| Safe Haskell | Safe-Inferred |
|---|---|
| Language | Haskell2010 |
Data.Yaml.TH
Contents
Synopsis
- yamlQQ :: QuasiQuoter
- decodeFile :: forall a. (Lift a, FromJSON a) => FilePath -> Q (TExp a)
- data Value
- data Parser a
- type Object = KeyMap Value
- type Array = Vector Value
- object :: [Pair] -> Value
- array :: [Value] -> Value
- (.=) :: (KeyValue kv, ToJSON v) => Key -> v -> kv
- (.:) :: FromJSON a => Object -> Key -> Parser a
- (.:?) :: FromJSON a => Object -> Key -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- class FromJSON a where
Decoding
yamlQQ :: QuasiQuoter Source #
A QuasiQuoter for YAML.
Examples
Expand
{-# LANGUAGE QuasiQuotes #-}
import Data.Yaml.TH
value :: Value
value = [yamlQQ|
name: John Doe
age: 23
|]
Since: 0.8.28.0
decodeFile :: forall a. (Lift a, FromJSON a) => FilePath -> Q (TExp a) Source #
Decode a YAML file at compile time. Only available on GHC version 7.8.1
or higher.
Examples
Expand
{-# LANGUAGE TemplateHaskell #-}
config :: Config
config = $$(decodeFile "config.yaml")
Since: 0.8.19.0
Re-exports from Data.Yaml
A JSON value represented as a Haskell value.
Instances
Instances details
Since: aeson-2.0.3.0
Instance details
Defined in Data.Aeson.Types.Internal
Methods
coarbitrary :: Value -> Gen b -> Gen b #
Instance details
Defined in Data.Aeson.Types.ToJSON
Instance details
Defined in Data.Aeson.Types.Internal
Methods
gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Value -> c Value #
gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c Value #
dataTypeOf :: Value -> DataType #
dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c Value) #
dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c Value) #
gmapT :: (forall b. Data b => b -> b) -> Value -> Value #
gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Value -> r #
gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Value -> r #
gmapQ :: (forall d. Data d => d -> u) -> Value -> [u] #
gmapQi :: Int -> (forall d. Data d => d -> u) -> Value -> u #
gmapM :: Monad m => (forall d. Data d => d -> m d) -> Value -> m Value #
gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Value -> m Value #
gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Value -> m Value #
Since version 1.5.6.0 version object values are printed in lexicographic key order
>>>toJSON $ H.fromList [("a", True), ("z", False)]Object (fromList [("a",Bool True),("z",Bool False)])
>>>toJSON $ H.fromList [("z", False), ("a", True)]Object (fromList [("a",Bool True),("z",Bool False)])
The ordering is total, consistent with Eq instance.
However, nothing else about the ordering is specified,
and it may change from environment to environment and version to version
of either this package or its dependencies (hashable and 'unordered-containers').
Since: aeson-1.5.2.0
Instance details
Defined in Data.Aeson.Types.Internal
Instance details
Defined in Data.Aeson.Types.ToJSON
Instance details
Defined in Data.Aeson.Types.ToJSON
Instance details
Defined in Data.Aeson.Types.Internal
A JSON parser. N.B. This might not fit your usual understanding of
"parser". Instead you might like to think of Parser as a "parse result",
i.e. a parser to which the input has already been applied.
Instances
Instances details
(.:) :: FromJSON a => Object -> Key -> Parser a #
Retrieve the value associated with the given key of an Object .
The result is empty if the key is not present or the value cannot
be converted to the desired type.
This accessor is appropriate if the key and value must be present
in an object for it to be valid. If the key and value are
optional, use .:? instead.
(.:?) :: FromJSON a => Object -> Key -> Parser (Maybe a) #
Retrieve the value associated with the given key of an Object . The
result is Nothing if the key is not present or if its value is Null ,
or empty if the value cannot be converted to the desired type.
This accessor is most useful if the key and value can be absent
from an object without affecting its validity. If the key and
value are mandatory, use .: instead.
(.!=) :: Parser (Maybe a) -> a -> Parser a #
Helper for use in combination with .:? to provide default
values for optional JSON object fields.
This combinator is most useful if the key and value can be absent
from an object without affecting its validity and we know a default
value to assign in that case. If the key and value are mandatory,
use .: instead.
Example usage:
v1 <- o.:?"opt_field_with_dfl" .!= "default_val" v2 <- o.:"mandatory_field" v3 <- o.:?"opt_field2"
A type that can be converted from JSON, with the possibility of failure.
In many cases, you can get the compiler to generate parsing code for you (see below). To begin, let's cover writing an instance by hand.
There are various reasons a conversion could fail. For example, an
Object could be missing a required key, an Array could be of
the wrong size, or a value could be of an incompatible type.
The basic ways to signal a failed conversion are as follows:
failyields a custom error message: it is the recommended way of reporting a failure;empty(ormzero) is uninformative: use it when the error is meant to be caught by some(;<|>)typeMismatchcan be used to report a failure when the encountered value is not of the expected JSON type;unexpectedis an appropriate alternative when more than one type may be expected, or to keep the expected type implicit.
prependFailure (or modifyFailure ) add more information to a parser's
error messages.
An example type and instance using typeMismatch and prependFailure :
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSONCoord whereparseJSON(Objectv) = Coord<$>v.:"x"<*>v.:"y" -- We do not expect a non-Objectvalue here. -- We could useemptyto fail, buttypeMismatch-- gives a much more informative error message.parseJSONinvalid =prependFailure"parsing Coord failed, " (typeMismatch"Object" invalid)
For this common case of only being concerned with a single
type of JSON value, the functions withObject , withScientific , etc.
are provided. Their use is to be preferred when possible, since
they are more terse. Using withObject , we can rewrite the above instance
(assuming the same language extension and data type) as:
instanceFromJSONCoord whereparseJSON=withObject"Coord" $ \v -> Coord<$>v.:"x"<*>v.:"y"
Instead of manually writing your FromJSON instance, there are two options
to do it automatically:
- Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time. The generated instance is optimized for your type so it will probably be more efficient than the following option.
- The compiler can provide a default generic implementation for
parseJSON.
To use the second, simply add a deriving clause to your
datatype and declare a Generic FromJSON instance for your datatype without giving
a definition for parseJSON .
For example, the previous example can be simplified to just:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord = Coord { x :: Double, y :: Double } deriving Generic
instance FromJSON Coord
or using the DerivingVia extension
deriving viaGenericallyCoord instanceFromJSONCoord
The default implementation will be equivalent to
parseJSON = ; if you need different
options, you can customize the generic decoding by defining:genericParseJSON defaultOptions
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceFromJSONCoord whereparseJSON=genericParseJSONcustomOptions
Minimal complete definition
Nothing
Instances
Instances details
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser CalendarDiffDays #
parseJSONList :: Value -> Parser [CalendarDiffDays] #
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser QuarterOfYear #
parseJSONList :: Value -> Parser [QuarterOfYear] #
This instance includes a bounds check to prevent maliciously
large inputs to fill up the memory of the target system. You can
newtype Scientific and provide your own instance using
withScientific if you want to allow larger inputs.
This instance includes a bounds check to prevent maliciously
large inputs to fill up the memory of the target system. You can
newtype Scientific and provide your own instance using
withScientific if you want to allow larger inputs.
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser NominalDiffTime #
parseJSONList :: Value -> Parser [NominalDiffTime] #
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser CalendarDiffTime #
parseJSONList :: Value -> Parser [CalendarDiffTime] #
Supported string formats:
YYYY-MM-DD HH:MM Z
YYYY-MM-DD HH:MM:SS Z
YYYY-MM-DD HH:MM:SS.SSS Z
The first space may instead be a T, and the second space is
optional. The Z represents UTC. The Z may be replaced with a
time zone offset of the form +0000 or -08:00, where the first
two digits are hours, the : is optional and the second two digits
(also optional) are minutes.
This instance includes a bounds check to prevent maliciously
large inputs to fill up the memory of the target system. You can
newtype Scientific and provide your own instance using
withScientific if you want to allow larger inputs.
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser (WrappedMonoid a) #
parseJSONList :: Value -> Parser [WrappedMonoid a] #
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser (Generically a) #
parseJSONList :: Value -> Parser [Generically a] #
Instance details
Defined in Data.Aeson.Types.FromJSON
Methods
parseJSON :: Value -> Parser (SmallArray a) #
parseJSONList :: Value -> Parser [SmallArray a] #
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
This instance includes a bounds check to prevent maliciously
large inputs to fill up the memory of the target system. You can
newtype Scientific and provide your own instance using
withScientific if you want to allow larger inputs.
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON
Instance details
Defined in Data.Aeson.Types.FromJSON