Safe Haskell | None |
---|---|
Language | Haskell98 |
- toDyn :: Typeable * a => a -> Dynamic
- data Dynamic :: *
- data WrapData = forall a . Data a => WrapData a
- wrapData :: Data a => a -> WrapData
- data WrapData2 = forall a . Data a => WrapData2 a a
- class Typeable * a => Data a
- data TestCase = forall a prop . (Testable prop, Data a) => TestCase (((String, a, a) -> Property) -> prop)
- newtype QCInt = QCInt Int
- newtype QCNat = QCNat Int
- newtype QCBool = QCBool Bool
Documentation
toDyn :: Typeable * a => a -> Dynamic
Converts an arbitrary value into an object of type Dynamic
.
The type of the object must be an instance of Typeable
, which
ensures that only monomorphically-typed objects may be converted to
Dynamic
. To convert a polymorphic object into Dynamic
, give it
a monomorphic type signature. For example:
toDyn (id :: Int -> Int)
data Dynamic :: *
A value of type Dynamic
is an object encapsulated together with its type.
A Dynamic
may only represent a monomorphic value; an attempt to
create a value of type Dynamic
from a polymorphically-typed
expression will result in an ambiguity error (see toDyn
).
Show
ing a value of type Dynamic
returns a pretty-printed representation
of the object's type; useful for debugging.
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.