Why does using try in conjuntion with affjax's XHRError result in a mismatch on the Error type? - purescript

Question

I'd like to be able to catch communications errors when using affjax in purescript. I try it like this:
module Main where
import Prelude
import Affjax as AX
import Affjax.ResponseFormat as ResponseFormat
import Data.Argonaut.Core (stringify, fromString)
import Data.Either (Either(..))
import Data.HTTP.Method (Method(..))
import Effect.Aff (launchAff)
import Effect.Class.Console (log)
import Effect.Exception(message)
import Control.Monad.Error.Class (try)
main = void $ launchAff $ do
resultEi <- try $ AX.get ResponseFormat.string "/api"
let result = case resultEi of
Left err -> Left $ AX.XHRError err
rightRes#(Right _)-> rightRes
case result of
Left err -> log $ "GET /api response failed to decode: " <> AX.printError err
Right response -> log $ "GET /api response: " <> response.body
This results in the somewhat confusing error, particularly so because I'm not really sure what would be here other than Effect.Exception.Error since both Aff and XHRError seem to use this Error datatype.
Error 1 of 1
Could not match type
Error
with type
Error
while trying to match type Either Error
with type Either Error
while checking that expression rightRes
has type t0
in value declaration main
where t0 is an unknown type

Answer 1

The branches of your case have different types.
The Left branch has type Either Effect.Exception.Error a, because that's how you construct it.
But your Right branch has the same type as rightRes, because that's what you're returning from it, and that type is Either Affjax.Error (Response _).
So the types don't match.
To make them match, you have to unwrap rightRes and rewrap it again:
let result = case resultEi of
Left err -> Left $ AX.XHRError err
Right res -> Right res
Yes, it will have the exact same content, but the type will be different.
But if your intent is just to change the type of the error, then I'd recommend lmap from Bifunctor:
let result = lmap AX.XHRError resultEi