Text.JSON.Generic

json-0.9.1: Support for serialising Haskell to and from JSON

Copyright | (c) Lennart Augustsson, 2008-2009
License | BSD3
Maintainer | Sigbjorn Finne <sof@galois.com>
Stability | provisional
Portability | portable
Safe Haskell | None
Language | Haskell98

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.

Minimal complete definition

gunfold, toConstr, dataTypeOf

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 Void |
Data SpecConstrAnnotation |
Data Version |
Data ByteString |
Data ByteString |
Data IntSet |
Data a => Data [a] |
(Data a, Integral a) => Data (Ratio a) |
(Data a, Typeable * a) => Data (Ptr a) |
(Data a, Typeable * a) => Data (ForeignPtr a) |
Data a => Data (Identity a) |
Data a => Data (Maybe a) |
Data a => Data (IntMap a) |
(Data a, Ord a) => Data (Set a) |
Data a => Data (Seq a) |
Data a => Data (ViewL a) |
Data a => Data (ViewR a) |
(Data a, Data b) => Data (Either a b) |
(Data a, Data b) => Data (a, b) |
(Typeable * a, Data a, Data b, Ix a) => Data (Array a b) |
Data t => Data (Proxy * t) |
(Data k, Data a, Ord k) => Data (Map k a) |
(Data a, Data b, Data c) => Data (a, b, c) |
(Coercible * a b, Data a, Data b) => Data (Coercion * a b) |
((~) * a b, Data a) => Data ((:~:) * a b) |
(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.

Minimal complete definition

typeRep#

toJSON :: Data a => a -> JSValue

Convert anything to a JSON value.

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

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

encodeJSON :: Data a => a -> String

Encode a value as a string.

decodeJSON :: Data a => String -> a

Decode a string as a value.

toJSON_generic :: Data a => a -> JSValue

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