json-autotype: Automatic type declaration for JSON input data

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Generates datatype declarations with Aeson's FromJSON instances from a set of example ".json" files.

To get started you need to install the package, and run "json-autotype" binary on an input ".json" file. That will generate a new Aeson-based JSON parser.

"$ json-autotype input.json -o JSONTypes.hs"

Feel free to tweak the by changing types of the fields - any field type that is instance of FromJSON should work.

You may immediately test the parser by calling it as a script:

"$ runghc JSONTypes.hs input.json"

One can now use multiple input files to generate better type description.

Now with Elm code generation support! (If you want your favourite programming language supported too - name your price and mail the author.)

See introduction on https://github.com/mgajda/json-autotype for details.

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Versions0.2.0.0,,,,,,,,,,,,,,,,,,,,,, 0.3, 0.4, 0.5, 1.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.0.6, 1.0.7, 1.0.8, 1.0.9, 1.0.10, 1.0.11, 1.0.12, 1.0.13, 1.0.14, 1.0.15, 1.0.16, 1.0.17, 1.0.18, 1.1.0, 1.1.1, 1.1.2, 2.0.0, 3.0.0, 3.0.1, 3.0.1
Change logchangelog.md
Dependenciesaeson (>=1.2.1 && <1.5), base (>=4.3 && <5), bytestring (>=0.9 && <0.11), containers (>=0.3 && <0.7), filepath (>=1.3 && <1.5), GenericPretty (==1.2.*), hashable (==1.2.*), json-alt, json-autotype, lens (>=4.1 && <4.18), mtl (>=2.1 && <2.3), optparse-applicative (>=0.12 && <1.0), pretty (>=1.1 && <1.3), process (>=1.1 && <1.7), QuickCheck (>=2.4 && <3.0), scientific (>=0.3 && <0.5), smallcheck (>=1.0 && <1.2), template-haskell, text (>=1.1 && <1.4), uniplate (==1.6.*), unordered-containers (==0.2.*), vector (>=0.9 && <0.13), yaml (>=0.8 && <0.12) [details]
CopyrightCopyright by Michal J. Gajda '2014-'2018
AuthorMichal J. Gajda
Maintainersimons@cryp.to mjgajda@gmail.com
CategoryData, Tools
Home pagehttps://github.com/mgajda/json-autotype
Bug trackerhttps://github.com/mgajda/json-autotype/issues
Source repositoryhead: git clone https://github.com/mgajda/json-autotype.git
UploadedWed Dec 5 16:13:00 UTC 2018 by PeterSimons




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For package maintainers and hackage trustees

Readme for json-autotype-3.0.1

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Takes a JSON format input, and generates automatic Haskell type declarations.

Parser and printer instances are derived using Aeson.

The program uses union type unification to trim output declarations. The types of same attribute tag and similar attribute set, are automatically unified using recognition by attribute set matching. (This option can be optionally turned off, or a set of unified types may be given explicitly.) :|: alternatives (similar to Either) are used to assure that all JSON inputs seen in example input file are handled correctly.

I should probably write a short paper to explain the methodology.

Build matrix across GHC compiler versions Release build Hackage Hackage Dependencies Docker Automated build Docker image size

Details on official releases are on Hackage We currently support code generation to Haskell, and Elm.

Please volunteer help or provide financial support, if you want your favourite language supported too! Expression of interest in particular feature may be filed as GitHub issue.


After installing with cabal install json-autotype, you might generate stub code for the parser:

    json-autotype input1.json ... inputN.json -o MyFormat.hs

Then you might test the parser by running it on an input file:

    runghc MyFormat.hs input.json

At this point you may see data structure generated automatically for you. The more input files you give to the inference engine json-autotype, the more precise type description will be.

Algorithm will also suggest which types look similar, based on a set of attribute names, and unify them unless specifically instructed otherwise.

The goal of this program is to make it easy for users of big JSON APIs to generate entries from example data.

Occasionally you might find a valid JSON for which json-autotype doesn't generate a correct parser. You may either edit the resulting file and send it to the author as a test case for future release.

Patches and suggestions are welcome.

You can run with Docker:

docker run -it migamake/json-autotype


The most simple example:


It will produce the module with the following datatypes and TH calls for JSON parser derivations:

    data ColorsArray = ColorsArray {
        colorsArrayHexValue    :: Text,
        colorsArrayColorName :: Text
      } deriving (Show,Eq)

    data TopLevel = TopLevel {
        topLevelColorsArray :: ColorsArray
      } deriving (Show,Eq)

Note that attribute names match the names of JSON dictionary keys.

Another example with ambiguous types:

                "parameterValue":"site API"

It will produce quite intuitive result (plus extra parentheses, and class derivations):

    data Parameter = Parameter {
        parameterParameterValue :: Bool :|: Int :|: Text,
        parameterParameterName :: Text

    data TopLevel = TopLevel {
        topLevelParameter :: Parameter

Real-world use case examples are provided in the package source repository.


  1. JSON-Autotype uses its own union type system to derive types from JSON documents as the first step.
  2. Then it finds all those records that have 90% of the same key names, and suggest them as similar enough to merit treating as instances of the same type. (Note that this is optional, and can be tuned manually.)
  3. Last step is to derive unique-ish type names - we currently do it by concatenating the name of the container and name of the key. (Please open PR, if you want something fancy about that - initial version used just key name, when it was unique.)
  4. Finally it generates Haskell or Elm code for the type.

Combination of robust union type system, and heuristic makes this system extremely reliable. Main test is QuickCheck-based generation of random JSON documents, and checking that they are all correctly parsed by resulting parser.

More details are described in Haskell.SG meetup presentation.

Other approaches: