--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
--- Portability : non-portable
+-- Portability : non-portable (local universal quantification)
--
--- \"Scrap your boilerplate\" --- Generic programming in Haskell
--- See <http://www.cs.vu.nl/boilerplate/>. The present module provides
--- the Data class with its primitives for generic programming.
+-- \"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.
--
-----------------------------------------------------------------------------
toConstr, -- :: a -> Constr
dataTypeOf, -- :: a -> DataType
dataCast1, -- mediate types and unary type constructors
- dataCast2 -- mediate types and binary type constructors
+ dataCast2, -- mediate types and binary type constructors
+ -- Generic maps defined in terms of gfoldl
+ gmapT,
+ gmapQ,
+ gmapQl,
+ gmapQr,
+ gmapQi,
+ gmapM,
+ gmapMp,
+ gmapMo
),
-- * Datatype representations
DataType, -- abstract, instance of: Show
- Constr, -- abstract, instance of: Eq, Show
- DataRep(..), -- instance of: Eq, Show
- ConstrRep(..), -- instance of: Eq, Show
- ConIndex, -- alias for Int, start at 1
- Fixity(..), -- instance of: Eq, Show
-
- -- * Observers for datatype representations
+ -- ** Constructors
+ mkDataType, -- :: String -> [Constr] -> DataType
+ mkIntType, -- :: String -> DataType
+ mkFloatType, -- :: String -> DataType
+ mkStringType, -- :: String -> DataType
+ mkNorepType, -- :: String -> DataType
+ -- ** Observers
dataTypeName, -- :: DataType -> String
+ DataRep(..), -- instance of: Eq, Show
dataTypeRep, -- :: DataType -> DataRep
- constrType, -- :: Constr -> DataType
- constrRep, -- :: Constr -> ConstrRep
- repConstr, -- :: DataType -> ConstrRep -> Constr
-
- -- * Representations of algebraic data types
- mkDataType, -- :: String -> [Constr] -> DataType
- mkConstr, -- :: DataType -> String -> Fixity -> Constr
- dataTypeConstrs,-- :: DataType -> [Constr]
- constrFields, -- :: Constr -> [String]
- constrFixity, -- :: Constr -> Fixity
-
- -- * From strings to constr's and vice versa: all data types
- showConstr, -- :: Constr -> String
- readConstr, -- :: DataType -> String -> Maybe Constr
-
- -- * Convenience funtions: algebraic data types
+ -- ** Convenience functions
+ repConstr, -- :: DataType -> ConstrRep -> Constr
isAlgType, -- :: DataType -> Bool
+ dataTypeConstrs,-- :: DataType -> [Constr]
indexConstr, -- :: DataType -> ConIndex -> Constr
- constrIndex, -- :: Constr -> ConIndex
maxConstrIndex, -- :: DataType -> ConIndex
+ isNorepType, -- :: DataType -> Bool
- -- * Representation of primitive types
- mkIntType, -- :: String -> DataType
- mkFloatType, -- :: String -> DataType
- mkStringType, -- :: String -> DataType
+ -- * Data constructor representations
+ Constr, -- abstract, instance of: Eq, Show
+ ConIndex, -- alias for Int, start at 1
+ Fixity(..), -- instance of: Eq, Show
+ -- ** Constructors
+ mkConstr, -- :: DataType -> String -> Fixity -> Constr
mkIntConstr, -- :: DataType -> Integer -> Constr
mkFloatConstr, -- :: DataType -> Double -> Constr
mkStringConstr, -- :: DataType -> String -> Constr
-
- -- * Non-representations for non-presentable types
- mkNorepType, -- :: String -> DataType
- isNorepType, -- :: DataType -> Bool
+ -- ** Observers
+ constrType, -- :: Constr -> DataType
+ ConstrRep(..), -- instance of: Eq, Show
+ constrRep, -- :: Constr -> ConstrRep
+ constrFields, -- :: Constr -> [String]
+ constrFixity, -- :: Constr -> Fixity
+ -- ** Convenience function: algebraic data types
+ constrIndex, -- :: Constr -> ConIndex
+ -- ** From strings to constructors and vice versa: all data types
+ showConstr, -- :: Constr -> String
+ readConstr, -- :: DataType -> String -> Maybe Constr
-- * Convenience functions: take type constructors apart
tyconUQname, -- :: String -> String
tyconModule, -- :: String -> String
- -- * Generic maps defined in terms of gfoldl
- gmapT,
- gmapQ,
- gmapQl,
- gmapQr,
- gmapQi,
- gmapM,
- gmapMp,
- gmapMo,
-
- -- * Generic operation(s) defined in terms of gunfold
+ -- * Generic operations defined in terms of 'gunfold'
fromConstr, -- :: Constr -> a
fromConstrB, -- :: ... -> Constr -> a
fromConstrM -- :: Monad m => ... -> Constr -> m a
--
------------------------------------------------------------------------------
-{-
-
-The Data class comprehends a fundamental primitive "gfoldl" for
+{- |
+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 module. 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.
+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.
-}
class Typeable a => Data a where
-{-
-
-Folding constructor applications ("gfoldl")
-
-The combinator takes two arguments "k" and "z" to fold over a term
-"x". The result type is defined in terms of "x" but variability is
-achieved by means of type constructor "c" for the construction of the
-actual result type. The purpose of the argument "z" is to define how
-the empty constructor application is folded. So "z" is like the
-neutral / start element for list folding. The purpose of the argument
-"k" is to define how the nonempty constructor application is
-folded. That is, "k" takes the folded "tail" of the constructor
-application and its head, i.e., an immediate subterm, and combines
-them in some way. See the Data instances in this file for an
-illustration of "gfoldl". Conclusion: the type of gfoldl is a
-headache, but operationally it is simple generalisation of a list
-fold.
-
--}
-
- -- | Left-associative fold operation for constructor applications
+ -- | 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.
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)
- -> a -> c a
+ -- ^ 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.
+
+ -- See the 'Data' instances in this file for an illustration of 'gfoldl'.
- -- Default definition for gfoldl
- -- which copes immediately with basic datatypes
- --
gfoldl _ z = z
-- | Unfolding constructor applications
-- 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).
- --
toConstr :: a -> Constr
- -- | Provide access to list of all constructors
+ -- | The outer type constructor of the type
dataTypeOf :: a -> DataType
--
------------------------------------------------------------------------------
- -- | Mediate types and unary type constructors
+ -- | 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.
dataCast1 :: Typeable1 t
=> (forall a. Data a => c (t a))
-> Maybe (c a)
dataCast1 _ = Nothing
- -- | Mediate types and binary 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 non-binary type constructors.
dataCast2 :: Typeable2 t
=> (forall a b. (Data a, Data b) => c (t a b))
-> Maybe (c a)
--
------------------------------------------------------------------------------
-{-
-
-The combinators gmapT, gmapQ, gmapM, ... can all be defined in terms
-of gfoldl. We provide corresponding default definitions 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.
-
--}
-
-- | 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.
gmapT :: (forall b. Data b => b -> b) -> a -> a
-- Use an identity datatype constructor ID (see below)
k c x = CONST $ (unCONST c) `o` f x
z _ = CONST r
-{-
-
-In the definition of gmapQ? 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.
-
--}
-
-- | A generic query with a right-associative binary operator
gmapQr :: (r' -> r -> r) -> r -> (forall a. Data a => a -> r') -> a -> r
gmapQr o r f x = unQr (gfoldl k (const (Qr id)) x) r
-- | 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 '>>='.
gmapM :: Monad m => (forall a. Data a => a -> m a) -> a -> m a
-- Use immediately the monad datatype constructor
z = ID
--- | Monadic variation on \"fromConstrB\"
+-- | Monadic variation on 'fromConstrB'
fromConstrM :: (Monad m, Data a)
=> (forall a. Data a => m a)
-> Constr
--
-- | Representation of datatypes.
--- | A package of constructor representations with names of type and module.
--- | The list of constructors could be an array, a balanced tree, or others.
+-- A package of constructor representations with names of type and module.
--
data DataType = DataType
{ tycon :: String
| NoRep
deriving (Eq,Show)
+-- The list of constructors could be an array, a balanced tree, or others.
-- | Public representation of constructors
deriving (Eq,Show)
---
--- | Unique index for datatype constructors.
--- | Textual order is respected. Starts at 1.
---
+-- | Unique index for datatype constructors,
+-- counting from 1 in the order they are given in the program text.
type ConIndex = Int
--- | Gets the public presentation of datatypes
+-- | Gets the public presentation of a datatype
dataTypeRep :: DataType -> DataRep
dataTypeRep = datarep
showConstr c == str ]
--- | Gets the constructors
+-- | Gets the constructors of an algebraic datatype
dataTypeConstrs :: DataType -> [Constr]
dataTypeConstrs dt = case datarep dt of
(AlgRep cons) -> cons
_ -> False
--- | Gets the constructor for an index
+-- | Gets the constructor for an index (algebraic datatypes only)
indexConstr :: DataType -> ConIndex -> Constr
indexConstr dt idx = case datarep dt of
(AlgRep cs) -> cs !! (idx-1)
_ -> error "indexConstr"
--- | Gets the index of a constructor
+-- | Gets the index of a constructor (algebraic datatypes only)
constrIndex :: Constr -> ConIndex
constrIndex con = case constrRep con of
(AlgConstr idx) -> idx
_ -> error "constrIndex"
--- | Gets the maximum constructor index
+-- | Gets the maximum constructor index of an algebraic datatype
maxConstrIndex :: DataType -> ConIndex
maxConstrIndex dt = case dataTypeRep dt of
AlgRep cs -> length cs
------------------------------------------------------------------------------
--- | Constructs the Int type
+-- | Constructs the 'Int' type
mkIntType :: String -> DataType
mkIntType = mkPrimType IntRep
--- | Constructs the Float type
+-- | Constructs the 'Float' type
mkFloatType :: String -> DataType
mkFloatType = mkPrimType FloatRep
--- | Constructs the String type
+-- | Constructs the 'String' type
mkStringType :: String -> DataType
mkStringType = mkPrimType StringRep
--- | Helper for mkIntType, mkFloatType, mkStringType
+-- | Helper for 'mkIntType', 'mkFloatType', 'mkStringType'
mkPrimType :: DataRep -> String -> DataType
mkPrimType dr str = DataType
{ tycon = str
------------------------------------------------------------------------------
--- | Constructs a non-representation
+-- | Constructs a non-representation for a non-presentable type
mkNorepType :: String -> DataType
mkNorepType str = DataType
{ tycon = str
------------------------------------------------------------------------------
--- | Gets the unqualified type constructor
--- Drop *.*.*... before name
+-- | Gets the unqualified type constructor:
+-- drop *.*.*... before name
--
tyconUQname :: String -> String
tyconUQname x = let x' = dropWhile (not . (==) '.') x
in if x' == [] then x else tyconUQname (tail x')
--- | Gets the module of a type constructor
--- Take *.*.*... before name
+-- | Gets the module of a type constructor:
+-- take *.*.*... before name
tyconModule :: String -> String
tyconModule x = let (a,b) = break ((==) '.') x
in if b == ""
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