Data.Yaml.TH

A QuasiQuoter for YAML.

Examples

{-# LANGUAGE QuasiQuotes #-}
import Data.Yaml.TH

value :: Value
value = [yamlQQ|
name: John Doe
age: 23
|]

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")

A JSON value represented as a Haskell value.

A JSON parser. N.B. This might not fit your usual understanding of "parser". Instead you might like to think of Parser as a "parse result", i.e. a parser to which the input has already been applied.

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

A JSON "array" (sequence).

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

Construct a new Value from a list of Values.

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.

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.

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:

• fail yields a custom error message: it is the recommended way of reporting a failure;

• empty (or mzero) is uninformative: use it when the error is meant to be caught by some (<|>);

• typeMismatch can be used to report a failure when the encountered value is not of the expected JSON type; unexpected is an appropriate alternative when more than one type may be expected, or to keep the expected type implicit.

prependFailure (or modifyFailure) add more information to a parser's error messages.

An example type and instance using typeMismatch and prependFailure:

-- 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 empty to fail, but typeMismatch
    -- gives a much more informative error message.
    parseJSON invalid    =
        prependFailure "parsing Coord failed, "
            (typeMismatch "Object" invalid)

For this common case of only being concerned with a single type of JSON value, the functions withObject, withScientific, 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:

instance FromJSON Coord where
    parseJSON = 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 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

or using the DerivingVia extension

deriving via Generically Coord instance FromJSON Coord

The default implementation will be equivalent to parseJSON = genericParseJSON defaultOptions; if you need different options, you can customize the generic decoding by defining:

customOptions = defaultOptions
                { fieldLabelModifier = map toUpper
                }

instance FromJSON Coord where
    parseJSON = genericParseJSON customOptions