The json-autotype package


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

See introduction on 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
Dependenciesaeson (==0.7.*), base (>=4.3 && <4.8), bytestring (>=0.9 && <0.11), containers (>=0.3 && <0.6), filepath (==1.3.*), hashable (==1.2.*), hflags (==0.4.*), lens (==4.1.*), MissingH (==1.2.*), mtl (==2.1.*), text (==1.1.*), uniplate (==1.6.*), unordered-containers (==0.2.*), vector (>=0.9 && <0.11) [details]
CopyrightCopyright by Michal J. Gajda '2014
AuthorMichal J. Gajda
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Source repositoryhead: git clone
UploadedSun Oct 26 23:23:38 UTC 2014 by MichalGajda


Maintainers' corner

For package maintainers and hackage trustees

Readme for json-autotype-


Takes a JSON format input, and generates automatic Haskell type declarations. Goal is to also generate parser, and pretty-printer instances.

It uses union type unification. Types inferred may be automatically trimmed and unified using attribute set matching.

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

Build Status Hackage

Details on official releases will be on Hackage


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

json-autotype input.json -o MyFormat.hs

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

runghc MyFormat.hs input.json

If everything is correct, then feel free to inspect the data structure generated automatically for you! The goal of this program is to make it easy for users of big JSON APIs to generate entries from example data.