring a Traversable instance).

makeMapM :: Name -> Q Exp

Generates a lambda expression which behaves like mapM (without requiring a Traversable instance).

makeSequence :: Name -> Q Exp

Generates a lambda expression which behaves like sequence (without requiring a Traversable instance).

deriveTraversable 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 Traversable instance.
• Type variables of kind * -> * are assumed to have Traversable constraints. If this is not desirable, use makeTraverse.