Add `sequence` function

[cobalamin]

See the docs for an explanation. It essentially turns a list of decoders into a decoder that returns a list, by zipping each decoder with one value in a JSON list.

[lukewestby]

If you need to be sure of the length of the list at compile time, say it's 3 like in this example, couldn't you use tuple3 instead and then only have to worry about the decoder at each position?

decoder : Decoder (List FloatOrInt)
decoder =
  Decode.tuple3 (\a b c -> [a, b, c]) intDecoder floatDecoder intDecoder

[cobalamin]

It's precisely not about compile time. The motivating problem someone had on Slack were:

• "ibizaman" had some pretty odd JSON where one property was a list of column names and types, the other was a list of complete rows, and they needed this function after dynamically generating a list of decoders from the first property.
• @ashishnegi had a different use case that I don't remember exactly, but it's described here (together with the code that solved the problem above): https://gist.github.com/cobalamin/5f63b6cbd67722c6eda525d5562d646c

I'll agree that my example code isn't ideal for demonstrating this, as it seems to do something that could be done hard-coded. I didn't think about that earlier and I felt like that was a minimal example to not get too long-winded with the docs, but it isn't great.

[lukewestby]

moving some discussion from slack over here for the sake of having it:

cobalamin [12:27 PM] [at]luke thanks for the feedback, I added a response :slightly_smiling_face:

luke [12:27 PM]
awesome! i totally get it now, i hadn’t considered the case of wanting to dynamically generate a sequence of decoders

cobalamin [12:27 PM]
yeah, it seems like that would be a better example. Would also be a lot of code though. Ideas?

luke [12:28 PM]
i think if the doc comment just mentions that idea of being able to generate your list of decoders dynamically we’ll be good to go

[lukewestby]

Beautiful. Thanks!