Safe Haskell | None |
---|---|
Language | Haskell98 |
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.
- 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
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- 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
data Value :: *
A JSON value represented as a Haskell value.
data Parser a :: * -> *
A JSON parser.
data ParseException Source
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
YamlException String | |
YamlParseException | problem, context, index, position line, position column |
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 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
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:
-- Allow ourselves to writeText
literals. {-# 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 Generic
ToJSON
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 } derivingGeneric
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
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
class FromJSON a where
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 uninformativefail
yields a custom error messagetypeMismatch
produces an informative message for cases when the value encountered is not of the expected type
An example type and instance:
-- Allow ourselves to writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instance FromJSON Coord where parseJSON (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
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 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
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
Nothing
Encoding/decoding
encode :: ToJSON a => a -> ByteString Source
encodeFile :: ToJSON a => FilePath -> a -> IO () Source
decode :: FromJSON a => ByteString -> Maybe a 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
More control over decoding
decodeHelper :: FromJSON a => Source Parse Event -> IO (Either ParseException (Either String a)) Source