json-0.5: Support for serialising Haskell to and from JSON

Portability | portable
Stability | provisional
Maintainer | Sigbjorn Finne <sof@galois.com>
Safe Haskell | None

Text.JSON.Generic

Description

JSON serializer and deserializer using Data.Generics. The functions here handle algebraic data types and primitive types. It uses the same representation as Text.JSON for Prelude types.

Synopsis

- module Text.JSON
- class Typeable a => Data a
- class Typeable a
- toJSON :: Data a => a -> JSValue
- fromJSON :: Data a => JSValue -> Result a
- encodeJSON :: Data a => a -> String
- decodeJSON :: Data a => String -> a
- toJSON_generic :: Data a => a -> JSValue
- fromJSON_generic :: Data a => JSValue -> Result a

# Documentation

module Text.JSON

class Typeable a => Data a

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.

Instances

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 Handle |
Data DataType |
Data ThreadId |
Data TypeRep |
Data TyCon |
Data ByteString |
Data ByteString |
Data IntSet |
Data a => Data [a] |
(Data a, Integral a) => Data (Ratio a) |
Typeable a => Data (StablePtr a) |
Typeable a => Data (IO a) |
Typeable a => Data (Ptr a) |
Typeable a => Data (ForeignPtr a) |
Typeable a => Data (STM a) |
Typeable a => Data (TVar a) |
Typeable a => Data (IORef a) |
Typeable a => Data (MVar a) |
Data a => Data (Maybe a) |
Data a => Data (IntMap a) |
(Data a, Ord a) => Data (Set a) |
(Data a, Data b) => Data (a -> b) |
(Data a, Data b) => Data (Either a b) |
(Data a, Data b) => Data (a, b) |
(Typeable s, Typeable a) => Data (ST s a) |
(Typeable a, Data b, Ix a) => Data (Array a b) |
(Data k, Data a, Ord k) => Data (Map k a) |
(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.

Instances

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 Handle |
Typeable Handle__ |
Typeable DataType |
Typeable ThreadId |
Typeable TypeRep |
Typeable TyCon |
Typeable ByteString |
Typeable ByteString |
Typeable IntSet |
Typeable JSString |
Typeable JSValue |
(Typeable1 s, Typeable a) => Typeable (s a) | One Typeable instance for all Typeable1 instances

toJSON :: Data a => a -> JSValue Source

Convert anything to a JSON value.

fromJSON :: Data a => JSValue -> Result aSource

Convert a JSON value to anything (fails if the types do not match).

encodeJSON :: Data a => a -> String Source

Encode a value as a string.

decodeJSON :: Data a => String -> aSource

Decode a string as a value.

toJSON_generic :: Data a => a -> JSValue Source

fromJSON_generic :: Data a => JSValue -> Result aSource