
Data.Generics.Basics  Portability  nonportable (local universal quantification)  Stability  experimental  Maintainer  libraries@haskell.org 





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 reexported for convenience


module Data.Typeable 

The Data class for processing constructor applications



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 toplevel 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 datatypespecific 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 higherorder
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 leftassociative anyway and so it is
readily suited to fold a leftassociative binary operation over the
immediate subterms. In the definition of gmapQr, extra effort is
needed. We use a higherorder accumulation trick to mediate between
leftassociative constructor application vs. rightassociative 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 fglasgowexts 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   :: (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.
 Leftassociative 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
   Obtaining the constructor from a given datum.
For proper terms, this is meant to be the toplevel constructor.
Primitive datatypes are here viewed as potentially infinite sets of
values (i.e., constructors).
   The outer type constructor of the type
   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 nonunary type constructors.
   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 nonbinary 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 leftassociative binary operator
  gmapQr :: (r' > r > r) > r > (forall a . Data a => a > r') > a > r  Source 
 A generic query with a rightassociative 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.
   A generic query that processes one child by index (zerobased)
   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 >>=.
   Transformation of at least one immediate subterm does not fail
   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



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


Constructors



Constructs an algebraic datatype



Constructs the Int type



Constructs the Float type



Constructs the String type



Constructs a nonrepresentation for a nonpresentable type


Observers



Gets the type constructor including the module



Public representation of datatypes
 Constructors  AlgRep [Constr]   IntRep   FloatRep   StringRep   NoRep  
 Instances  



Gets the public presentation of a datatype


Convenience functions



Look up a constructor by its representation



Test for an algebraic type



Gets the constructors of an algebraic datatype



Gets the constructor for an index (algebraic datatypes only)



Gets the maximum constructor index of an algebraic datatype



Test for a nonrepresentable type


Data constructor representations



Representation of constructors
 Instances  



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



Fixity of constructors
 Constructors   Instances  


Constructors



Constructs a constructor








Observers



Gets the datatype of a constructor



Public representation of constructors
 Constructors   Instances  



Gets the public presentation of constructors



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.



Gets the fixity of a constructor


Convenience function: algebraic data types



Gets the index of a constructor (algebraic datatypes only)


From strings to constructors and vice versa: all data types



Gets the string for a constructor



Lookup a constructor via a string


Convenience functions: take type constructors apart



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



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


Generic operations defined in terms of gunfold



Build a term skeleton



Build a term and use a generic function for subterms



Monadic variation on fromConstrB


Produced by Haddock version 0.8 