Safe Haskell | Safe-Inferred |
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# Documentation

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 `-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.

Data Bool | |

Data Char | |

Data Double | |

Data Float | |

Data Int | |

Data Int8 | |

Data Int16 | |

Data Int32 | |

Data Int64 | |

Data Integer | |

Data Ordering | |

Data Word | |

Data Word8 | |

Data Word16 | |

Data Word32 | |

Data Word64 | |

Data () | |

Data Rational | |

Data a => Data [a] | |

(Data a, Integral a) => Data (Ratio a) | |

Typeable a => Data (Ptr a) | |

Data a => Data (Maybe a) | |

Typeable a => Data (ForeignPtr a) | |

Data a => Data (Maybe a) | |

(Data a, Data b) => Data (Either a b) | |

(Data a, Data b) => Data (a, b) | |

(Typeable a, Data b, Ix a) => Data (Array 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) |

class Typeable a

The class `Typeable`

allows a concrete representation of a type to
be calculated.

Typeable Bool | |

Typeable Char | |

Typeable Double | |

Typeable Float | |

Typeable Int | |

Typeable Int8 | |

Typeable Int16 | |

Typeable Int32 | |

Typeable Int64 | |

Typeable Integer | |

Typeable Ordering | |

Typeable RealWorld | |

Typeable Word | |

Typeable Word8 | |

Typeable Word16 | |

Typeable Word32 | |

Typeable Word64 | |

Typeable () | |

Typeable TypeRep | |

Typeable TyCon | |

Typeable Rational | |

(Typeable1 s, Typeable a) => Typeable (s a) | One Typeable instance for all Typeable1 instances |

Typeable a => Typeable (Maybe a) |