| base-3.0.2.0: Basic libraries | Source code | Contents | Index

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 | 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 `-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 | Source

:: (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 | 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 :: Typeable1 t => (forall a . Data a => c (t a)) -> 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 ``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) | 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 ``const` `Nothing``, which is appropriate for non-binary type constructors.

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

A generic query with a left-associative binary operator

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

A generic query with a right-associative binary operator

gmapQ :: (forall a . Data a => a -> 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 declaratoin of the data constructors.

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

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

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

Transformation of at least one immediate subterm does not fail

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

Transformation of one immediate subterm with success

Instances

Data Bool
Data Char
Data DataType
Data Double
Data Float
Data Handle
Data Int
Data Int16
Data Int32
Data Int64
Data Int8
Data Integer
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)
(RealFloat a, Data a) => Data (Complex a)
Typeable a => Data (ForeignPtr a)
Typeable a => Data (IO a)
Typeable a => Data (IORef 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)
Typeable a => Data (StablePtr a)
Typeable a => Data (TVar a)
Data a => Data [a]
(Typeable a, Data b, Ix a) => Data (Array a b)
(Data a, Data b) => Data (Either a b)
(Typeable s, Typeable a) => Data (ST s a)

Datatype representations

data DataType | Source

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 | Source

Constructs an algebraic datatype

mkIntType :: String -> DataType | Source

Constructs the `Int` type

mkFloatType :: String -> DataType | Source

Constructs the `Float` type

mkStringType :: String -> DataType | Source

Constructs the `String` type

mkNorepType :: String -> DataType | Source

Constructs a non-representation for a non-presentable type
Observers

dataTypeName :: DataType -> String | Source

Gets the type constructor including the module

data DataRep | Source

Public representation of datatypes
Constructors

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

Instances

Eq DataRep
Show DataRep

dataTypeRep :: DataType -> DataRep | Source

Gets the public presentation of a datatype
Convenience functions

repConstr :: DataType -> ConstrRep -> Constr | Source

Look up a constructor by its representation

isAlgType :: DataType -> Bool | Source

Test for an algebraic type

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

data Constr | Source

Representation of constructors
Instances

Eq Constr
Show Constr

type ConIndex = Int | Source

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

data Fixity | Source

Fixity of constructors
Constructors

Prefix |
Infix |

Instances

Eq Fixity
Show Fixity

Constructors

mkConstr :: DataType -> String -> [String] -> Fixity -> Constr | Source

Constructs a constructor

mkIntConstr :: DataType -> Integer -> Constr | Source

mkFloatConstr :: DataType -> Double -> Constr | Source

mkStringConstr :: DataType -> String -> Constr | Source

Observers

constrType :: Constr -> DataType | Source

Gets the datatype of a constructor

data ConstrRep | Source

Public representation of constructors
Constructors

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

Instances

Eq ConstrRep
Show ConstrRep

constrRep :: Constr -> ConstrRep | Source

Gets the public presentation 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

readConstr :: DataType -> String -> Maybe Constr | Source

Lookup a constructor via a string
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 a . Data a => a) -> Constr -> a | Source

Build a term and use a generic function for subterms

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

Monadic variation on `fromConstrB`
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