Copyright | (c) The University of Glasgow, CWI 2001--2004 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | experimental |

Portability | non-portable (local universal quantification) |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

"Scrap your boilerplate" --- Generic programming in Haskell. See
http://www.haskell.org/haskellwiki/Research_papers/Generics#Scrap_your_boilerplate.21.
This module provides the `Data`

class with its primitives for
generic programming, along with instances for many datatypes. It
corresponds to a merge between the previous Data.Generics.Basics
and almost all of Data.Generics.Instances. The instances that are
not present in this module were moved to the
`Data.Generics.Instances`

module in the `syb`

package.

For more information, please visit the new SYB wiki: http://www.cs.uu.nl/wiki/bin/view/GenericProgramming/SYB.

- module Data.Typeable
- class Typeable a => Data a where
- gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> a -> c a
- gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c a
- toConstr :: a -> Constr
- dataTypeOf :: a -> DataType
- dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c a)
- dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c a)
- gmapT :: (forall b. Data b => b -> b) -> a -> a
- gmapQl :: forall r r'. (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> a -> r
- gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> a -> r
- gmapQ :: (forall d. Data d => d -> u) -> a -> [u]
- gmapQi :: forall u. Int -> (forall d. Data d => d -> u) -> a -> u
- gmapM :: forall m. Monad m => (forall d. Data d => d -> m d) -> a -> m a
- gmapMp :: forall m. MonadPlus m => (forall d. Data d => d -> m d) -> a -> m a
- gmapMo :: forall m. MonadPlus m => (forall d. Data d => d -> m d) -> a -> m a

- data DataType
- mkDataType :: String -> [Constr] -> DataType
- mkIntType :: String -> DataType
- mkFloatType :: String -> DataType
- mkCharType :: String -> DataType
- mkNoRepType :: String -> DataType
- dataTypeName :: DataType -> String
- data DataRep
- dataTypeRep :: DataType -> DataRep
- repConstr :: DataType -> ConstrRep -> Constr
- isAlgType :: DataType -> Bool
- dataTypeConstrs :: DataType -> [Constr]
- indexConstr :: DataType -> ConIndex -> Constr
- maxConstrIndex :: DataType -> ConIndex
- isNorepType :: DataType -> Bool
- data Constr
- type ConIndex = Int
- data Fixity
- mkConstr :: DataType -> String -> [String] -> Fixity -> Constr
- mkIntegralConstr :: (Integral a, Show a) => DataType -> a -> Constr
- mkRealConstr :: (Real a, Show a) => DataType -> a -> Constr
- mkCharConstr :: DataType -> Char -> Constr
- constrType :: Constr -> DataType
- data ConstrRep
- constrRep :: Constr -> ConstrRep
- constrFields :: Constr -> [String]
- constrFixity :: Constr -> Fixity
- constrIndex :: Constr -> ConIndex
- showConstr :: Constr -> String
- readConstr :: DataType -> String -> Maybe Constr
- tyconUQname :: String -> String
- tyconModule :: String -> String
- fromConstr :: Data a => Constr -> a
- fromConstrB :: Data a => (forall d. Data d => d) -> Constr -> a
- fromConstrM :: forall m a. (Monad m, Data a) => (forall d. Data d => m d) -> Constr -> m a

# Module Data.Typeable re-exported for convenience

module Data.Typeable

# The Data class for processing constructor applications

class Typeable a => Data a where Source

The `Data`

class comprehends a fundamental primitive `gfoldl`

for
folding over constructor applications, say terms. This primitive can
be instantiated in several ways to map over the immediate subterms
of a term; see the `gmap`

combinators later in this class. Indeed, a
generic programmer does not necessarily need to use the ingenious gfoldl
primitive but rather the intuitive `gmap`

combinators. The `gfoldl`

primitive is completed by means to query top-level constructors, to
turn constructor representations into proper terms, and to list all
possible datatype constructors. This completion allows us to serve
generic programming scenarios like read, show, equality, term generation.

The combinators `gmapT`

, `gmapQ`

, `gmapM`

, etc are all provided with
default definitions in terms of `gfoldl`

, leaving open the opportunity
to provide datatype-specific definitions.
(The inclusion of the `gmap`

combinators as members of class `Data`

allows the programmer or the compiler to derive specialised, and maybe
more efficient code per datatype. *Note*: `gfoldl`

is more higher-order
than the `gmap`

combinators. This is subject to ongoing benchmarking
experiments. It might turn out that the `gmap`

combinators will be
moved out of the class `Data`

.)

Conceptually, the definition of the `gmap`

combinators in terms of the
primitive `gfoldl`

requires the identification of the `gfoldl`

function
arguments. Technically, we also need to identify the type constructor
`c`

for the construction of the result type from the folded term type.

In the definition of `gmapQ`

*x* combinators, we use phantom type
constructors for the `c`

in the type of `gfoldl`

because the result type
of a query does not involve the (polymorphic) type of the term argument.
In the definition of `gmapQl`

we simply use the plain constant type
constructor because `gfoldl`

is left-associative anyway and so it is
readily suited to fold a left-associative binary operation over the
immediate subterms. In the definition of gmapQr, extra effort is
needed. We use a higher-order accumulation trick to mediate between
left-associative constructor application vs. right-associative binary
operation (e.g., `(:)`

). When the query is meant to compute a value
of type `r`

, then the result type withing generic folding is `r -> r`

.
So the result of folding is a function to which we finally pass the
right unit.

With the `-XDeriveDataTypeable`

option, GHC can generate instances of the
`Data`

class automatically. For example, given the declaration

data T a b = C1 a b | C2 deriving (Typeable, Data)

GHC will generate an instance that is equivalent to

instance (Data a, Data b) => Data (T a b) where gfoldl k z (C1 a b) = z C1 `k` a `k` b gfoldl k z C2 = z C2 gunfold k z c = case constrIndex c of 1 -> k (k (z C1)) 2 -> z C2 toConstr (C1 _ _) = con_C1 toConstr C2 = con_C2 dataTypeOf _ = ty_T con_C1 = mkConstr ty_T "C1" [] Prefix con_C2 = mkConstr ty_T "C2" [] Prefix ty_T = mkDataType "Module.T" [con_C1, con_C2]

This is suitable for datatypes that are exported transparently.

:: (forall d b. Data d => c (d -> b) -> d -> c b) | defines how nonempty constructor applications are folded. It takes the folded tail of the constructor application and its head, i.e., an immediate subterm, and combines them in some way. |

-> (forall g. g -> c g) | defines how the empty constructor application is folded, like the neutral / start element for list folding. |

-> a | structure to be folded. |

-> c a | result, with a type defined in terms of |

Left-associative fold operation for constructor applications.

The type of `gfoldl`

is a headache, but operationally it is a simple
generalisation of a list fold.

The default definition for `gfoldl`

is

, which is
suitable for abstract datatypes with no substructures.`const`

`id`

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c a Source

Unfolding constructor applications

toConstr :: a -> Constr Source

Obtaining the constructor from a given datum. For proper terms, this is meant to be the top-level constructor. Primitive datatypes are here viewed as potentially infinite sets of values (i.e., constructors).

dataTypeOf :: a -> DataType Source

The outer type constructor of the type

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c a) Source

Mediate types and unary type constructors.
In `Data`

instances of the form `T a`

, `dataCast1`

should be defined
as `gcast1`

.

The default definition is

, which is appropriate
for non-unary type constructors.`const`

`Nothing`

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c a) Source

Mediate types and binary type constructors.
In `Data`

instances of the form `T a b`

, `dataCast2`

should be
defined as `gcast2`

.

The default definition is

, which is appropriate
for non-binary type constructors.`const`

`Nothing`

gmapT :: (forall b. Data b => b -> b) -> a -> a Source

A generic transformation that maps over the immediate subterms

The default definition instantiates the type constructor `c`

in the
type of `gfoldl`

to an identity datatype constructor, using the
isomorphism pair as injection and projection.

gmapQl :: forall r r'. (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> a -> r Source

A generic query with a left-associative binary operator

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> a -> r Source

A generic query with a right-associative binary operator

gmapQ :: (forall d. Data d => d -> u) -> a -> [u] Source

A generic query that processes the immediate subterms and returns a list of results. The list is given in the same order as originally specified in the declaration of the data constructors.

gmapQi :: forall u. Int -> (forall d. Data d => d -> u) -> a -> u Source

A generic query that processes one child by index (zero-based)

gmapM :: forall m. Monad m => (forall d. Data d => d -> m d) -> a -> m a Source

A generic monadic transformation that maps over the immediate subterms

The default definition instantiates the type constructor `c`

in
the type of `gfoldl`

to the monad datatype constructor, defining
injection and projection using `return`

and `>>=`

.

gmapMp :: forall m. MonadPlus m => (forall d. Data d => d -> m d) -> a -> m a Source

Transformation of at least one immediate subterm does not fail

gmapMo :: forall m. MonadPlus m => (forall d. Data d => d -> m d) -> a -> m a Source

Transformation of one immediate subterm with success

# Datatype representations

Representation of datatypes. A package of constructor representations with names of type and module.

## Constructors

mkDataType :: String -> [Constr] -> DataType Source

Constructs an algebraic datatype

mkFloatType :: String -> DataType Source

Constructs the `Float`

type

mkCharType :: String -> DataType Source

Constructs the `Char`

type

mkNoRepType :: String -> DataType Source

Constructs a non-representation for a non-representable type

## Observers

dataTypeName :: DataType -> String Source

Gets the type constructor including the module

Public representation of datatypes

dataTypeRep :: DataType -> DataRep Source

Gets the public presentation of a datatype

## Convenience functions

dataTypeConstrs :: DataType -> [Constr] Source

Gets the constructors of an algebraic datatype

indexConstr :: DataType -> ConIndex -> Constr Source

Gets the constructor for an index (algebraic datatypes only)

maxConstrIndex :: DataType -> ConIndex Source

Gets the maximum constructor index of an algebraic datatype

isNorepType :: DataType -> Bool Source

Test for a non-representable type

# Data constructor representations

Unique index for datatype constructors, counting from 1 in the order they are given in the program text.

## Constructors

## Observers

constrType :: Constr -> DataType Source

Gets the datatype of a constructor

Public representation of constructors

constrFields :: Constr -> [String] Source

Gets the field labels of a constructor. The list of labels is returned in the same order as they were given in the original constructor declaration.

constrFixity :: Constr -> Fixity Source

Gets the fixity of a constructor

## Convenience function: algebraic data types

constrIndex :: Constr -> ConIndex Source

Gets the index of a constructor (algebraic datatypes only)

## From strings to constructors and vice versa: all data types

showConstr :: Constr -> String Source

Gets the string for a constructor

# Convenience functions: take type constructors apart

tyconUQname :: String -> String Source

Gets the unqualified type constructor: drop *.*.*... before name

tyconModule :: String -> String Source

Gets the module of a type constructor: take *.*.*... before name

# Generic operations defined in terms of `gunfold`

fromConstr :: Data a => Constr -> a Source

Build a term skeleton

fromConstrB :: Data a => (forall d. Data d => d) -> Constr -> a Source

Build a term and use a generic function for subterms

fromConstrM :: forall m a. (Monad m, Data a) => (forall d. Data d => m d) -> Constr -> m a Source

Monadic variation on `fromConstrB`