How to provide an alternative error message?

[Profpatsch]

I have a row that requires two fields to be set.

Something akin to

  foo <- exactly "foo"
  bar <- exactly "bar"

If I give it "foo", it will return UnexpectedEndOfRow; however, I can make this error nicer, because I have some local information about what is missing.

There is a function

onError :: Decode e s a -> (DecodeErrors e -> Decode e s a) -> Decode e s a

but similar to the last issue (#47) it’s not at all clear to me how to go from e.g. decodeError :: DecodeError e -> DecodeValidation e a to a full Decode e s a, i.e how to fill that continuation of onError and provide an alternative error message.