| Safe Haskell | None |
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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.
Synopsis
- data Value
- data Parser a
- type Object = HashMap Text Value
- type Array = Vector Value
- data ParseException
- = NonScalarKey
- | UnknownAlias { }
- | UnexpectedEvent { }
- | InvalidYaml (Maybe YamlException)
- object :: [Pair] -> Value
- array :: [Value] -> Value
- (.=) :: ToJSON a => Text -> a -> Pair
- (.:) :: 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
- decodeHelper :: FromJSON a => Source Parse Event -> IO (Either ParseException (Either String a))
Types
data Value
A JSON value represented as a Haskell value.
data Parser a
A continuation-based parser type.
data ParseException Source
Constructors
Fields
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 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 bSource
parseMaybe :: (a -> Parser b) -> a -> Maybe b
Classes
class ToJSON a where
A type that can be converted to JSON.
An example type and instance:
{-# LANGUAGE OverloadedStrings #-}
data Coord { x :: Double, y :: Double }
instance ToJSON Coord where
toJSON (Coord x y) = object ["x" .= x, "y" .= y]
Note the use of the OverloadedStrings language extension which enables
Text values to be written as string literals.
Instead of manually writing your ToJSON instance, there are three 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:
- Data.Aeson.Generic provides a generic
toJSONfunction that accepts any type which is an instance ofData. - If your compiler has support for the
DeriveGenericandDefaultSignatureslanguage extensions (GHC 7.2 and newer),toJSONwill have a default generic implementation.
To use the latter option, simply add a deriving clause to your
datatype and declare a Generic ToJSON instance for your datatype without giving a
definition for toJSON.
For example the previous example can be simplified to just:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord { x :: Double, y :: Double } deriving Generic
instance ToJSON Coord
Instances
class FromJSON a where
A type that can be converted from JSON, with the possibility of failure.
When writing an instance, use empty , mzero, or fail to make a
conversion fail, e.g. if an Object is missing a required key, or
the value is of the wrong type.
An example type and instance:
{-# LANGUAGE OverloadedStrings #-}
data Coord { x :: Double, y :: Double }
instance FromJSON Coord where
parseJSON (Object v) = Coord <$>
v .: "x" <*>
v .: "y"
-- A non-Object value is of the wrong type, so use mzero to fail.
parseJSON _ = mzero
Note the use of the OverloadedStrings language extension which enables
Text values to be written as string literals.
Instead of manually writing your FromJSON instance, there are three 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:
- Data.Aeson.Generic provides a generic
fromJSONfunction that parses to any type which is an instance ofData. - If your compiler has support for the
DeriveGenericandDefaultSignatureslanguage extensions,parseJSONwill have a default generic implementation.
To use this, 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 { x :: Double, y :: Double } deriving Generic
instance FromJSON Coord
Instances
Encoding/decoding
encode :: ToJSON a => a -> ByteString Source
encodeFile :: ToJSON a => FilePath -> a -> IO ()Source
decode :: FromJSON a => ByteString -> Maybe aSource
Better error information
decodeEither :: FromJSON a => ByteString -> Either String aSource
More control over decoding
decodeHelper :: FromJSON a => Source Parse Event -> IO (Either ParseException (Either String a))Source