Copyright | (c) 2012-2016 Bryan O'Sullivan (c) 2011 MailRank Inc. |
---|---|
License | BSD3 |
Maintainer | Bryan O'Sullivan <bos@serpentine.com> |
Stability | experimental |
Portability | portable |
Safe Haskell | Safe-Inferred |
Language | Haskell2010 |
Efficiently and correctly parse a JSON string. The string must be encoded as UTF-8.
It can be useful to think of parsing as occurring in two phases:
- Identification of the textual boundaries of a JSON value. This is always strict, so that an invalid JSON document can be rejected as soon as possible.
- Conversion of a JSON value to a Haskell value. This may be either immediate (strict) or deferred (lazy); see below for details.
The question of whether to choose a lazy or strict parser is subtle, but it can have significant performance implications, resulting in changes in CPU use and memory footprint of 30% to 50%, or occasionally more. Measure the performance of your application with each!
Synopsis
- json :: Parser Value
- value :: Parser Value
- jstring :: Parser Text
- scientific :: Parser Scientific
- jsonWith :: ([(Key, Value)] -> Either String Object) -> Parser Value
- jsonLast :: Parser Value
- jsonAccum :: Parser Value
- jsonNoDup :: Parser Value
- json' :: Parser Value
- value' :: Parser Value
- jsonWith' :: ([(Key, Value)] -> Either String Object) -> Parser Value
- jsonLast' :: Parser Value
- jsonAccum' :: Parser Value
- jsonNoDup' :: Parser Value
- decodeWith :: Parser Value -> (Value -> Result a) -> ByteString -> Maybe a
- decodeStrictWith :: Parser Value -> (Value -> Result a) -> ByteString -> Maybe a
- eitherDecodeWith :: Parser Value -> (Value -> IResult a) -> ByteString -> Either (JSONPath, String) a
- eitherDecodeStrictWith :: Parser Value -> (Value -> IResult a) -> ByteString -> Either (JSONPath, String) a
Lazy parsers
The json
and value
parsers decouple identification from
conversion. Identification occurs immediately (so that an invalid
JSON document can be rejected as early as possible), but conversion
to a Haskell value is deferred until that value is needed.
This decoupling can be time-efficient if only a smallish subset of elements in a JSON value need to be inspected, since the cost of conversion is zero for uninspected elements. The trade off is an increase in memory usage, due to allocation of thunks for values that have not yet been converted.
Parse any JSON value.
The conversion of a parsed value to a Haskell value is deferred until the Haskell value is needed. This may improve performance if only a subset of the results of conversions are needed, but at a cost in thunk allocation.
This function is an alias for value
. In aeson 0.8 and earlier, it
parsed only object or array types, in conformance with the
now-obsolete RFC 4627.
Warning
If an object contains duplicate keys, only the first one will be kept.
For a more flexible alternative, see jsonWith
.
scientific :: Parser Scientific Source #
Parse a JSON number.
Handling objects with duplicate keys
jsonWith :: ([(Key, Value)] -> Either String Object) -> Parser Value Source #
Parse any JSON value.
This parser is parameterized by a function to construct an Object
from a raw list of key-value pairs, where duplicates are preserved.
The pairs appear in reverse order from the source.
Examples
json
keeps only the first occurrence of each key, using fromList
.
json
=jsonWith
(Right
.
fromList
)
jsonLast
keeps the last occurrence of each key, using
.fromListWith
(const
id
)
jsonLast
=jsonWith
(Right
.
fromListWith
(const
id
))
jsonAccum
keeps wraps all values in arrays to keep duplicates, using
fromListAccum
.
jsonAccum
=jsonWith
(Right
.fromListAccum
)
jsonNoDup
fails if any object contains duplicate keys, using parseListNoDup
.
jsonNoDup
=jsonWith
parseListNoDup
jsonNoDup :: Parser Value Source #
Variant of json
which fails if any object contains duplicate keys.
Strict parsers
The json'
and value'
parsers combine identification with
conversion. They consume more CPU cycles up front, but have a
smaller memory footprint.
json' :: Parser Value Source #
Parse any JSON value.
This is a strict version of json
which avoids building up thunks
during parsing; it performs all conversions immediately. Prefer
this version if most of the JSON data needs to be accessed.
This function is an alias for value'
. In aeson 0.8 and earlier, it
parsed only object or array types, in conformance with the
now-obsolete RFC 4627.
Warning
If an object contains duplicate keys, only the first one will be kept.
For a more flexible alternative, see jsonWith'
.
Handling objects with duplicate keys
jsonWith' :: ([(Key, Value)] -> Either String Object) -> Parser Value Source #
Strict version of jsonWith
.
jsonLast' :: Parser Value Source #
Variant of json'
which keeps only the last occurrence of every key.
jsonAccum' :: Parser Value Source #
jsonNoDup' :: Parser Value Source #
Variant of json'
which fails if any object contains duplicate keys.
Decoding without FromJSON instances
decodeWith :: Parser Value -> (Value -> Result a) -> ByteString -> Maybe a Source #
decodeStrictWith :: Parser Value -> (Value -> Result a) -> ByteString -> Maybe a Source #