|Maintainer||Sigbjorn Finne <firstname.lastname@example.org>|
Data class comprehends a fundamental primitive
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
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.
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
allows the programmer or the compiler to derive specialised, and maybe
more efficient code per datatype. Note:
gfoldl is more higher-order
gmap combinators. This is subject to ongoing benchmarking
experiments. It might turn out that the
gmap combinators will be
moved out of the class
Conceptually, the definition of the
gmap combinators in terms of the
gfoldl requires the identification of the
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
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
(:)). When the query is meant to compute a value
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
-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.
class Typeable a
Typeable allows a concrete representation of a type to
Convert a JSON value to anything (fails if the types do not match).