Copyright | (C) 2008-2016 Edward Kmett (C) 2015-2016 Ryan Scott |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | Edward Kmett <ekmett@gmail.com> |

Stability | provisional |

Portability | portable |

Safe Haskell | Unsafe |

Language | Haskell98 |

Functions to mechanically derive `Bifunctor`

, `Bifoldable`

,
or `Bitraversable`

instances, or to splice their functions directly into
source code. You need to enable the `TemplateHaskell`

language extension
in order to use this module.

- deriveBifunctor :: Name -> Q [Dec]
- makeBimap :: Name -> Q Exp
- deriveBifoldable :: Name -> Q [Dec]
- makeBifold :: Name -> Q Exp
- makeBifoldMap :: Name -> Q Exp
- makeBifoldr :: Name -> Q Exp
- makeBifoldl :: Name -> Q Exp
- deriveBitraversable :: Name -> Q [Dec]
- makeBitraverse :: Name -> Q Exp
- makeBisequenceA :: Name -> Q Exp
- makeBimapM :: Name -> Q Exp
- makeBisequence :: Name -> Q Exp

`derive`

- functions

`deriveBifunctor`

, `deriveBifoldable`

, and `deriveBitraversable`

automatically
generate their respective class instances for a given data type, newtype, or data
family instance that has at least two type variable. Examples:

{-# LANGUAGE TemplateHaskell #-} import Data.Bifunctor.TH data Pair a b = Pair a b $(`deriveBifunctor`

''Pair) -- instance Bifunctor Pair where ... data WrapLeftPair f g a b = WrapLeftPair (f a) (g a b) $(`deriveBifoldable`

''WrapLeftPair) -- instance (Foldable f, Bifoldable g) => Bifoldable (WrapLeftPair f g) where ...

If you are using `template-haskell-2.7.0.0`

or later (i.e., GHC 7.4 or later),
the `derive`

functions can be used data family instances (which requires the
`-XTypeFamilies`

extension). To do so, pass the name of a data or newtype instance
constructor (NOT a data family name!) to a `derive`

function. Note that the
generated code may require the `-XFlexibleInstances`

extension. Example:

```
{-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-}
import Data.Bifunctor.TH
class AssocClass a b c where
data AssocData a b c
instance AssocClass Int b c where
data AssocData Int b c = AssocDataInt1 Int | AssocDataInt2 b c
$(
````deriveBitraversable`

'AssocDataInt1) -- instance Bitraversable (AssocData Int) where ...
-- Alternatively, one could use $(deriveBitraversable 'AssocDataInt2)

Note that there are some limitations:

- The
`Name`

argument to a`derive`

function must not be a type synonym. - With a
`derive`

function, the last two type variables must both be of kind`*`

. Other type variables of kind`* -> *`

are assumed to require a`Functor`

,`Foldable`

, or`Traversable`

constraint (depending on which`derive`

function is used), and other type variables of kind`* -> * -> *`

are assumed to require an`Bifunctor`

,`Bifoldable`

, or`Bitraversable`

constraint. If your data type doesn't meet these assumptions, use a`make`

function. - If using the
`-XDatatypeContexts`

,`-XExistentialQuantification`

, or`-XGADTs`

extensions, a constraint cannot mention either of the last two type variables. For example,`data Illegal2 a b where I2 :: Ord a => a -> b -> Illegal2 a b`

cannot have a derived`Bifunctor`

instance. - If either of the last two type variables is used within a constructor argument's
type, it must only be used in the last two type arguments. For example,
`data Legal a b = Legal (Int, Int, a, b)`

can have a derived`Bifunctor`

instance, but`data Illegal a b = Illegal (a, b, a, b)`

cannot. - Data family instances must be able to eta-reduce the last two type variables. In other words, if you have a instance of the form:

data family Family a1 ... an t1 t2 data instance Family e1 ... e2 v1 v2 = ...

Then the following conditions must hold:

`v1`

and`v2`

must be distinct type variables.- Neither
`v1`

not`v2`

must be mentioned in any of`e1`

, ...,`e2`

.

There may be scenarios in which you want to, say, `bimap`

over an arbitrary data type
or data family instance without having to make the type an instance of `Bifunctor`

. For
these cases, this module provides several functions (all prefixed with `make`

-) that
splice the appropriate lambda expression into your source code.

This is particularly useful for creating instances for sophisticated data types. For
example, `deriveBifunctor`

cannot infer the correct type context for
`newtype HigherKinded f a b c = HigherKinded (f a b c)`

, since `f`

is of kind
`* -> * -> * -> *`

. However, it is still possible to create a `Bifunctor`

instance for
`HigherKinded`

without too much trouble using `makeBimap`

:

{-# LANGUAGE FlexibleContexts, TemplateHaskell #-} import Data.Bifunctor import Data.Bifunctor.TH newtype HigherKinded f a b c = HigherKinded (f a b c) instance Bifunctor (f a) => Bifunctor (HigherKinded f a) where bimap = $(makeBimap ''HigherKinded)

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

Generates a `Bifunctor`

instance declaration for the given data type or data
family instance.

makeBimap :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bimap`

(without requiring a
`Bifunctor`

instance).

`Bifoldable`

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

Generates a `Bifoldable`

instance declaration for the given data type or data
family instance.

makeBifold :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bifold`

(without requiring a
`Bifoldable`

instance).

makeBifoldMap :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bifoldMap`

(without requiring a
`Bifoldable`

instance).

makeBifoldr :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bifoldr`

(without requiring a
`Bifoldable`

instance).

makeBifoldl :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bifoldl`

(without requiring a
`Bifoldable`

instance).

`Bitraversable`

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

Generates a `Bitraversable`

instance declaration for the given data type or data
family instance.

makeBitraverse :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bitraverse`

(without requiring a
`Bitraversable`

instance).

makeBisequenceA :: Name -> Q Exp Source #

Generates a lambda expression which behaves like `bisequenceA`

(without requiring a
`Bitraversable`

instance).