aeson-better-errors-0.9.1.0: Better error messages when decoding JSON values.

Safe Haskell | None
Language | Haskell2010

Data.Aeson.BetterErrors

Contents

- The Parser type
- Basic parsers
- Traversing JSON
- Custom validations
  - Monadic validators
- Running parsers
  - Monadic parsers
- Errors
- Miscellaneous

Description

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.

Synopsis

- data ParseT err m a
- type Parse err a = ParseT err Identity a
- type Parse' a = Parse Void a
- mapError :: Functor m => (err -> err') -> ParseT err m a -> ParseT err' m a
- (.!) :: Functor m => ParseT err m a -> (err -> err') -> ParseT err' m a
- (<|>) :: Monad m => ParseT err m a -> ParseT err m a -> ParseT err m a
- asValue :: (Functor m, Monad m) => ParseT err m Value
- asText :: (Functor m, Monad m) => ParseT err m Text
- asString :: (Functor m, Monad m) => ParseT err m String
- asScientific :: (Functor m, Monad m) => ParseT err m Scientific
- asIntegral :: (Functor m, Monad m, Integral a) => ParseT err m a
- asRealFloat :: (Functor m, Monad m, RealFloat a) => ParseT err m a
- asBool :: (Functor m, Monad m) => ParseT err m Bool
- asNull :: (Functor m, Monad m) => ParseT err m ()
- asObject :: (Functor m, Monad m) => ParseT err m Object
- asArray :: (Functor m, Monad m) => ParseT err m Array
- perhaps :: (Functor m, Monad m) => ParseT err m a -> ParseT err m (Maybe a)
- key :: (Functor m, Monad m) => Text -> ParseT err m a -> ParseT err m a
- keyOrDefault :: (Functor m, Monad m) => Text -> a -> ParseT err m a -> ParseT err m a
- keyMay :: (Functor m, Monad m) => Text -> ParseT err m a -> ParseT err m (Maybe a)
- nth :: (Functor m, Monad m) => Int -> ParseT err m a -> ParseT err m a
- nthOrDefault :: (Functor m, Monad m) => Int -> a -> ParseT err m a -> ParseT err m a
- nthMay :: (Functor m, Monad m) => Int -> ParseT err m a -> ParseT err m (Maybe a)
- eachInArray :: (Functor m, Monad m) => ParseT err m a -> ParseT err m [a]
- forEachInObject :: (Functor m, Monad m) => (Text -> ParseT err m a) -> ParseT err m [a]
- eachInObject :: (Functor m, Monad m) => ParseT err m a -> ParseT err m [(Text, a)]
- eachInObjectWithKey :: (Functor m, Monad m) => (Text -> Either err k) -> ParseT err m a -> ParseT err m [(k, a)]
- withValue :: (Functor m, Monad m) => (Value -> Either err a) -> ParseT err m a
- withText :: (Functor m, Monad m) => (Text -> Either err a) -> ParseT err m a
- withString :: (Functor m, Monad m) => (String -> Either err a) -> ParseT err m a
- withScientific :: (Functor m, Monad m) => (Scientific -> Either err a) -> ParseT err m a
- withIntegral :: (Functor m, Monad m, Integral a) => (a -> Either err b) -> ParseT err m b
- withRealFloat :: (Functor m, Monad m, RealFloat a) => (a -> Either err b) -> ParseT err m b
- withBool :: (Functor m, Monad m) => (Bool -> Either err a) -> ParseT err m a
- withObject :: (Functor m, Monad m) => (Object -> Either err a) -> ParseT err m a
- withArray :: (Functor m, Monad m) => (Array -> Either err a) -> ParseT err m a
- throwCustomError :: (Functor m, Monad m) => err -> ParseT err m a
- withValueM :: (Functor m, Monad m) => (Value -> m (Either err a)) -> ParseT err m a
- withTextM :: (Functor m, Monad m) => (Text -> m (Either err a)) -> ParseT err m a
- withStringM :: (Functor m, Monad m) => (String -> m (Either err a)) -> ParseT err m a
- withScientificM :: (Functor m, Monad m) => (Scientific -> m (Either err a)) -> ParseT err m a
- withIntegralM :: (Functor m, Monad m, Integral a) => (a -> m (Either err b)) -> ParseT err m b
- withRealFloatM :: (Functor m, Monad m, RealFloat a) => (a -> m (Either err b)) -> ParseT err m b
- withBoolM :: (Functor m, Monad m) => (Bool -> m (Either err a)) -> ParseT err m a
- withObjectM :: (Functor m, Monad m) => (Object -> m (Either err a)) -> ParseT err m a
- withArrayM :: (Functor m, Monad m) => (Array -> m (Either err a)) -> ParseT err m a
- parse :: Parse err a -> ByteString -> Either (ParseError err) a
- parseStrict :: Parse err a -> ByteString -> Either (ParseError err) a
- parseValue :: Parse err a -> Value -> Either (ParseError err) a
- parseM :: Monad m => ParseT err m a -> ByteString -> m (Either (ParseError err) a)
- parseStrictM :: Monad m => ParseT err m a -> ByteString -> m (Either (ParseError err) a)
- parseValueM :: Monad m => ParseT err m a -> Value -> m (Either (ParseError err) a)
- data ParseError err
  - = InvalidJSON String
  - | BadSchema [PathPiece] (ErrorSpecifics err)
- type ParseError' = ParseError Void
- data PathPiece
  - = ObjectKey Text
  - | ArrayIndex Int
- data ErrorSpecifics err
  - = KeyMissing Text
  - | OutOfBounds Int
  - | WrongType JSONType Value
  - | ExpectedIntegral Double
  - | FromAeson String
  - | CustomError err
- type ErrorSpecifics' = ErrorSpecifics Void
- displayError :: (err -> Text) -> ParseError err -> [Text]
- displayError' :: ParseError' -> [Text]
- displayPath :: [PathPiece] -> Text
- displaySpecifics :: (err -> Text) -> ErrorSpecifics err -> [Text]
- displaySpecifics' :: ErrorSpecifics' -> [Text]
- toAesonParser :: (err -> Text) -> Parse err a -> Value -> Parser a
- toAesonParser' :: Parse' a -> Value -> Parser a
- fromAesonParser :: (Functor m, Monad m) => FromJSON a => ParseT e m a
- data JSONType
  - = TyObject
  - | TyArray
  - | TyString
  - | TyNumber
  - | TyBool
  - | TyNull
- jsonTypeOf :: Value -> JSONType

# The Parser type

data ParseT err m 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

The `m` parameter allows you to run the parser within an abitrary underlying Monad. You may want to use `Parse` in most cases instead, and all functions in this module work on either.

Instances

Monad m => MonadReader ParseReader (ParseT err m) # |
Methods ask :: ParseT err m ParseReader # local :: (ParseReader -> ParseReader) -> ParseT err m a -> ParseT err m a # reader :: (ParseReader -> a) -> ParseT err m a #
MonadTrans (ParseT err) # |
Methods lift :: Monad m => m a -> ParseT err m a #
Monad m => MonadError (ParseError err) (ParseT err m) # |
Methods throwError :: ParseError err -> ParseT err m a # catchError :: ParseT err m a -> (ParseError err -> ParseT err m a) -> ParseT err m a #
Monad m => Monad (ParseT err m) # |
Methods (>>=) :: ParseT err m a -> (a -> ParseT err m b) -> ParseT err m b # (>>) :: ParseT err m a -> ParseT err m b -> ParseT err m b # return :: a -> ParseT err m a # fail :: String -> ParseT err m a #
Functor m => Functor (ParseT err m) # |
Methods fmap :: (a -> b) -> ParseT err m a -> ParseT err m b # (<$) :: a -> ParseT err m b -> ParseT err m a #
Monad m => Applicative (ParseT err m) # |
Methods pure :: a -> ParseT err m a # (<*>) :: ParseT err m (a -> b) -> ParseT err m a -> ParseT err m b # (*>) :: ParseT err m a -> ParseT err m b -> ParseT err m b # (<*) :: ParseT err m a -> ParseT err m b -> ParseT err m a #

type Parse err a = ParseT err Identity a #

This is the standard version of `ParseT` over the `Identity` Monad, for running pure parsers.

type Parse' a = Parse Void a #

The type of parsers which never produce custom validation errors.

mapError :: Functor m => (err -> err') -> ParseT err m a -> ParseT err' m a #

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

(.!) :: Functor m => ParseT err m a -> (err -> err') -> ParseT err' m a #

An infix version of `mapError`.

(<|>) :: Monad m => ParseT err m a -> ParseT err m a -> ParseT err m a infixl 3 #

First try the left parser, if that fails try the right. | If both fail, the error will come from the right one.

# Basic parsers

asValue :: (Functor m, Monad m) => ParseT err m Value #

Return the current JSON `Value` as is. This does no error checking and thus always succeeds. You probably don't want this parser unless the JSON at the current part of your structure is truly arbitrary. You should prefer to use more specific parsers, like `asText` or `asIntegral`, where possible.

asText :: (Functor m, Monad m) => ParseT err m Text #

Parse a single JSON string as `Text`.

asString :: (Functor m, Monad m) => ParseT err m String #

Parse a single JSON string as a `String`.

asScientific :: (Functor m, Monad m) => ParseT err m Scientific #

Parse a single JSON number as a `Scientific`.

asIntegral :: (Functor m, Monad m, Integral a) => ParseT err m a #

Parse a single JSON number as any `Integral` type.

asRealFloat :: (Functor m, Monad m, RealFloat a) => ParseT err m a #

Parse a single JSON number as any `RealFloat` type.

asBool :: (Functor m, Monad m) => ParseT err m Bool #

Parse a single JSON boolean as a `Bool`.

asNull :: (Functor m, Monad m) => ParseT err m () #

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

asObject :: (Functor m, Monad m) => ParseT err m 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 :: (Functor m, Monad m) => ParseT err m 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 :: (Functor m, Monad m) => ParseT err m a -> ParseT err m (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 :: (Functor m, Monad m) => Text -> ParseT err m a -> ParseT err m a #

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

keyOrDefault :: (Functor m, Monad m) => Text -> a -> ParseT err m a -> ParseT err m 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 :: (Functor m, Monad m) => Text -> ParseT err m a -> ParseT err m (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 :: (Functor m, Monad m) => Int -> ParseT err m a -> ParseT err m a #

Take the nth value of the current array.

nthOrDefault :: (Functor m, Monad m) => Int -> a -> ParseT err m a -> ParseT err m a #

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

nthMay :: (Functor m, Monad m) => Int -> ParseT err m a -> ParseT err m (Maybe a) #

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

eachInArray :: (Functor m, Monad m) => ParseT err m a -> ParseT err m [a] #

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

forEachInObject :: (Functor m, Monad m) => (Text -> ParseT err m a) -> ParseT err m [a] #

Parse each property in an object with the given parser, given the key as an argument, and collect the results.

eachInObject :: (Functor m, Monad m) => ParseT err m a -> ParseT err m [(Text, a)] #

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

eachInObjectWithKey :: (Functor m, Monad m) => (Text -> Either err k) -> ParseT err m a -> ParseT err m [(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

withValue :: (Functor m, Monad m) => (Value -> Either err a) -> ParseT err m a #

Lifts a function attempting to validate an arbitrary JSON value into a parser. You should only use this if absolutely necessary; the other functions in this module will generally give better error reporting.

withText :: (Functor m, Monad m) => (Text -> Either err a) -> ParseT err m a #

withString :: (Functor m, Monad m) => (String -> Either err a) -> ParseT err m a #

withScientific :: (Functor m, Monad m) => (Scientific -> Either err a) -> ParseT err m a #

withIntegral :: (Functor m, Monad m, Integral a) => (a -> Either err b) -> ParseT err m b #

withRealFloat :: (Functor m, Monad m, RealFloat a) => (a -> Either err b) -> ParseT err m b #

withBool :: (Functor m, Monad m) => (Bool -> Either err a) -> ParseT err m a #

withObject :: (Functor m, Monad m) => (Object -> Either err a) -> ParseT err m a #

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

withArray :: (Functor m, Monad m) => (Array -> Either err a) -> ParseT err m a #

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

throwCustomError :: (Functor m, Monad m) => err -> ParseT err m a #

Throw a custom validation error.

## Monadic validators

withValueM :: (Functor m, Monad m) => (Value -> m (Either err a)) -> ParseT err m a #

withTextM :: (Functor m, Monad m) => (Text -> m (Either err a)) -> ParseT err m a #

withStringM :: (Functor m, Monad m) => (String -> m (Either err a)) -> ParseT err m a #

withScientificM :: (Functor m, Monad m) => (Scientific -> m (Either err a)) -> ParseT err m a #

withIntegralM :: (Functor m, Monad m, Integral a) => (a -> m (Either err b)) -> ParseT err m b #

withRealFloatM :: (Functor m, Monad m, RealFloat a) => (a -> m (Either err b)) -> ParseT err m b #

withBoolM :: (Functor m, Monad m) => (Bool -> m (Either err a)) -> ParseT err m a #

withObjectM :: (Functor m, Monad m) => (Object -> m (Either err a)) -> ParseT err m a #

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

withArrayM :: (Functor m, Monad m) => (Array -> m (Either err a)) -> ParseT err m 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`.

## Monadic parsers

parseM :: Monad m => ParseT err m a -> ByteString -> m (Either (ParseError err) a) #

Like `parse` but runs the parser on an arbitrary underlying Monad.

parseStrictM :: Monad m => ParseT err m a -> ByteString -> m (Either (ParseError err) a) #

Like `parseStrict` but runs the parser on an arbitrary underlying Monad.

parseValueM :: Monad m => ParseT err m a -> Value -> m (Either (ParseError err) a) #

Like `parseValue` but runs the parser on an arbitrary underlying Monad.

# 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 # |
Methods fmap :: (a -> b) -> ParseError a -> ParseError b # (<$) :: a -> ParseError b -> ParseError a #
Eq err => Eq (ParseError err) # |
Methods (==) :: ParseError err -> ParseError err -> Bool # (/=) :: ParseError err -> ParseError err -> Bool #
Show err => Show (ParseError err) # |
Methods showsPrec :: Int -> ParseError err -> ShowS # show :: ParseError err -> String # showList :: [ParseError err] -> ShowS #
Monad m => MonadError (ParseError err) (ParseT err m) # |
Methods throwError :: ParseError err -> ParseT err m a # catchError :: ParseT err m a -> (ParseError err -> ParseT err m a) -> ParseT err m a #

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 # |
Methods (==) :: PathPiece -> PathPiece -> Bool # (/=) :: PathPiece -> PathPiece -> Bool #
Ord PathPiece # |
Methods compare :: PathPiece -> PathPiece -> Ordering # (<) :: PathPiece -> PathPiece -> Bool # (<=) :: PathPiece -> PathPiece -> Bool # (>) :: PathPiece -> PathPiece -> Bool # (>=) :: PathPiece -> PathPiece -> Bool # max :: PathPiece -> PathPiece -> PathPiece # min :: PathPiece -> PathPiece -> PathPiece #
Show PathPiece # |
Methods showsPrec :: Int -> PathPiece -> ShowS # show :: PathPiece -> String # showList :: [PathPiece] -> ShowS #

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 # |
Methods fmap :: (a -> b) -> ErrorSpecifics a -> ErrorSpecifics b # (<$) :: a -> ErrorSpecifics b -> ErrorSpecifics a #
Eq err => Eq (ErrorSpecifics err) # |
Methods (==) :: ErrorSpecifics err -> ErrorSpecifics err -> Bool # (/=) :: ErrorSpecifics err -> ErrorSpecifics err -> Bool #
Show err => Show (ErrorSpecifics err) # |
Methods showsPrec :: Int -> ErrorSpecifics err -> ShowS # show :: ErrorSpecifics err -> String # showList :: [ErrorSpecifics err] -> ShowS #

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 :: (Functor m, Monad m) => FromJSON a => ParseT e m 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

Bounded JSONType # |
Methods minBound :: JSONType # maxBound :: JSONType #
Enum JSONType # |
Methods succ :: JSONType -> JSONType # pred :: JSONType -> JSONType # toEnum :: Int -> JSONType # fromEnum :: JSONType -> Int # enumFrom :: JSONType -> [JSONType] # enumFromThen :: JSONType -> JSONType -> [JSONType] # enumFromTo :: JSONType -> JSONType -> [JSONType] # enumFromThenTo :: JSONType -> JSONType -> JSONType -> [JSONType] #
Eq JSONType # |
Methods (==) :: JSONType -> JSONType -> Bool # (/=) :: JSONType -> JSONType -> Bool #
Ord JSONType # |
Methods compare :: JSONType -> JSONType -> Ordering # (<) :: JSONType -> JSONType -> Bool # (<=) :: JSONType -> JSONType -> Bool # (>) :: JSONType -> JSONType -> Bool # (>=) :: JSONType -> JSONType -> Bool # max :: JSONType -> JSONType -> JSONType # min :: JSONType -> JSONType -> JSONType #
Show JSONType # |
Methods showsPrec :: Int -> JSONType -> ShowS # show :: JSONType -> String # showList :: [JSONType] -> ShowS #

jsonTypeOf :: Value -> JSONType #

Get the type of a JSON value.