Data.Aeson.BetterErrors

A module for decoding JSON, and generating good error messages. Note, however, that this package only deals with generating good error messages after the JSON has been parsed into a Value - unfortunately, invalid JSON will still produce poor error messages.

See http://harry.garrood.me/blog/aeson-better-errors/ for a tutorial.

Any kind of feedback is very welcome: suggestions for a better designed API, bug reports, whatever - the best place for it is probably the GitHub issue tracker: https://github.com/hdgarrood/aeson-better-errors/issues.

The Parser type

data Parse err a

The type of parsers: things which consume JSON values and produce either detailed errors or successfully parsed values (of other types).

The err type parameter is for custom validation errors; for parsers that don't produce any custom validation errors, I recommend you just stick a type variable in for full generality:

    asTuple :: Parse e (Int, Int)
    asTuple = (,) <$> nth 0 asIntegral <*> nth 1 asIntegral

Instances

MonadReader ParseReader (Parse err)

Monad (Parse err)

Functor (Parse err)

Applicative (Parse err)

MonadError (ParseError err) (Parse err)

type Parse' = Parse Void

The type of parsers which never produce custom validation errors.

mapError :: (err -> err') -> Parse err a -> Parse err' a

Transform the error of a parser according to the given function.

(.!) :: Parse err a -> (err -> err') -> Parse err' a

An infix version of mapError.

Basic parsers

asText :: Parse err Text

Parse a single JSON string as Text.

asString :: Parse err String

Parse a single JSON string as a String.

asScientific :: Parse err Scientific

Parse a single JSON number as a Scientific.

asIntegral :: Integral a => Parse err a

Parse a single JSON number as any Integral type.

asRealFloat :: RealFloat a => Parse err a

Parse a single JSON number as any RealFloat type.

asBool :: Parse err Bool

Parse a single JSON boolean as a Bool.

asNull :: Parse err ()

Parse a single JSON null value. Useful if you want to throw an error in the case where something is not null.

asObject :: Parse err Object

Parse a JSON object, as an Object. You should prefer functions like eachInObject where possible, since they will usually generate better error messages.

asArray :: Parse err Array

Parse a JSON array, as an Array. You should prefer functions like eachInArray where possible, since they will usually generate better error messages.

Traversing JSON

perhaps :: Parse err a -> Parse err (Maybe a)

Given a parser, transform it into a parser which returns Nothing when supplied with a JSON null, and otherwise, attempts to parse with the original parser; if this succeeds, the result becomes a Just value.

key :: Text -> Parse err a -> Parse err a

Take the value corresponding to a given key in the current object.

keyOrDefault :: Text -> a -> Parse err a -> Parse err a

Take the value corresponding to a given key in the current object, or if no property exists with that key, use the supplied default.

keyMay :: Text -> Parse err a -> Parse err (Maybe a)

Take the value corresponding to a given key in the current object, or if no property exists with that key, return Nothing .

nth :: Int -> Parse err a -> Parse err a

Take the nth value of the current array.

nthOrDefault :: Int -> a -> Parse err a -> Parse err a

Take the nth value of the current array, or if no value exists with that index, use the supplied default.

nthMay :: Int -> Parse err a -> Parse err (Maybe a)

Take the nth value of the current array, or if no value exists with that index, return Nothing.

eachInArray :: Parse err a -> Parse err [a]

Attempt to parse each value in the array with the given parser, and collect the results.

eachInObject :: Parse err a -> Parse err [(Text, a)]

Attempt to parse each property value in the object with the given parser, and collect the results.

eachInObjectWithKey :: (Text -> Either err k) -> Parse err a -> Parse err [(k, a)]

Attempt to parse each property in the object: parse the key with the given validation function, parse the value with the given parser, and collect the results.

Custom validations

withText :: (Text -> Either err a) -> Parse err a

withString :: (String -> Either err a) -> Parse err a

withScientific :: (Scientific -> Either err a) -> Parse err a

withIntegral :: Integral a => (a -> Either err b) -> Parse err b

withRealFloat :: RealFloat a => (a -> Either err b) -> Parse err b

withBool :: (Bool -> Either err a) -> Parse err a

withObject :: (Object -> Either err a) -> Parse err a

Prefer to use functions like key or eachInObject to this one where possible, as they will generate better error messages.

withArray :: (Array -> Either err a) -> Parse err a

Prefer to use functions like nth or eachInArray to this one where possible, as they will generate better error messages.

Running parsers

parse :: Parse err a -> ByteString -> Either (ParseError err) a

Run a parser with a lazy ByteString containing JSON data. Note that the normal caveat applies: the JSON supplied must contain either an object or an array for this to work.

parseStrict :: Parse err a -> ByteString -> Either (ParseError err) a

Run a parser with a strict ByteString containing JSON data. Note that the normal caveat applies: the JSON supplied must contain either an object or an array for this to work.

parseValue :: Parse err a -> Value -> Either (ParseError err) a

Run a parser with a pre-parsed JSON Value.

Errors

data ParseError err

A value indicating that the JSON could not be decoded successfully.

Constructors

InvalidJSON String

Indicates a syntax error in the JSON string. Unfortunately, in this case, Aeson's errors are not very helpful.

BadSchema [PathPiece] (ErrorSpecifics err)

Indicates a decoding error; the input was parsed as JSON successfully, but a value of the required type could not be constructed, perhaps because of a missing key or type mismatch.

Instances

Functor ParseError

Eq err => Eq (ParseError err)

Show err => Show (ParseError err)

MonadError (ParseError err) (Parse err)

type ParseError' = ParseError Void

The type of parse errors which never involve custom validation errors.

data PathPiece

A piece of a path leading to a specific part of the JSON data. Internally, a list of these is maintained as the parser traverses the JSON data. This list is included in the error if one occurs.

Constructors

ObjectKey Text

ArrayIndex Int

Instances

Eq PathPiece

Ord PathPiece

Show PathPiece

data ErrorSpecifics err

Detailed information in the case where a value could be parsed as JSON, but a value of the required type could not be constructed from it, for some reason.

Constructors

KeyMissing Text

OutOfBounds Int

WrongType JSONType Value

Expected type, actual value

ExpectedIntegral Double

FromAeson String

An error arising inside a FromJSON instance.

CustomError err

Instances

Functor ErrorSpecifics

Eq err => Eq (ErrorSpecifics err)

Show err => Show (ErrorSpecifics err)

type ErrorSpecifics' = ErrorSpecifics Void

The type of error specifics which never involve custom validation errors.

displayError :: (err -> Text) -> ParseError err -> [Text]

Turn a ParseError into a human-readable list of Text values. They will be in a sensible order. For example, you can feed the result to mapM putStrLn, or unlines.

displayError' :: ParseError' -> [Text]

A version of displayError for parsers which do not produce custom validation errors.

displayPath :: [PathPiece] -> Text

displaySpecifics :: (err -> Text) -> ErrorSpecifics err -> [Text]

displaySpecifics' :: ErrorSpecifics' -> [Text]

A version of displaySpecifics for parsers which do not produce custom validation errors.

Miscellaneous

toAesonParser :: (err -> Text) -> Parse err a -> Value -> Parser a

This function is useful when you have a Parse err a and you want to obtain an instance for FromJSON a. Simply define:

   parseJSON = toAesonParser showMyCustomError myParser

toAesonParser' :: Parse' a -> Value -> Parser a

Take a parser which never produces custom validation errors and turn it into an Aeson parser. Note that in this case, there is no need to provide a display function.

fromAesonParser :: FromJSON a => Parse e a

Create a parser for any type, using its FromJSON instance. Generally, you should prefer to write parsers using the other functions in this module; key, asString, etc, since they will usually generate better error messages. However this function is also useful occasionally.

data JSONType

An enumeration of the different types that JSON values may take.

Constructors

TyObject

TyArray

TyString

TyNumber

TyBool

TyNull

Instances

Eq JSONType

Ord JSONType

Show JSONType

jsonTypeOf :: Value -> JSONType

Get the type of a JSON value.