Safe Haskell | None |
---|---|
Language | Haskell2010 |
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
{-# 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
{-# 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
Eq Value | |
Data Value | |
Defined in Data.Aeson.Types.Internal 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 # | |
Ord Value | The ordering is total, consistent with Since: aeson-1.5.2.0 |
Read Value | |
Show Value | Since version 1.5.6.0 version object values are printed in lexicographic key order
|
IsString Value | |
Defined in Data.Aeson.Types.Internal fromString :: String -> Value # | |
Generic Value | |
Function Value | Since: aeson-2.0.3.0 |
Arbitrary Value | Since: aeson-2.0.3.0 |
CoArbitrary Value | Since: aeson-2.0.3.0 |
Defined in Data.Aeson.Types.Internal coarbitrary :: Value -> Gen b -> Gen b # | |
Hashable Value | |
Defined in Data.Aeson.Types.Internal | |
ToJSON Value | |
Defined in Data.Aeson.Types.ToJSON | |
FromJSON Value | |
NFData Value | |
Defined in Data.Aeson.Types.Internal | |
FromString Encoding | |
Defined in Data.Aeson.Types.ToJSON fromString :: String -> Encoding | |
FromString Value | |
Defined in Data.Aeson.Types.ToJSON fromString :: String -> Value | |
Lift Value | Since: aeson-0.11.0.0 |
GToJSON' Encoding arity (U1 :: Type -> Type) | |
GToJSON' Value arity (V1 :: Type -> Type) | |
GToJSON' Value arity (U1 :: Type -> Type) | |
ToJSON1 f => GToJSON' Encoding One (Rec1 f) | |
ToJSON1 f => GToJSON' Value One (Rec1 f) | |
ToJSON a => GToJSON' Encoding arity (K1 i a :: Type -> Type) | |
(EncodeProduct arity a, EncodeProduct arity b) => GToJSON' Encoding arity (a :*: b) | |
ToJSON a => GToJSON' Value arity (K1 i a :: Type -> Type) | |
(WriteProduct arity a, WriteProduct arity b, ProductSize a, ProductSize b) => GToJSON' Value arity (a :*: b) | |
(ToJSON1 f, GToJSON' Encoding One g) => GToJSON' Encoding One (f :.: g) | |
(ToJSON1 f, GToJSON' Value One g) => GToJSON' Value One (f :.: g) | |
FromPairs Value (DList Pair) | |
Defined in Data.Aeson.Types.ToJSON | |
v ~ Value => KeyValuePair v (DList Pair) | |
Defined in Data.Aeson.Types.ToJSON | |
(GToJSON' Encoding arity a, ConsToJSON Encoding arity a, Constructor c) => SumToJSON' TwoElemArray Encoding arity (C1 c a) | |
Defined in Data.Aeson.Types.ToJSON | |
(GToJSON' Value arity a, ConsToJSON Value arity a, Constructor c) => SumToJSON' TwoElemArray Value arity (C1 c a) | |
Defined in Data.Aeson.Types.ToJSON | |
type Rep Value | |
Defined in Data.Aeson.Types.Internal type Rep Value = D1 ('MetaData "Value" "Data.Aeson.Types.Internal" "aeson-2.1.0.0-GtLAVgwlFqYH8dPM6JskwV" 'False) ((C1 ('MetaCons "Object" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Object)) :+: (C1 ('MetaCons "Array" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Array)) :+: C1 ('MetaCons "String" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Text)))) :+: (C1 ('MetaCons "Number" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Scientific)) :+: (C1 ('MetaCons "Bool" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Bool)) :+: C1 ('MetaCons "Null" 'PrefixI 'False) (U1 :: Type -> Type)))) |
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
Monad Parser | |
Functor Parser | |
MonadFix Parser | Since: aeson-2.1.0.0 |
Defined in Data.Aeson.Types.Internal | |
MonadFail Parser | |
Defined in Data.Aeson.Types.Internal | |
Applicative Parser | |
Alternative Parser | |
MonadPlus Parser | |
Semigroup (Parser a) | |
Monoid (Parser a) | |
(.:) :: 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:
fail
yields 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(
;<|>
)typeMismatch
can be used to report a failure when the encountered value is not of the expected JSON type;unexpected
is 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 writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSON
Coord whereparseJSON
(Object
v) = Coord<$>
v.:
"x"<*>
v.:
"y" -- We do not expect a non-Object
value here. -- We could useempty
to fail, buttypeMismatch
-- gives a much more informative error message.parseJSON
invalid =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:
instanceFromJSON
Coord 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 } derivingGeneric
instanceFromJSON
Coord
or using the DerivingVia extension
deriving viaGenerically
Coord instanceFromJSON
Coord
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
=map
toUpper
} instanceFromJSON
Coord whereparseJSON
=genericParseJSON
customOptions
Nothing