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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Moduleattoparsec-aeson-2.2.2.0Haskell2010

Data.Aeson.Parser

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!

  • 18 values

Lazy parsers

4 declarations

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.

valuejson :: Parser Value
#

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.

Handling objects with duplicate keys

valuejsonWith :: ([(Key, Value)] -> Either String Object) -> Parser Value
#

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 . H.fromList)

jsonLast keeps the last occurrence of each key, using HashMap.Lazy.fromListWith (const id).

jsonLast = jsonWith (Right . HashMap.Lazy.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

Strict parsers

2 declarations

The json' and value' parsers combine identification with conversion. They consume more CPU cycles up front, but have a smaller memory footprint.

valuejson' :: Parser Value
#

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

Decoding without FromJSON instances

4 declarations