| Safe Haskell | None |
|---|---|
| Language | Haskell98 |
Data.Yaml
Contents
Description
Provides a high-level interface for processing YAML files.
This module reuses most of the infrastructure from the aeson package.
This means that you can use all of the existing tools for JSON
processing for processing YAML files. As a result, much of the
documentation below mentions JSON; do not let that confuse you, it's
intentional.
For the most part, YAML content translates directly into JSON, and therefore there is very little data loss. If you need to deal with YAML more directly (e.g., directly deal with aliases), you should use the Text.Libyaml module instead.
For documentation on the aeson types, functions, classes, and
operators, please see the Data.Aeson module of the aeson package.
Look in the examples directory of the source repository for some initial pointers on how to use this library.
- data Value :: *
- data Parser a :: * -> *
- type Object = HashMap Text Value
- type Array = Vector Value
- data ParseException
- prettyPrintParseException :: ParseException -> String
- data YamlException
- data YamlMark = YamlMark {}
- 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
- parseMonad :: Monad m => (a -> Parser b) -> a -> m b
- parseEither :: (a -> Parser b) -> a -> Either String b
- parseMaybe :: (a -> Parser b) -> a -> Maybe b
- class ToJSON a where
- class FromJSON a where
- encode :: ToJSON a => a -> ByteString
- encodeFile :: ToJSON a => FilePath -> a -> IO ()
- decode :: FromJSON a => ByteString -> Maybe a
- decodeFile :: FromJSON a => FilePath -> IO (Maybe a)
- decodeEither :: FromJSON a => ByteString -> Either String a
- decodeEither' :: FromJSON a => ByteString -> Either ParseException a
- decodeFileEither :: FromJSON a => FilePath -> IO (Either ParseException a)
- decodeHelper :: FromJSON a => Source Parse Event -> IO (Either ParseException (Either String a))
Types
A JSON value represented as a Haskell value.
A JSON parser.
data ParseException Source #
Constructors
| NonScalarKey | |
| UnknownAlias | |
Fields | |
| UnexpectedEvent | |
| InvalidYaml (Maybe YamlException) | |
| AesonException String | |
| OtherParseException SomeException | |
| NonStringKeyAlias AnchorName Value | |
| CyclicIncludes | |
Instances
prettyPrintParseException :: ParseException -> String Source #
Alternative to show to display a ParseException on the screen.
Instead of displaying the data constructors applied to their arguments,
a more textual output is returned. For example, instead of printing:
InvalidYaml (Just (YamlParseException {yamlProblem = "did not find expected ',' or '}'", yamlContext = "while parsing a flow mapping", yamlProblemMark = YamlMark {yamlIndex = 42, yamlLine = 2, yamlColumn = 12}})))It looks more pleasant to print:
YAML parse exception at line 2, column 12, while parsing a flow mapping: did not find expected ',' or '}'
Since 0.8.11
data YamlException Source #
Constructors
| YamlException String | |
| YamlParseException | problem, context, index, position line, position column |
Fields | |
Instances
The pointer position
Constructors and accessors
(.:) :: 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"
Parsing
parseMonad :: Monad m => (a -> Parser b) -> a -> m b Source #
parseEither :: (a -> Parser b) -> a -> Either String b #
Classes
A type that can be converted to JSON.
An example type and instance:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instance ToJSON Coord where toJSON (Coord x y) =object["x".=x, "y".=y] toEncoding (Coord x y) =pairs("x".=x<>"y".=y)
Instead of manually writing your ToJSON 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 will probably be more efficient than the following two options:
- The compiler can provide a default generic implementation for
toJSON.
To use the second, simply add a deriving clause to your
datatype and declare a GenericToJSON instance for your datatype without giving
definitions for toJSON or toEncoding.
For example, the previous example can be simplified to a more minimal instance:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord = Coord { x :: Double, y :: Double } deriving Generic
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
Why do we provide an implementation for toEncoding here? The
toEncoding function is a relatively new addition to this class.
To allow users of older versions of this library to upgrade without
having to edit all of their instances or encounter surprising
incompatibilities, the default implementation of toEncoding uses
toJSON. This produces correct results, but since it performs an
intermediate conversion to a Value, it will be less efficient
than directly emitting an Encoding. Our one-liner definition of
toEncoding above bypasses the intermediate Value.
If DefaultSignatures doesn't give exactly the results you want,
you can customize the generic encoding with only a tiny amount of
effort, using genericToJSON and genericToEncoding with your
preferred Options:
instance ToJSON Coord where
toJSON = genericToJSON defaultOptions
toEncoding = genericToEncoding defaultOptions
Methods
Convert a Haskell value to a JSON-friendly intermediate type.
toEncoding :: a -> Encoding #
Encode a Haskell value as JSON.
The default implementation of this method creates an
intermediate Value using toJSON. This provides
source-level compatibility for people upgrading from older
versions of this library, but obviously offers no performance
advantage.
To benefit from direct encoding, you must provide an
implementation for this method. The easiest way to do so is by
having your types implement Generic using the DeriveGeneric
extension, and then have GHC generate a method body as follows.
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
toJSONList :: [a] -> Value #
toEncodingList :: [a] -> Encoding #
Instances
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:
emptyandmzerowork, but are terse and uninformativefailyields a custom error messagetypeMismatchproduces an informative message for cases when the value encountered is not of the expected type
An example type and instance:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instance FromJSON Coord where parseJSON (Objectv) = Coord<$>v.:"x"<*>v.:"y" -- We do not expect a non-Objectvalue here. -- We could usemzeroto fail, buttypeMismatch-- gives a much more informative error message. parseJSON invalid =typeMismatch"Coord" invalid
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 will probably be more efficient than the following two options:
- 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 GenericFromJSON 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
If DefaultSignatures doesn't give exactly the results you want,
you can customize the generic decoding with only a tiny amount of
effort, using genericParseJSON with your preferred Options:
instance FromJSON Coord where
parseJSON = genericParseJSON defaultOptions
Instances
Encoding/decoding
encode :: ToJSON a => a -> ByteString Source #
Better error information
decodeEither :: FromJSON a => ByteString -> Either String a Source #
decodeEither' :: FromJSON a => ByteString -> Either ParseException a Source #
More helpful version of decodeEither which returns the YamlException.
Since 0.8.3
decodeFileEither :: FromJSON a => FilePath -> IO (Either ParseException a) Source #
A version of decodeFile which should not throw runtime exceptions.
Since 0.8.4