License | BSD-style |
---|---|
Maintainer | palkovsky.ondrej@gmail.com |
Stability | experimental |
Portability | portable |
Safe Haskell | None |
Language | Haskell2010 |
An incremental applicative-style JSON parser, suitable for high performance memory efficient stream parsing.
The parser is using Data.Aeson types and FromJSON
instance, it can be
easily combined with aeson monadic parsing instances when appropriate.
- data Parser a
- data ParseOutput a
- = ParseYield a (ParseOutput a)
- | ParseNeedData (ByteString -> ParseOutput a)
- | ParseFailed String
- | ParseDone ByteString
- runParser :: Parser a -> ParseOutput a
- runParser' :: Parser a -> ByteString -> ParseOutput a
- parseByteString :: Parser a -> ByteString -> [a]
- parseLazyByteString :: Parser a -> ByteString -> [a]
- decode :: FromJSON a => ByteString -> Maybe a
- eitherDecode :: FromJSON a => ByteString -> Either String a
- decodeStrict :: FromJSON a => ByteString -> Maybe a
- eitherDecodeStrict :: FromJSON a => ByteString -> Either String a
- value :: FromJSON a => Parser a
- string :: Parser Text
- safeString :: Int -> Parser Text
- number :: Parser Scientific
- integer :: forall i. (Integral i, Bounded i) => Parser i
- real :: RealFloat a => Parser a
- bool :: Parser Bool
- jNull :: Parser ()
- (.:) :: Text -> Parser a -> Parser a
- (.:?) :: Text -> Parser a -> Parser (Maybe a)
- (.|) :: Parser a -> a -> Parser a
- (.!) :: Int -> Parser a -> Parser a
- objectWithKey :: Text -> Parser a -> Parser a
- objectItems :: Parser a -> Parser (Text, a)
- objectValues :: Parser a -> Parser a
- arrayOf :: Parser a -> Parser a
- arrayWithIndexOf :: Int -> Parser a -> Parser a
- indexedArrayOf :: Parser a -> Parser (Int, a)
- nullable :: Parser a -> Parser (Maybe a)
- filterI :: (a -> Bool) -> Parser a -> Parser a
- takeI :: Int -> Parser a -> Parser a
- mapWithFailure :: (a -> Either String b) -> Parser a -> Parser b
- arrayFound :: a -> a -> Parser a -> Parser a
- objectFound :: a -> a -> Parser a -> Parser a
How to use this library
>>>
parseByteString value "[1,2,3]" :: [[Int]]
[[1,2,3]]
The value
parser matches any FromJSON
value. The above command is essentially
identical to the aeson decode function; the parsing process can generate more
objects, therefore the results is [a].
Example of json-stream style parsing:
>>>
parseByteString (arrayOf integer) "[1,2,3]" :: [Int]
[1,2,3]
Parsers can be combinated using <*>
and <|>
operators. The parsers are
run in parallel and return combinations of the parsed values.
>>>
let text = "[{\"name\": \"John\", \"age\": 20}, {\"age\": 30, \"name\": \"Frank\"} ]"
>>>
let parser = arrayOf $ (,) <$> "name" .: string <*> "age" .: integer
>>>
parseByteString parser text :: [(T.Text,Int)]
[("John",20),("Frank",30)]
When parsing larger values, it is advisable to use lazy ByteStrings. The parsing is then more memory efficient as less lexical state is needed to be held in memory for parallel parsers.
More examples are available on https://github.com/ondrap/json-stream.
Performance
The parser tries to do the least amount of work to get the job done, skipping over items that are not required. General guidelines to get best performance:
Do not use the value
parser for the whole object if the object is big. Do not use json-stream
applicative parsing for creating objects if they have lots of records, unless you are skipping
large part of the structure. Every <*>
causes parallel parsing, too many parallel parsers
kill performance.
arrayOf value :: Parser MyStructure -- MyStructure with FromJSON instance
will probably behave better than
arrayOf $ MyStructure <$> "field1" .: string <*> "field2" .: integer <*> .... <*> "field20" .: string
and also better (at least memory-wise) than
value :: Parser [MyStructure]
unless the structure has hundreths of fields and you are parsing only a substructure.
The integer
parser was optimized in such
a way that the integer numbers skip the conversion to Scientific
, resulting in a slightly
faster speed.
It is possible to use the *>
operator to filter objects based on a condition, e.g.:
arrayOf $ id <$> "error" .: number *> "name" .: string
This will return all objects that contain attribute error with number content. The parser will skip trying to decode the name attribute if error is not found.
Constant space decoding
Constant space decoding is possible if the grammar does not specify non-constant
operations. The non-constant operations are value
, string
, many
and in some instances
<*>
.
The value
parser works by creating an aeson AST and passing it to the
parseJSON
method. The AST can consume a lot of memory before it is rejected
in parseJSON
. To achieve constant space the parsers safeString
, number
, integer
,
real
and bool
must be used; these parsers reject and do not parse data if it does not match the
type.
The object key length is limited to ~64K. Object records with longer key are ignored and unparsed.
Numbers are limited to 200.000 digits. Longer numbers will make the parsing fail.
The many
parser works by accumulating all matched values. Obviously, number
of such values influences the amount of used memory.
The <*>
operator runs both parsers in parallel and when they are both done, it
produces combinations of the received values. It is constant-space as long as the
number of element produced by child parsers is limited by a constant. This can be achieved by using
.!
and .:
functions combined with constant space
parsers or limiting the number of returned elements with takeI
.
If the source object contains an object with multiple keys with a same name,
json-stream matches the key multiple times. The only exception
is objectWithKey
(.:
and .:?
) that return at most one value for a given key.
Aeson compatibility
The parser uses internally Data.Aeson types, so that the FromJSON instances are
directly usable with the value
parser. It may be more convenient to parse the
outer structure with json-stream and the inner objects with aeson as long as constant-space
decoding is not required.
Json-stream defines the object-access operators .:
, .:?
but in a slightly different albeit more natural way. New operators are .!
for
array access and .|
to handle missing values.
>>>
let test = "[{\"name\": \"test1\", \"value\": 1}, {\"name\": \"test2\", \"value\": null}, {\"name\": \"test3\"}]"
>>>
let person = (,) <$> "name" .: string <*> "value" .: integer .| (-1)
>>>
let people = arrayOf person
>>>
parseByteString people test :: [(T.Text, Int)]
[("test1",1),("test2",-1),("test3",-1)]
The Parser
type
A representation of the parser.
Functor Parser Source # | |
Applicative Parser Source # |
It behaves as a list functor (produces all combinations), but the typical use is:
|
Alternative Parser Source # | Match items from the first parser, if none is matched, return items
from the second parser. Constant-space if second parser returns
constant number of items.
|
Monoid (Parser a) Source # |
|
data ParseOutput a Source #
Result of parsing. Contains continuations to continue parsing.
ParseYield a (ParseOutput a) | Returns a value from a parser. |
ParseNeedData (ByteString -> ParseOutput a) | Parser needs more data to continue parsing. |
ParseFailed String | Parsing failed, error is reported. |
ParseDone ByteString | Parsing finished, unparsed data is returned. |
Parsing functions
runParser :: Parser a -> ParseOutput a Source #
Run streaming parser, immediately returns ParseNeedData
.
runParser' :: Parser a -> ByteString -> ParseOutput a Source #
Run streaming parser with initial input.
parseByteString :: Parser a -> ByteString -> [a] Source #
Parse a bytestring, generate lazy list of parsed values. If an error occurs, throws an exception.
>>>
parseByteString (arrayOf integer) "[1,2,3,4]" :: [Int]
[1,2,3,4]
>>>
parseByteString (arrayOf ("name" .: string)) "[{\"name\":\"KIWI\"}, {\"name\":\"BIRD\"}]"
["KIWI","BIRD"]
parseLazyByteString :: Parser a -> ByteString -> [a] Source #
Parse a lazy bytestring, generate lazy list of parsed values. If an error occurs, throws an exception.
Aeson in-place replacement functions
decode :: FromJSON a => ByteString -> Maybe a Source #
Deserialize a JSON value from lazy ByteString
.
If this fails due to incomplete or invalid input, Nothing
is returned.
The input must consist solely of a JSON document, with no trailing data except for whitespace.
eitherDecode :: FromJSON a => ByteString -> Either String a Source #
Like decode
but returns an error message when decoding fails.
decodeStrict :: FromJSON a => ByteString -> Maybe a Source #
Like decode
, but on strict ByteString
eitherDecodeStrict :: FromJSON a => ByteString -> Either String a Source #
Like eitherDecode
, but on strict ByteString
FromJSON parser
value :: FromJSON a => Parser a Source #
Match FromJSON
value. Calls parseJSON on the parsed value.
>>>
let json = "[{\"key1\": [1,2], \"key2\": [5,6]}]"
>>>
parseByteString (arrayOf value) json :: [AE.Value]
[Object (fromList [("key2",Array [Number 5.0,Number 6.0]),("key1",Array [Number 1.0,Number 2.0])])]
Constant space parsers
safeString :: Int -> Parser Text Source #
Stops parsing string after the limit is reached. The string will not be matched if it exceeds the size. The size is the size of escaped string including escape characters.
number :: Parser Scientific Source #
Parse number, return in scientific format.
integer :: forall i. (Integral i, Bounded i) => Parser i Source #
Parse to bounded integer type (not Integer
).
If you are using integer numbers, use this parser.
It skips the conversion JSON -> Scientific
-> Int
and uses an Int
directly.
Structure operators
(.:) :: Text -> Parser a -> Parser a infixr 7 Source #
Synonym for objectWithKey
. Matches key in an object. The .:
operators can be chained.
>>>
let json = "{\"key1\": {\"nested-key\": 3}}"
>>>
parseByteString ("key1" .: "nested-key" .: integer) json :: [Int]
[3]
(.|) :: Parser a -> a -> Parser a infixl 6 Source #
Return default value if the parsers on the left hand didn't produce a result.
p .| defval = p <|> pure defval
The operator works on complete left side, the following statements are equal:
Record <$> "key1" .: "nested-key" .: value .| defaultValue Record <$> (("key1" .: "nested-key" .: value) .| defaultValue)
(.!) :: Int -> Parser a -> Parser a infixr 7 Source #
Synonym for arrayWithIndexOf
. Matches n-th item in array.
>>>
parseByteString (arrayOf (1 .! bool)) "[ [1,true,null], [2,false], [3]]" :: [Bool]
[True,False]
Structure parsers
objectWithKey :: Text -> Parser a -> Parser a Source #
Match only specific key of an object. This function will return only the first matched value in an object even if the source JSON defines the key multiple times (in violation of the specification).
objectItems :: Parser a -> Parser (Text, a) Source #
Match all key-value pairs of an object, return them as a tuple. If the source object defines same key multiple times, all values are matched.
objectValues :: Parser a -> Parser a Source #
Match all key-value pairs of an object, return only values. If the source object defines same key multiple times, all values are matched. Keys are ignored.
indexedArrayOf :: Parser a -> Parser (Int, a) Source #
Match all items of an array, add index to output.
Parsing modifiers
filterI :: (a -> Bool) -> Parser a -> Parser a Source #
Let only items matching a condition pass.
>>>
parseByteString (filterI (>5) $ arrayOf integer) "[1,2,3,4,5,6,7,8,9,0]" :: [Int]
[6,7,8,9]
takeI :: Int -> Parser a -> Parser a Source #
Take maximum n matching items.
>>>
parseByteString (takeI 3 $ arrayOf integer) "[1,2,3,4,5,6,7,8,9,0]" :: [Int]
[1,2,3]
mapWithFailure :: (a -> Either String b) -> Parser a -> Parser b Source #
A back-door for lifting of possibly failing actions. If an action fails with Left value, convert it into failure of parsing
SAX-like parsers
arrayFound :: a -> a -> Parser a -> Parser a Source #
Generate start/end values when an array is found, in between run a parser. The inner parser is not run if an array is not found.
>>>
let test = "[[1,2,3],true,[],false,{\"key\":1}]" :: BS.ByteString
>>>
parseByteString (arrayOf (arrayFound 10 20 (1 .! integer))) test :: [Int]
[10,2,20,10,20]
objectFound :: a -> a -> Parser a -> Parser a Source #
Generate start/end values when an object is found, in between run a parser. The inner parser is not run if an array is not found.