Parsing Foreign Records fails

[Rembane]

Good morning,

I have a record in a Foreign object, and when I try to turn it into a Purescript record using read' I get the following error:

Error at property "channel": Type mismatch: expected String, found Undefined

The interesting thing is that it just works if I first convert the Foreign object to a string.

Why does it behave like this?

Here's a quite large minimum failing example:

Main.js:

exports.rcrd = "{\"channel\":\"Hej där!\",\"glorp\":27,\"event\":\"mystisk uppenbarelse\",\"payload\":\"Just det...\"}";

Main.purs:

module Main where

import Prelude

import Control.Monad.Except (runExcept)
import Data.Either (Either(..))
import Data.Foldable (foldMap)
import Effect (Effect)
import Effect.Console (log, logShow)
import Foreign (Foreign, readString, renderForeignError, unsafeFromForeign)
import Simple.JSON (read', readJSON')

foreign import rcrd :: Foreign

iPrintRecords :: { channel :: String, glorp :: Int, event :: String, payload :: Foreign } -> Effect Unit
iPrintRecords r = log r.channel *> logShow r.glorp

main :: Effect Unit
main = do
  log "Hi! :D"
  log $ unsafeFromForeign rcrd
  log "----------------------------------------------------------------------------------------"
  log "This fails:"
  case runExcept (read' rcrd) of
    Left es -> log (foldMap renderForeignError es)
    Right r -> iPrintRecords r

  log "\nThis works:"
  case runExcept (readJSON' =<< readString rcrd) of
    Left es -> log (foldMap renderForeignError es)
    Right r -> iPrintRecords r

[justinwoo]

read' works on Foreign values, but Foreign objects can be anything -- including being a String as you have defined. Your foreign import definition should actually define a JS object or your foreign import type declaration should be of type String.

[justinwoo]

If you look at the types here, you'll see how the second works, by using the JSON string:

readString :: Foreign -> F String

readJSON' :: forall a. ReadForeign a => String -> F a

bind :: forall a b m. Bind m => m a -> (a -> m b) -> m b

r1 :: F String
r1 = readString (rcrd :: Foreign)

r2 :: forall b. (String -> F b) -> F b
r2 = bind r1

r3 :: forall a. ReadJSON a => F a
r3 = r2 readJSON'