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spago install yaml-next

YAML to Data Type Usage

Assuming we have the following Point data type and YAML string...

data Point = Point Int Int

yamlPoint :: String
yamlPoint = """
X: 1
Y: 1

We can read a Point from the YAML by converting the YAML into JSON and then using purescript-argonaut's encoding functionality to get the type we need (specifically, purescript-argonaut-codecs functionality).

getPoint :: Either String Point
getPoint = case runExcept $ parseYAMLToJson yamlPoint of
  Left err -> Left "Could not parse yaml"
  Right json -> decodeJson json

instance pointJson :: DecodeJson Point where
  decodeJson s = do
    obj <- maybe (Left "Point is not an object.") Right (toObject s)
    x <- getField obj "X"
    y <- getField obj "Y"
    pure $ Point x y

Data Type to YAML Usage

YAML is represented with the following data type.

data YValue
    = YObject (M.Map String YValue)
    | YArray (Array YValue)
    | YString String
    | YNumber Number
    | YInt Int
    | YBoolean Boolean
    | YNull

To convert data into a YValue, create instances of the ToYAML class for your data types.

class ToYAML a where
    toYAML :: a -> YValue

For example to take a Point to YValue

import Data.YAML.Foreign.Encode (object, entry, class ToYAML)

instance pointToYAML :: ToYAML Point where
    toYAML (Point x y) =
            [ "X" `entry` x
            , "Y" `entry` y

You can find helper functions for converting basic types into YValue in the Data.YAML.Foreign.Encode module.

Finally, if you want to convert YValue into a String, you can use the printYAML function from Data.YAML.Foreign.Encode.

printYAML :: forall a. (ToYAML a) => a -> String


Using the previous code and the type classes we defined earlier, we can go full circle from a YAML string to a PureScript Data Type and back to a YAML string.

fullCircle :: String -> Either String String
fullCircle yamlString = (readPoint yamlString) >>= pure <<< printYAML


  1. Check out the repo
  2. Run npm install
  3. Make changes
  4. Test them by running npm test