fromVar should return an Either String a

[dmjio]

For more parse failure information, we should use the latest and greatest readEither function from Text.Read.readEither.

[dmjio]

Also note for decoding basic type the failure information returned isn't that useful, so this change might not be worth it.

>>> readEither "hello" :: Either String Int
Left "Prelude.read: no parse"

[cideM]

I'm trying to use URI from Text.URI together with Envy but I don't know how to line up the types and preserve error information. I started by wrapping URI in a newtype wrapper Url so that I don't get orphan instance warnings. GHC then tells me that

No instance for (Envy.Var Url)

which makes sense. So I implement Var but the fromVar method has the type fromVar :: String -> Maybe a so I'm immediately stuck because I need String -> Either Whatever Url or any type that let's me add a custom error message or error information.

Then I started wondering about the purpose of fromEnv :: Maybe a -> Parser a since Parser a clearly let's me return more error information. But that information is already lost when I'm trying to implement fromVar... 🤔

So long story short: the fact that fromVar already throws out all error information is to me a complete show stopper when it comes to Envy. Which is a shame because it seems that Haskell doesn't have many nice libraries in the env var parsing space 😞

[dmjio]

@cideM we can change fromVar to be fromVar :: String -> Either String a. I'd support that change.