Generics.Deriving.TH

generic-deriving-1.14.6: Generic programming library for generalised deriving.

Copyright | (c) 2008--2009 Universiteit Utrecht
License | BSD3
Maintainer | generics@haskell.org
Stability | experimental
Portability | non-portable
Safe Haskell | None
Language | Haskell2010

Generics.Deriving.TH

Contents

- `derive`- functions
- `make`- functions
- Options
  - Functions with optional arguments

Description

This module contains Template Haskell code that can be used to automatically generate the boilerplate code for the generic deriving library.

To use these functions, pass the name of a data type as an argument:

    {-# LANGUAGE TemplateHaskell #-}

    data Example a = Example Int Char a
    $(deriveAll0     ''Example) -- Derives Generic instance
    $(deriveAll1     ''Example) -- Derives Generic1 instance
    $(deriveAll0And1 ''Example) -- Derives Generic and Generic1 instances

On GHC 7.4 or later, this code can also be used with data families. To derive for a data family instance, pass the name of one of the instance's constructors:

    {-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-}

    data family Family a b
    newtype instance Family Char x = FamilyChar Char
    data    instance Family Bool x = FamilyTrue | FamilyFalse

    $(deriveAll0 'FamilyChar) -- instance Generic (Family Char b) where ...
    $(deriveAll1 'FamilyTrue) -- instance Generic1 (Family Bool) where ...
    -- Alternatively, one could type $(deriveAll1 'FamilyFalse)

Synopsis

- deriveMeta :: Name -> Q [Dec]
- deriveData :: Name -> Q [Dec]
- deriveConstructors :: Name -> Q [Dec]
- deriveSelectors :: Name -> Q [Dec]
- deriveAll :: Name -> Q [Dec]
- deriveAll0 :: Name -> Q [Dec]
- deriveAll1 :: Name -> Q [Dec]
- deriveAll0And1 :: Name -> Q [Dec]
- deriveRepresentable0 :: Name -> Q [Dec]
- deriveRepresentable1 :: Name -> Q [Dec]
- deriveRep0 :: Name -> Q [Dec]
- deriveRep1 :: Name -> Q [Dec]
- makeRep0Inline :: Name -> Q Type -> Q Type
- makeRep0 :: Name -> Q Type
- makeRep0FromType :: Name -> Q Type -> Q Type
- makeFrom :: Name -> Q Exp
- makeFrom0 :: Name -> Q Exp
- makeTo :: Name -> Q Exp
- makeTo0 :: Name -> Q Exp
- makeRep1Inline :: Name -> Q Type -> Q Type
- makeRep1 :: Name -> Q Type
- makeRep1FromType :: Name -> Q Type -> Q Type
- makeFrom1 :: Name -> Q Exp
- makeTo1 :: Name -> Q Exp
- data Options = Options {
  - repOptions :: RepOptions
  - kindSigOptions :: KindSigOptions
  - emptyCaseOptions :: EmptyCaseOptions
  }
- defaultOptions :: Options
- data RepOptions
  - = InlineRep
  - | TypeSynonymRep
- defaultRepOptions :: RepOptions
- type KindSigOptions = Bool
- defaultKindSigOptions :: KindSigOptions
- type EmptyCaseOptions = Bool
- defaultEmptyCaseOptions :: EmptyCaseOptions
- deriveAll0Options :: Options -> Name -> Q [Dec]
- deriveAll1Options :: Options -> Name -> Q [Dec]
- deriveAll0And1Options :: Options -> Name -> Q [Dec]
- deriveRepresentable0Options :: Options -> Name -> Q [Dec]
- deriveRepresentable1Options :: Options -> Name -> Q [Dec]
- deriveRep0Options :: KindSigOptions -> Name -> Q [Dec]
- deriveRep1Options :: KindSigOptions -> Name -> Q [Dec]
- makeFrom0Options :: EmptyCaseOptions -> Name -> Q Exp
- makeTo0Options :: EmptyCaseOptions -> Name -> Q Exp
- makeFrom1Options :: EmptyCaseOptions -> Name -> Q Exp
- makeTo1Options :: EmptyCaseOptions -> Name -> Q Exp

# `derive`- functions

deriveMeta :: Name -> Q [Dec] #

Given the type and the name (as string) for the type to derive, generate the `Data` instance, the `Constructor` instances, and the `Selector` instances.

On GHC 7.11 and up, this functionality is no longer used in GHC generics, so this function generates no declarations.

deriveData :: Name -> Q [Dec] #

Given a datatype name, derive a datatype and instance of class `Datatype`.

On GHC 7.11 and up, this functionality is no longer used in GHC generics, so this function generates no declarations.

deriveConstructors :: Name -> Q [Dec] #

Given a datatype name, derive datatypes and instances of class `Constructor`.

On GHC 7.11 and up, this functionality is no longer used in GHC generics, so this function generates no declarations.

deriveSelectors :: Name -> Q [Dec] #

Given a datatype name, derive datatypes and instances of class `Selector`.

On GHC 7.11 and up, this functionality is no longer used in GHC generics, so this function generates no declarations.

deriveAll :: Name -> Q [Dec] #

A backwards-compatible synonym for `deriveAll0`.

deriveAll0 :: Name -> Q [Dec] #

Given the type and the name (as string) for the type to derive, generate the `Data` instance, the `Constructor` instances, the `Selector` instances, and the `Representable0` instance.

deriveAll1 :: Name -> Q [Dec] #

Given the type and the name (as string) for the type to derive, generate the `Data` instance, the `Constructor` instances, the `Selector` instances, and the `Representable1` instance.

deriveAll0And1 :: Name -> Q [Dec] #

Given the type and the name (as string) for the type to derive, generate the `Data` instance, the `Constructor` instances, the `Selector` instances, the `Representable0` instance, and the `Representable1` instance.

deriveRepresentable0 :: Name -> Q [Dec] #

Given the type and the name (as string) for the Representable0 type synonym to derive, generate the `Representable0` instance.

deriveRepresentable1 :: Name -> Q [Dec] #

Given the type and the name (as string) for the Representable1 type synonym to derive, generate the `Representable1` instance.

deriveRep0 :: Name -> Q [Dec] #

Derive only the `Rep0` type synonym. Not needed if `deriveRepresentable0` is used.

deriveRep1 :: Name -> Q [Dec] #

Derive only the `Rep1` type synonym. Not needed if `deriveRepresentable1` is used.

# `make`- functions

There are some data types for which the Template Haskell deriver functions in this module are not sophisticated enough to infer the correct `Generic` or `Generic1` instances. As an example, consider this data type:

    newtype Fix f a = Fix (f (Fix f a))

A proper `Generic1` instance would look like this:

    instance Functor f => Generic1 (Fix f) where ...

Unfortunately, `deriveRepresentable1` cannot infer the `Functor f` constraint. One can still define a `Generic1` instance for `Fix`, however, by using the functions in this module that are prefixed with `make`-. For example:

    $(deriveMeta ''Fix)
    $(deriveRep1 ''Fix)
    instance Functor f => Generic1 (Fix f) where
      type Rep1 (Fix f) = $(makeRep1Inline ''Fix [t| Fix f |])
      from1 = $(makeFrom1 ''Fix)
      to1   = $(makeTo1   ''Fix)

Note that due to the lack of type-level lambdas in Haskell, one must manually apply ``makeRep1Inline` ''Fix` to the type `Fix f`.

Be aware that there is a bug on GHC 7.0, 7.2, and 7.4 which might prevent you from using `makeRep0Inline` and `makeRep1Inline`. In the `Fix` example above, you would experience the following error:

        Kinded thing f used as a type
        In the Template Haskell quotation [t| Fix f |]

Then a workaround is to use `makeRep1` instead, which requires you to:

1. Invoke `deriveRep1` beforehand
2. Pass as arguments the type variables that occur in the instance, in order from left to right, topologically sorted, excluding duplicates. (Normally, `makeRep1Inline` would figure this out for you.)

Using the above example:

    $(deriveMeta ''Fix)
    $(deriveRep1 ''Fix)
    instance Functor f => Generic1 (Fix f) where
      type Rep1 (Fix f) = $(makeRep1 ''Fix) f
      from1 = $(makeFrom1 ''Fix)
      to1   = $(makeTo1   ''Fix)

On GHC 7.4, you might encounter more complicated examples involving data families. For instance:

    data family Fix a b c d
    newtype instance Fix b (f c) (g b) a = Fix (f (Fix b (f c) (g b) a))

    $(deriveMeta ''Fix)
    $(deriveRep1 ''Fix)
    instance Functor f => Generic1 (Fix b (f c) (g b)) where
      type Rep1 (Fix b (f c) (g b)) = $(makeRep1 'Fix) b f c g
      from1 = $(makeFrom1 'Fix)
      to1   = $(makeTo1   'Fix)

Note that you don't pass `b` twice, only once.

makeRep0Inline :: Name -> Q Type -> Q Type #

Generates the full `Rep` type inline. Since this type can be quite large, it is recommended you only use this to define `Rep`, e.g.,

    type Rep (Foo (a :: k) b) = $(makeRep0Inline ''Foo [t| Foo (a :: k) b |])

You can then simply refer to `Rep (Foo a b)` elsewhere.

Note that the type passed as an argument to `makeRep0Inline` must match the type argument of `Rep` exactly, even up to including the explicit kind signature on `a`. This is due to a limitation of Template Haskell—without the kind signature, `makeRep0Inline` has no way of figuring out the kind of `a`, and the generated type might be completely wrong as a result!

makeRep0 :: Name -> Q Type #

Generates the `Rep` type synonym constructor (as opposed to `deriveRep0`, which generates the type synonym declaration). After splicing it into Haskell source, it expects types as arguments. For example:

    type Rep (Foo a b) = $(makeRep0 ''Foo) a b

The use of `makeRep0` is generally discouraged, as it can sometimes be difficult to predict the order in which you are expected to pass type variables. As a result, `makeRep0Inline` is recommended instead. However, `makeRep0Inline` is not usable on GHC 7.0, 7.2, or 7.4 due to a GHC bug, so `makeRep0` still exists for GHC 7.0, 7.2, and 7.4 users.

makeRep0FromType :: Name -> Q Type -> Q Type #

Generates the `Rep` type synonym constructor (as opposed to `deriveRep0`, which generates the type synonym declaration) applied to its type arguments. Unlike `makeRep0`, this also takes a quoted `Type` as an argument, e.g.,

    type Rep (Foo (a :: k) b) = $(makeRep0FromType ''Foo [t| Foo (a :: k) b |])

Note that the type passed as an argument to `makeRep0FromType` must match the type argument of `Rep` exactly, even up to including the explicit kind signature on `a`. This is due to a limitation of Template Haskell—without the kind signature, `makeRep0FromType` has no way of figuring out the kind of `a`, and the generated type might be completely wrong as a result!

The use of `makeRep0FromType` is generally discouraged, since `makeRep0Inline` does exactly the same thing but without having to go through an intermediate type synonym, and as a result, `makeRep0Inline` tends to be less buggy.

makeFrom :: Name -> Q Exp #

A backwards-compatible synonym for `makeFrom0`.

makeFrom0 :: Name -> Q Exp #

Generates a lambda expression which behaves like `from`.

makeTo :: Name -> Q Exp #

A backwards-compatible synonym for `makeTo0`.

makeTo0 :: Name -> Q Exp #

Generates a lambda expression which behaves like `to`.

makeRep1Inline :: Name -> Q Type -> Q Type #

Generates the full `Rep1` type inline. Since this type can be quite large, it is recommended you only use this to define `Rep1`, e.g.,

    type Rep1 (Foo (a :: k)) = $(makeRep0Inline ''Foo [t| Foo (a :: k) |])

You can then simply refer to `Rep1 (Foo a)` elsewhere.

Note that the type passed as an argument to `makeRep1Inline` must match the type argument of `Rep1` exactly, even up to including the explicit kind signature on `a`. This is due to a limitation of Template Haskell—without the kind signature, `makeRep1Inline` has no way of figuring out the kind of `a`, and the generated type might be completely wrong as a result!

makeRep1 :: Name -> Q Type #

Generates the `Rep1` type synonym constructor (as opposed to `deriveRep1`, which generates the type synonym declaration). After splicing it into Haskell source, it expects types as arguments. For example:

    type Rep1 (Foo a) = $(makeRep1 ''Foo) a

The use of `makeRep1` is generally discouraged, as it can sometimes be difficult to predict the order in which you are expected to pass type variables. As a result, `makeRep1Inline` is recommended instead. However, `makeRep1Inline` is not usable on GHC 7.0, 7.2, or 7.4 due to a GHC bug, so `makeRep1` still exists for GHC 7.0, 7.2, and 7.4 users.

makeRep1FromType :: Name -> Q Type -> Q Type #

Generates the `Rep1` type synonym constructor (as opposed to `deriveRep1`, which generates the type synonym declaration) applied to its type arguments. Unlike `makeRep1`, this also takes a quoted `Type` as an argument, e.g.,

    type Rep1 (Foo (a :: k)) = $(makeRep1FromType ''Foo [t| Foo (a :: k) |])

Note that the type passed as an argument to `makeRep1FromType` must match the type argument of `Rep` exactly, even up to including the explicit kind signature on `a`. This is due to a limitation of Template Haskell—without the kind signature, `makeRep1FromType` has no way of figuring out the kind of `a`, and the generated type might be completely wrong as a result!

The use of `makeRep1FromType` is generally discouraged, since `makeRep1Inline` does exactly the same thing but without having to go through an intermediate type synonym, and as a result, `makeRep1Inline` tends to be less buggy.

makeFrom1 :: Name -> Q Exp #

Generates a lambda expression which behaves like `from1`.

makeTo1 :: Name -> Q Exp #

Generates a lambda expression which behaves like `to1`.

# Options

`Options` gives you a way to further tweak derived `Generic` and `Generic1` instances:

- `RepOptions`: By default, all derived `Rep` and `Rep1` type instances emit the code directly (the `InlineRep` option). One can also choose to emit a separate type synonym for the `Rep` type (this is the functionality of `deriveRep0` and `deriveRep1`) and define a `Rep` instance in terms of that type synonym (the `TypeSynonymRep` option).

- `EmptyCaseOptions`: By default, all derived instances for empty data types (i.e., data types with no constructors) use `error` in `from(1)`/`to(1)`. For instance, `data Empty` would have this derived `Generic` instance:

      instance Generic Empty where
        type Rep Empty = D1 ('MetaData ...) V1
        from _ = M1 (error "No generic representation for empty datatype Empty")
        to (M1 _) = error "No generic representation for empty datatype Empty"

  This matches the behavior of GHC up until 8.4, when derived `Generic(1)` instances began to use the `EmptyCase` extension. In GHC 8.4, the derived `Generic` instance for `Empty` would instead be:

      instance Generic Empty where
        type Rep Empty = D1 ('MetaData ...) V1
        from x = M1 (case x of {})
        to (M1 x) = case x of {}

  This is a slightly better encoding since, for example, any divergent computations passed to `from` will actually diverge (as opposed to before, where the result would always be a call to `error`). On the other hand, using this encoding in `generic-deriving` has one large drawback: it requires enabling `EmptyCase`, an extension which was only introduced in GHC 7.8 (and only received reliable pattern-match coverage checking in 8.2).

  The `EmptyCaseOptions` field controls whether code should be emitted that uses `EmptyCase` (i.e., `EmptyCaseOptions` set to `True`) or not (`False`). The default value is `False`. Note that even if set to `True`, this option has no effect on GHCs before 7.8, as `EmptyCase` did not exist then.

- `KindSigOptions`: By default, all derived instances will use explicit kind signatures (when the `KindSigOptions` is `True`). You might wish to set the `KindSigOptions` to `False` if you want a `Generic`/`Generic1` instance at a particular kind that GHC will infer correctly, but the functions in this module won't guess correctly. You probably won't ever need this option unless you are a power user.

data Options #

Additional options for configuring derived `Generic`/`Generic1` instances using Template Haskell.

Constructors

Options |
Fields repOptions :: RepOptions kindSigOptions :: KindSigOptions emptyCaseOptions :: EmptyCaseOptions

#### Instances

Instances details

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

defaultOptions :: Options #

Sensible default `Options`.

data RepOptions #

Configures whether `Rep`/`Rep1` type instances should be defined inline in a derived `Generic`/`Generic1` instance (`InlineRep`) or defined in terms of a type synonym (`TypeSynonymRep`).

Constructors

InlineRep |
TypeSynonymRep |

#### Instances

Instances details

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

defaultRepOptions :: RepOptions #

`InlineRep`, a sensible default `RepOptions`.

type KindSigOptions = Bool #

`True` if explicit kind signatures should be used in derived `Generic`/`Generic1` instances, `False` otherwise.

defaultKindSigOptions :: KindSigOptions #

`True`, a sensible default `KindSigOptions`.

type EmptyCaseOptions = Bool #

`True` if generated code for empty data types should use the `EmptyCase` extension, `False` otherwise. This has no effect on GHCs before 7.8, since `EmptyCase` is only available in 7.8 or later.

defaultEmptyCaseOptions :: EmptyCaseOptions #

Sensible default `EmptyCaseOptions`.

## Functions with optional arguments

deriveAll0Options :: Options -> Name -> Q [Dec] #

Like `deriveAll0`, but takes an `Options` argument.

deriveAll1Options :: Options -> Name -> Q [Dec] #

Like `deriveAll1`, but takes an `Options` argument.

deriveAll0And1Options :: Options -> Name -> Q [Dec] #

Like `deriveAll0And1`, but takes an `Options` argument.

deriveRepresentable0Options :: Options -> Name -> Q [Dec] #

Like `deriveRepresentable0`, but takes an `Options` argument.

deriveRepresentable1Options :: Options -> Name -> Q [Dec] #

Like `deriveRepresentable1`, but takes an `Options` argument.

deriveRep0Options :: KindSigOptions -> Name -> Q [Dec] #

Like `deriveRep0`, but takes an `KindSigOptions` argument.

deriveRep1Options :: KindSigOptions -> Name -> Q [Dec] #

Like `deriveRep1`, but takes an `KindSigOptions` argument.

makeFrom0Options :: EmptyCaseOptions -> Name -> Q Exp #

Like `makeFrom0Options`, but takes an `EmptyCaseOptions` argument.

makeTo0Options :: EmptyCaseOptions -> Name -> Q Exp #

Like `makeTo0Options`, but takes an `EmptyCaseOptions` argument.

makeFrom1Options :: EmptyCaseOptions -> Name -> Q Exp #

Like `makeFrom1Options`, but takes an `EmptyCaseOptions` argument.

makeTo1Options :: EmptyCaseOptions -> Name -> Q Exp #

Like `makeTo1Options`, but takes an `EmptyCaseOptions` argument.