Vfrdtyky

I am using purescript-agronauth library to manually encode and decode the following types to json and back. But the following does not work

data Attributes 

   = TextAlignment TextAlign

   | TextScale String

   | LineHeight String

instance encodeAttributes :: EncodeJson Attributes where

encodeJson r = 

    case r of

        (TextAlignment p) -> 

            "key" := (fromString "text-align")

            ~> "value" := p

        (TextScale p) -> 

            "key" := (fromString "font-size")

            ~> "value" := p

        (LineHeight p) -> 

            "key" := (fromString "line-height")

            ~> "value" := p

instance decodeElementAttributes :: DecodeJson ElementAttributes where

  decodeJson json = do

    obj <- decodeJson json

    key <- getField obj "key"

    value <- getField obj "value"

    case key of

        "text-align" -> Right $ TextAlignment value 

        "font-size" -> Right $ TextScale value

        "line-height" -> Right $ LineHeight value

        _ -> Left "Unkown element property"



data TextAlign

    = LeftAlign

    | RightAlign

    | CenterAlign

    | Justify

instance encodeTextAlign :: EncodeJson TextAlign where

    encodeJson r = 

        case r of

            LeftAlign -> fromString "left"

            RightAlign -> fromString "right"

            CenterAlign -> fromString "center"

            Justify -> fromString "justify"

instance decodeTextAlign :: DecodeJson TextAlign where

    decodeJson obj = do

       case toString obj of

         Just "left" -> Right LeftAlign

         Just "right" -> Right RightAlign

         Just "center" -> Right CenterAlign

         Just "justify" -> Right Justify

         Just _ -> Left "Unknown alignment"

         Nothing -> Left "Unknown alignment"

This gives the following error

  Could not match type



    TextAlign



  with type



    String





while checking that type t0

  is at least as general as type String

while checking that expression value

  has type String

in value declaration decodeElementAttributes



where t0 is an unknown type

Basically, I would like to know what would be the proper way to decode a Sum type like Attributes in this case

(..) But the following does not work

TLDR; This should work:

instance decodeElementAttributes :: DecodeJson Attributes where

  decodeJson json = do

    obj <- decodeJson json

    key <- getField obj "key"

    case key of

      "text-align" -> TextAlignment <$> getField obj "value"

      "font-size" -> TextScale <$> getField obj "value"

      "line-height" -> LineHeight <$> getField obj "value"

      _ -> Left "Unkown element property"

Let's jump into compiler shoes for a moment and try to infer the value type.
In the monadic block in decodeJson there is a call to getField:

value <- getField obj "value"

getField is polymorphic on its return type:

getField :: forall a. DecodeJson a => Object Json -> String -> Either String a

So from this call alone we are not able to guess the type of the value. We need some more information / context.

But luckily few lines below we can find a usage of value which gives us some solution:

"text-align" -> Right $ TextAlignment value

So for sure our value has to be typed as TextAlign because TextAlignment constructor expects such a parameter.

But wait... just line underneath there is another usage of value:

"font-size" -> Right $ TextScale value

and here we have a problem because this tells us that value has type String and... TextAlign at the same time...
We have no other choice than tell the World about our discovery:

Could not match type



 TextAlign



with type



  String

Basically, I would like to know what would be the proper way to decode a Sum type like Attributes in this case

• Your approach is OK to me. It gives you full control over coding / decoding process. It can be error prone though...




• You can give a try and use fully generic solution like purescript-argounaut-generic.




• You can also try different generic approach and use purescript-simple-json. I wasn't able to find example for generic sum handling - here is only enum like type encoded / decoded: https://www.reddit.com/r/purescript/comments/7b5y7q/some_extra_examples_of_simplejson_usage/. You can always ask Justin Woo for suggestions - he is really responsive author :-)




• I haven't used purescript-codec-argonaut yet but it should help you minimize some duplication related to coding and decoding definitions. With this approach you are still responsible for defining everything by hand I think.




• Here is interesting post, which I think is mostly relevant if you do not have PureScript on both ends of the wire (consumer and producer) by @garyb about downsides of generic codecs: http://code.slipthrough.net/2018/03/13/thoughts-on-typeclass-codecs/

Do you have PureScript on both ends of the wire?