Copyright	(c) The University of Glasgow, CWI 2001--2004
License	BSD-style (see the file libraries/base/LICENSE)
Maintainer	[email protected]
Stability	experimental
Portability	non-portable (local universal quantification)
Safe Haskell	Trustworthy
Language	Haskell2010

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 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.

Datatype representations

data DataType Source

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

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

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

data DataRep Source

Public representation of datatypes

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. Note that equality on constructors with different types may not work -- i.e. the constructors for False and Nothing may compare equal.

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

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

Constructs a constructor

mkIntegralConstr :: (Integral a, Show a) => DataType -> a -> Constr Source

mkRealConstr :: (Real a, Show a) => DataType -> a -> Constr Source

mkCharConstr :: DataType -> Char -> Constr Source

Makes a constructor for Char.

Observers

constrType :: Constr -> DataType Source

Gets the datatype of a constructor

data ConstrRep Source

Public representation of constructors

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 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