| 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 |
Data.Data
Contents
Description
"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
- 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 #
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 gmapQx 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.
Minimal complete definition
Methods
gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> a -> c a #
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
gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c a #
Unfolding constructor applications
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 :: Typeable t => (forall d. Data d => c (t d)) -> 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
dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> 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
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 :: forall r r'. (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> a -> r #
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 #
A generic query with a right-associative binary operator
gmapQ :: (forall d. Data d => d -> u) -> a -> [u] #
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 #
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 #
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 #
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 #
Transformation of one immediate subterm with success
Instances
Datatype representations
Representation of datatypes. A package of constructor representations with names of type and module.
Constructors
mkDataType :: String -> [Constr] -> DataType #
Constructs an algebraic datatype
mkFloatType :: String -> DataType #
Constructs the Float type
mkCharType :: String -> DataType #
Constructs the Char type
mkNoRepType :: String -> DataType #
Constructs a non-representation for a non-representable type
Observers
dataTypeName :: DataType -> String #
Gets the type constructor including the module
Public representation of datatypes
dataTypeRep :: DataType -> DataRep #
Gets the public presentation of a datatype
Convenience functions
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
Unique index for datatype constructors, counting from 1 in the order they are given in the program text.
Constructors
Observers
constrType :: Constr -> DataType #
Gets the datatype of a constructor
Public representation of constructors
Constructors
| AlgConstr ConIndex | |
| IntConstr Integer | |
| FloatConstr Rational | |
| CharConstr Char |
constrFields :: Constr -> [String] #
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 #
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
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 d. Data d => d) -> Constr -> a #
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 #
Monadic variation on fromConstrB