Safe Haskell | None |
---|---|
Language | Haskell98 |
- decodeFile :: forall a. (Lift a, FromJSON a) => FilePath -> Q (TExp a)
- data Value :: *
- data Parser a :: * -> *
- type Object = HashMap Text Value
- type Array = Vector Value
- object :: [Pair] -> Value
- array :: [Value] -> Value
- (.=) :: KeyValue kv => forall v. ToJSON v => Text -> v -> kv
- (.:) :: FromJSON a => Object -> Text -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- class FromJSON a where
Decoding
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.
Eq Value | |
Data Value | |
Read Value | |
Show Value | |
IsString Value | |
Generic Value | |
Lift Value | |
FromString Encoding | |
FromString Value | |
Hashable Value | |
ToJSON Value | |
KeyValue Pair | |
FromJSON Value | |
NFData Value | |
FromPairs Encoding Series | |
GKeyValue Encoding Series | |
GToJSON Encoding arity (U1 *) | |
GToJSON Value arity (U1 *) | |
ToJSON1 f => GToJSON Encoding One (Rec1 * f) | |
ToJSON1 f => GToJSON Value One (Rec1 * f) | |
ToJSON a => GToJSON Encoding arity (K1 * i a) | |
(EncodeProduct arity a, EncodeProduct arity b) => GToJSON Encoding arity ((:*:) * a b) | |
ToJSON a => GToJSON Value arity (K1 * i a) | |
(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) | |
ToJSON v => GKeyValue v (DList Pair) | |
(GToJSON Encoding arity a, ConsToJSON Encoding arity a, Constructor Meta c) => SumToJSON' * TwoElemArray Encoding arity (C1 * c a) | |
(GToJSON Value arity a, ConsToJSON Value arity a, Constructor Meta c) => SumToJSON' * TwoElemArray Value arity (C1 * c a) | |
type Rep Value | |
A JSON parser.
(.:) :: FromJSON a => Object -> Text -> 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 -> Text -> 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:
empty
andmzero
work, but are terse and uninformative;fail
yields a custom error message;typeMismatch
produces an informative message for cases when the value encountered is not of the expected type.
An example type and instance using typeMismatch
:
-- 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 usemzero
to fail, buttypeMismatch
-- gives a much more informative error message.parseJSON
invalid =typeMismatch
"Coord" invalid
For this common case of only being concerned with a single
type of JSON value, the functions withObject
, withNumber
, 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
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
FromJSON Bool | |
FromJSON Char | |
FromJSON Double | |
FromJSON Float | |
FromJSON Int | |
FromJSON Int8 | |
FromJSON Int16 | |
FromJSON Int32 | |
FromJSON Int64 | |
FromJSON Integer | WARNING: Only parse Integers from trusted input since an
attacker could easily fill up the memory of the target system by
specifying a scientific number with a big exponent like
|
FromJSON Natural | |
FromJSON Ordering | |
FromJSON Word | |
FromJSON Word8 | |
FromJSON Word16 | |
FromJSON Word32 | |
FromJSON Word64 | |
FromJSON () | |
FromJSON Scientific | |
FromJSON Number | |
FromJSON Text | |
FromJSON UTCTime | |
FromJSON Value | |
FromJSON DotNetTime | |
FromJSON Text | |
FromJSON Version | |
FromJSON CTime | |
FromJSON IntSet | |
FromJSON ZonedTime | Supported string formats:
The first space may instead be a |
FromJSON LocalTime | |
FromJSON TimeOfDay | |
FromJSON NominalDiffTime | WARNING: Only parse lengths of time from trusted input
since an attacker could easily fill up the memory of the target
system by specifying a scientific number with a big exponent like
|
FromJSON DiffTime | WARNING: Only parse lengths of time from trusted input
since an attacker could easily fill up the memory of the target
system by specifying a scientific number with a big exponent like
|
FromJSON Day | |
FromJSON UUID | |
FromJSON a => FromJSON [a] | |
FromJSON a => FromJSON (Maybe a) | |
(FromJSON a, Integral a) => FromJSON (Ratio a) | |
HasResolution a => FromJSON (Fixed a) | WARNING: Only parse fixed-precision numbers from trusted input
since an attacker could easily fill up the memory of the target
system by specifying a scientific number with a big exponent like
|
FromJSON a => FromJSON (Min a) | |
FromJSON a => FromJSON (Max a) | |
FromJSON a => FromJSON (First a) | |
FromJSON a => FromJSON (Last a) | |
FromJSON a => FromJSON (WrappedMonoid a) | |
FromJSON a => FromJSON (Option a) | |
FromJSON a => FromJSON (NonEmpty a) | |
FromJSON a => FromJSON (Identity a) | |
FromJSON a => FromJSON (Dual a) | |
FromJSON a => FromJSON (First a) | |
FromJSON a => FromJSON (Last a) | |
FromJSON a => FromJSON (IntMap a) | |
FromJSON v => FromJSON (Tree v) | |
FromJSON a => FromJSON (Seq a) | |
(Ord a, FromJSON a) => FromJSON (Set a) | |
FromJSON a => FromJSON (DList a) | |
(Eq a, Hashable a, FromJSON a) => FromJSON (HashSet a) | |
(Vector Vector a, FromJSON a) => FromJSON (Vector a) | |
(Storable a, FromJSON a) => FromJSON (Vector a) | |
(Prim a, FromJSON a) => FromJSON (Vector a) | |
FromJSON a => FromJSON (Vector a) | |
(FromJSON a, FromJSON b) => FromJSON (Either a b) | |
(FromJSON a, FromJSON b) => FromJSON (a, b) | |
(FromJSON v, FromJSONKey k, Eq k, Hashable k) => FromJSON (HashMap k v) | |
(FromJSONKey k, Ord k, FromJSON v) => FromJSON (Map k v) | |
FromJSON (Proxy k a) | |
(FromJSON a, FromJSON b, FromJSON c) => FromJSON (a, b, c) | |
FromJSON a => FromJSON (Const k a b) | |
FromJSON b => FromJSON (Tagged k a b) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d) => FromJSON (a, b, c, d) | |
(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Product * f g a) | |
(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Sum * f g a) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e) => FromJSON (a, b, c, d, e) | |
(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Compose * * f g a) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f) => FromJSON (a, b, c, d, e, f) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g) => FromJSON (a, b, c, d, e, f, g) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h) => FromJSON (a, b, c, d, e, f, g, h) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i) => FromJSON (a, b, c, d, e, f, g, h, i) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j) => FromJSON (a, b, c, d, e, f, g, h, i, j) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k) => FromJSON (a, b, c, d, e, f, g, h, i, j, k) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n) | |
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n, FromJSON o) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) | |