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Data.Generics.Basics

Portability | non-portable (local universal quantification)
Stability | experimental
Maintainer | libraries@haskell.org

Contents

Module Data.Typeable re-exported for convenience
The Data class for processing constructor applications
Datatype representations
  Constructors
  Observers
  Convenience functions
Data constructor representations
  Constructors
  Observers
  Convenience function: algebraic data types
  From strings to constructors and vice versa: all data types
Convenience functions: take type constructors apart
Generic operations defined in terms of `gunfold`

Description
"Scrap your boilerplate" --- Generic programming in Haskell. See http://www.cs.vu.nl/boilerplate/. This module provides the `Data` class with its primitives for generic programming.
Synopsis

module Data.Typeable
class Typeable a => Data a where

gfoldl :: (forall a b . Data a => c (a -> b) -> a -> 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 :: Typeable1 t => (forall a . Data a => c (t a)) -> Maybe (c a)
dataCast2 :: Typeable2 t => (forall a b . (Data a, Data b) => c (t a b)) -> Maybe (c a)
gmapT :: (forall b . Data b => b -> b) -> a -> a
gmapQl :: (r -> r' -> r) -> r -> (forall a . Data a => a -> r') -> a -> r
gmapQr :: (r' -> r -> r) -> r -> (forall a . Data a => a -> r') -> a -> r
gmapQ :: (forall a . Data a => a -> u) -> a -> [u]
gmapQi :: Int -> (forall a . Data a => a -> u) -> a -> u
gmapM :: Monad m => (forall a . Data a => a -> m a) -> a -> m a
gmapMp :: MonadPlus m => (forall a . Data a => a -> m a) -> a -> m a
gmapMo :: MonadPlus m => (forall a . Data a => a -> m a) -> a -> m a

data DataType
mkDataType :: String -> [Constr] -> DataType
mkIntType :: String -> DataType
mkFloatType :: String -> DataType
mkStringType :: String -> DataType
mkNorepType :: String -> DataType
dataTypeName :: DataType -> String

data DataRep

= AlgRep [Constr]
| IntRep
| FloatRep
| StringRep
| NoRep

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

= Prefix
| Infix

mkConstr :: DataType -> String -> [String] -> Fixity -> Constr
mkIntConstr :: DataType -> Integer -> Constr
mkFloatConstr :: DataType -> Double -> Constr
mkStringConstr :: DataType -> String -> Constr
constrType :: Constr -> DataType

data ConstrRep

= AlgConstr ConIndex
| IntConstr Integer
| FloatConstr Double
| StringConstr String

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

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 `-fglasgow-exts` 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.

Methods

gfoldl

:: (forall a b . Data a => c (a -> b) -> a -> 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 `a`, but variability is achieved by means of type constructor `c` for the construction of the actual result type.
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 ``const` `id``, which is suitable for abstract datatypes with no substructures.

gunfold :: (forall b r . Data b => c (b -> r) -> c r) -> (forall r . r -> c r) -> Constr -> c a
Unfolding constructor applications
toConstr :: a -> Constr
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
The outer type constructor of the type
dataCast1 :: Typeable1 t => (forall a . Data a => c (t a)) -> Maybe (c a)
Mediate types and unary type constructors. In `Data` instances of the form `T a`, `dataCast1` should be defined as `gcast1`. The default definition is ``const` `Nothing``, which is appropriate for non-unary type constructors.
dataCast2 :: Typeable2 t => (forall a b . (Data a, Data b) => c (t a b)) -> Maybe (c a)
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 ``const` `Nothing``, which is appropriate for non-binary type constructors.
gmapT :: (forall b . Data b => b -> b) -> a -> a
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 :: (r -> r' -> r) -> r -> (forall a . Data a => a -> r') -> a -> r
A generic query with a left-associative binary operator
gmapQr :: (r' -> r -> r) -> r -> (forall a . Data a => a -> r') -> a -> r
A generic query with a right-associative binary operator
gmapQ :: (forall a . Data a => a -> u) -> a -> [u]
A generic query that processes the immediate subterms and returns a list
gmapQi :: Int -> (forall a . Data a => a -> u) -> a -> u
A generic query that processes one child by index (zero-based)
gmapM :: Monad m => (forall a . Data a => a -> m a) -> a -> m a
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 :: MonadPlus m => (forall a . Data a => a -> m a) -> a -> m a
Transformation of at least one immediate subterm does not fail
gmapMo :: MonadPlus m => (forall a . Data a => a -> m a) -> a -> m a
Transformation of one immediate subterm with success

Instances

Data Bool
Data ByteString
Data Char
Data DataType
Data Double
Data Float
Data Handle
Data Int
Data Int16
Data Int32
Data Int64
Data Int8
Data IntSet
Data Integer
Data LazyByteString
Data Ordering
Data ThreadId
Data TyCon
Data TypeRep
Data Word
Data Word16
Data Word32
Data Word64
Data Word8
Data ()
(Data a, Data b) => Data (a, b)
(Data a, Data b, Data c) => Data (a, b, c)
(Data a, Data b, Data c, Data d) => Data (a, b, c, d)
(Data a, Data b, Data c, Data d, Data e) => Data (a, b, c, d, e)
(Data a, Data b, Data c, Data d, Data e, Data f) => Data (a, b, c, d, e, f)
(Data a, Data b, Data c, Data d, Data e, Data f, Data g) => Data (a, b, c, d, e, f, g)
(Data a, Data b) => Data (a -> b)
Typeable a => Data (ForeignPtr a)
Typeable a => Data (IO a)
Typeable a => Data (IORef a)
Data a => Data (IntMap a)
Typeable a => Data (MVar a)
Data a => Data (Maybe a)
Typeable a => Data (Ptr a)
(Data a, Integral a) => Data (Ratio a)
Typeable a => Data (STM a)
Data a => Data (Seq a)
(Data a, Ord a) => Data (Set a)
Typeable a => Data (StablePtr a)
Typeable a => Data (TVar a)
Data a => Data (Tree a)
Data a => Data (ViewL a)
Data a => Data (ViewR a)
Data a => Data [a]
(Typeable a, Data b, Ix a) => Data (Array a b)
(Data a, Data b) => Data (Either a b)
(Data k, Data a, Ord k) => Data (Map k a)
(Typeable s, Typeable a) => Data (ST s a)

Datatype representations
data DataType

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

Data DataType
Show DataType
Typeable DataType

Constructors
mkDataType :: String -> [Constr] -> DataType
Constructs an algebraic datatype
mkIntType :: String -> DataType
Constructs the `Int` type
mkFloatType :: String -> DataType
Constructs the `Float` type
mkStringType :: String -> DataType
Constructs the `String` type
mkNorepType :: String -> DataType
Constructs a non-representation for a non-presentable type
Observers
dataTypeName :: DataType -> String
Gets the type constructor including the module
data DataRep

Public representation of datatypes
Constructors

AlgRep [Constr] |
IntRep |
FloatRep |
StringRep |
NoRep |

Instances

Eq DataRep
Show DataRep

dataTypeRep :: DataType -> DataRep
Gets the public presentation of a datatype
Convenience functions
repConstr :: DataType -> ConstrRep -> Constr
Look up a constructor by its representation
isAlgType :: DataType -> Bool
Test for an algebraic type
dataTypeConstrs :: DataType -> [Constr]
Gets the constructors of an algebraic datatype
indexConstr :: DataType -> ConIndex -> Constr
Gets the constructor for an index (algebraic datatypes only)
maxConstrIndex :: DataType -> ConIndex
Gets the maximum constructor index of an algebraic datatype
isNorepType :: DataType -> Bool
Test for a non-representable type
Data constructor representations
data Constr

Representation of constructors
Instances

Eq Constr
Show Constr

type ConIndex = Int
Unique index for datatype constructors, counting from 1 in the order they are given in the program text.
data Fixity

Fixity of constructors
Constructors

Prefix |
Infix |

Instances

Eq Fixity
Show Fixity

Constructors
mkConstr :: DataType -> String -> [String] -> Fixity -> Constr
Constructs a constructor
mkIntConstr :: DataType -> Integer -> Constr
mkFloatConstr :: DataType -> Double -> Constr
mkStringConstr :: DataType -> String -> Constr
Observers
constrType :: Constr -> DataType
Gets the datatype of a constructor
data ConstrRep

Public representation of constructors
Constructors

AlgConstr ConIndex |
IntConstr Integer |
FloatConstr Double |
StringConstr String |

Instances

Eq ConstrRep
Show ConstrRep

constrRep :: Constr -> ConstrRep
Gets the public presentation of constructors
constrFields :: Constr -> [String]
Gets the field labels of a constructor
constrFixity :: Constr -> Fixity
Gets the fixity of a constructor
Convenience function: algebraic data types
constrIndex :: Constr -> ConIndex
Gets the index of a constructor (algebraic datatypes only)
From strings to constructors and vice versa: all data types
showConstr :: Constr -> String
Gets the string for a constructor
readConstr :: DataType -> String -> Maybe Constr
Lookup a constructor via a string
Convenience functions: take type constructors apart
tyconUQname :: String -> String
Gets the unqualified type constructor: drop *.*.*... before name
tyconModule :: String -> String
Gets the module of a type constructor: take *.*.*... before name
Generic operations defined in terms of `gunfold`
fromConstr :: Data a => Constr -> a
Build a term skeleton
fromConstrB :: Data a => (forall a . Data a => a) -> Constr -> a
Build a term and use a generic function for subterms
fromConstrM :: (Monad m, Data a) => (forall a . Data a => m a) -> Constr -> m a
Monadic variation on `fromConstrB`
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