Improve decoder binding

[Profpatsch]

I’m trying to write a decoder which parses the whole remaining columns with the same decoder.

Something like remaining :: Decode e s a -> Decode e s [a].
and then use it like: remaining utf8 :: Decode e ByteString [Text].

Now there is a function row :: Decode e s (Vector s) which does basically that, but the return type is fixed to s.
So I’d expect to be able to bind a monadic function.

And indeed, there is two different bind functions:

(>>==) :: Decode e s a -> (a -> DecodeValidation e b) -> Decode e s b

bindDecode :: Decode e s a -> (a -> Decode e s b) -> Decode e s b

which both seem close, but none of them works.

row >>== ??? does not work, because >>== wants an a -> DecodeValidation e b, not a utf8 :: Decode e s a. Unfortunately, the DecodeValidation version of utf8 is not exported.

row bindDecode (\bs -> ???) doesn’t work, because it requires a function ByteString -> Decode e s a which I don’t know how to construct.

I think for now I’ll just copy & paste the inner DecodeValidation part of utf8, but the binding story is really not great right now, it doesn’t compose with any of the primitives the Decode module exposes.

[Profpatsch]

Alternatively, a way of converting a Decode e s a back to a s -> DecodeValidation e a would make these composable.

[Profpatsch]

I figured out that row would always take all rows, not like I expected the rest of the rows from the current index.

This function does that:

rowFromCurrentIndex :: Dec.Decode e a (Vector a)
rowFromCurrentIndex =
  Dec.Decode . Compose . Dec.DecodeState . ReaderT $ \v ->
    state (\(Dec.Ind i) -> (pure (Vector.drop i v), Dec.Ind (Vector.length v)))