I have:
> data Reference = Ref [String] String
> data ReferenceError = RefError
> { expectedType :: String -- type of element we were looking for (e.g. "type","package")
> , pointOfError :: Reference -- path to deepest parent element not found in path
> }
> type ReferenceMonad = Either ReferenceError
I want to write functions that use "Either ReferenceError a" as as the error monad, instead of the more common "Either String a". In particular, I want to use be able to write:
> type Model = [(String,Type)] > findType :: Model -> Reference -> ReferenceMonad Type > findType m -> r@(Ref [] name) = case lookup ((==name) . nameOf) m of > Nothing -> throwError r > Just x -> return x
I know that I could make this work by making ReferenceError an instance of the Error class, but I cannot provide meaningful implementations of "noMsg" and "strMsg" for ReferenceError. So, it seems instead I need to make (Either ReferenceError) an instance of MonadError. However, when I try, I get:
> instance MonadError (Either ReferenceError)
Kind error: `Either ReferenceError' is not applied to enough type arguments
When checking kinds in `MonadError (Either ReferenceError)'
In the instance declaration for `MonadError (Either ReferenceError)'
So, how do I get the effect I want for "findType"? Besides "throwError" I also want to use "catchError".
Thanks, Brian (Haskell newbie)
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