decoder stuff

decoder.gleam

import gleam/dynamic.{DecodeError, Dynamic}
import gleam/list
import gleam/io

type Foo {
  Foo(Int, Float, String)
}

pub opaque type Collector(dynamic, a, b) {
  Collector(data: dynamic, result: Result(a, List(DecodeError)), prev: b)
}

fn begin(data: dynamic) {
  Collector(data: data, result: Ok(Nil), prev: Nil)
}

fn then(
  prev: Collector(dynamic, a, b),
  decode: fn(dynamic) -> Result(val, List(DecodeError)),
) -> Collector(dynamic, val, Collector(dynamic, a, b)) {
  case decode(prev.data), prev.result {
    Ok(x), Ok(_) -> Collector(data: prev.data, result: Ok(x), prev: prev)
    Error(x), Error(y) ->
      Collector(data: prev.data, result: Error(list.append(x, y)), prev: prev)
    Error(x), _ -> Collector(data: prev.data, result: Error(x), prev: prev)
    _, Error(y) -> Collector(data: prev.data, result: Error(y), prev: prev)
  }
}

fn unwrap(
  collector: Collector(dynamic, a, b),
  cb: fn(a, b) -> Result(c, List(DecodeError)),
) -> Result(c, List(DecodeError)) {
  case collector.result {
    Ok(x) -> cb(x, collector.prev)
    Error(x) -> Error(x)
  }
}

fn example(data: Dynamic) -> Result(Foo, List(DecodeError)) {
  let collector =
    begin(data)
    |> then(dynamic.element(0, dynamic.int))
    |> then(dynamic.element(1, dynamic.float))
    |> then(dynamic.element(2, dynamic.string))
  use e2, collector <- unwrap(collector)
  use e1, collector <- unwrap(collector)
  use e0, _collector <- unwrap(collector)
  Ok(Foo(e0, e1, e2))
}

pub fn main() {
  #(0, 1.0, "2")
  |> dynamic.from
  |> example()
  |> io.debug()
  #(Nil, 1.0, "2")
  |> dynamic.from
  |> example()
  |> io.debug()
  #(0, Nil, "2")
  |> dynamic.from
  |> example()
  |> io.debug()
  #(0, 1.0, Nil)
  |> dynamic.from
  |> example()
  |> io.debug()
  #(Nil, Nil, "2")
  |> dynamic.from
  |> example()
  |> io.debug()
  #(Nil, 1.0, Nil)
  |> dynamic.from
  |> example()
  |> io.debug()
  #(0, Nil, Nil)
  |> dynamic.from
  |> example()
  |> io.debug()
  #(Nil, Nil, Nil)
  |> dynamic.from
  |> example()
  |> io.debug()
  Nil
}