Data.Attoparsec.ByteString.Streaming.parse uses left-biased Either

[torgeirsh]

Is there a reason, or just an oversight?

[michaelt]

It's an oversight. Here and in a couple other modules here I am refactoring pipes material, where generally Either is used as an unordered sum type - not envisaging a use of the monad instance or with any suggestion that material on the left is error-like. The results that are sometimes counter-intuitive. So I was bewitched into an indifference which is here pointlessly confusing: of course with a parser the result should be Either Message a! I'm not sure what the best path to repair this is. I am slowly accumulating material for a large version bump for all the streaming libraries but was thinking of delaying a few months.

[torgeirsh]

Since swapping the Either is a breaking change, I'm sure it can wait until the large bump. Another thing that would be good to have for the large bump is parsing with length, i.e. parseL and parsedL, similar to pipes-attoparsec.

By the way, how do you feel about splitting up the package? It currently brings in a lot of unrelated functionality. I wanted to stream an attoparsec parser, but was unable to compile streaming-utils on Windows because one of the JSON dependencies was POSIX only. The JSON dependency is fixed now, but it's still something that isn't needed for attoparsec support.

[fosskers]

Came here to open this issue, to find it already open! Just echoing that left-biased Either is weird.

[michaelt]

Right sorry, I got bogged down in an excess of choice points. My decision was just to make a separate streaming-attoparsec package. https://github.com/michaelt/streaming-attoparsec See what you think of the return type. There are various possibilities, alas.

[fosskers]

Ah, nice. I'd rather have that than the catch-all utils package. The return type seems to me to be the obvious choice, except for maybe the ambiguously named Message alias.

I'll also point out a typo / historical artifact in the docs, where your import alias is A but it's used as both A and AS in the examples.

[torgeirsh]

At a quick glance it looks great, thanks! Is it possible to easily recover parseL & parsedL, or are separate implementations required like in pipes-attoparsec?

[mboes]

👍 for splitting up this package. Over time the dependency list will just get huge. I'm thinking about contributing a connection to binary, but then if we put that in streaming-utils eventually we'll have to depend on every binary-like library (cereal etc) on Earth in the same package.

[fosskers]

Oh cool @mboes , this was you.

[fosskers]

A factored-out streaming-attoparsec has been added to the haskell-streaming organization. Along the way I fixed this Either issue, so this report can be closed.