Data.Yaml

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.

Types

data Value :: *

A JSON value represented as a Haskell value.

Constructors

Object !Object

Array !Array

String !Text

Number !Scientific

Bool !Bool

Null

Instances

Eq Value

Data Value

Read Value

Show Value

IsString Value

NFData Value

Hashable Value

data Parser a :: * -> *

A JSON parser.

Instances

Monad Parser

Functor Parser

Applicative Parser

Alternative Parser

MonadPlus Parser

Monoid (Parser a)

type Object = HashMap Text Value

A JSON "object" (key/value map).

type Array = Vector Value

A JSON "array" (sequence).

data ParseException

Constructors

NonScalarKey

UnknownAlias

Fields

_anchorName :: AnchorName

UnexpectedEvent

Fields

_received :: Maybe Event

_expected :: Maybe Event

InvalidYaml (Maybe YamlException)

AesonException String

OtherParseException SomeException

NonStringKeyAlias AnchorName Value

CyclicIncludes

Instances

Show ParseException

Exception ParseException

prettyPrintParseException :: ParseException -> String

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

Constructors

YamlException String

YamlParseException

problem, context, index, position line, position column

Fields

yamlProblem :: String

yamlContext :: String

yamlProblemMark :: YamlMark

Instances

Show YamlException

Exception YamlException

data YamlMark

The pointer position

Constructors

YamlMark

Fields

yamlIndex :: Int

yamlLine :: Int

yamlColumn :: Int

Instances

Show YamlMark

Constructors and accessors

object :: [Pair] -> Value

Create a Value from a list of name/value Pairs. If duplicate keys arise, earlier keys and their associated values win.

array :: [Value] -> Value

(.=) :: KeyValue kv => forall v. ToJSON v => Text -> v -> kv

(.:) :: 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

parseEither :: (a -> Parser b) -> a -> Either String b

Run a Parser with an Either result type. If the parse fails, the Left payload will contain an error message.

parseMaybe :: (a -> Parser b) -> a -> Maybe b

Run a Parser with a Maybe result type.

Classes

class ToJSON a where

A type that can be converted to JSON.

An example type and instance:

-- Allow ourselves to write Text 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 Generic clause to your datatype and declare a 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 } 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

Minimal complete definition

toJSON

Methods

toJSON :: a -> Value

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 and mzero 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:

-- Allow ourselves to write Text 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 use mzero to fail, but typeMismatch
  -- 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 Generic clause to your datatype and declare a 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

Minimal complete definition

Nothing

Methods

parseJSON :: Value -> Parser a

Encoding/decoding

encode :: ToJSON a => a -> ByteString

encodeFile :: ToJSON a => FilePath -> a -> IO ()

decode :: FromJSON a => ByteString -> Maybe a

decodeFile :: FromJSON a => FilePath -> IO (Maybe a)

Better error information

decodeEither :: FromJSON a => ByteString -> Either String a

decodeEither' :: FromJSON a => ByteString -> Either ParseException a

More helpful version of decodeEither which returns the YamlException.

Since 0.8.3

decodeFileEither :: FromJSON a => FilePath -> IO (Either ParseException a)

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))