Stream: beginners

Topic: ✔ backpassing style for non-IO

Steven Chen (Dec 12 2023 at 19:12):

Just my 2 cents...personally I don't favor backpassing for non-IO (Task) due to the indentation change. E.g. glancing below, it feels like the last line is the return value (instead of of List Str)

result =
    x <- nums |> List.map
    Num.toStr x

The old fashion way is cleaner keeping the lambda closer to List.map

result =
    nums |> List.map \x -> Num.toStr x

Brendan Hansknecht (Dec 12 2023 at 19:35):

It definitely can read weird at times. Though works great with Result.try for example

Brendan Hansknecht (Dec 12 2023 at 19:35):

I probably use it too much personally.

Steven Chen (Dec 12 2023 at 19:38):

also we can only do one backpassing block at a time, right? because the lambda body now takes that indentation

Steven Chen (Dec 12 2023 at 19:42):

Brendan Hansknecht said:

It definitely can read weird at times. Though works great with Result.try for example

because Result.try is a typical Monad (with my limited Haskell understanding). so it works naturally with the backpassing style (do-notation in Haskell)

Brendan Hansknecht (Dec 12 2023 at 19:44):

Just trying to point out that it works with more than just io. It works with anything monad like. Random number generation can also be nice for this.

Brendan Hansknecht (Dec 12 2023 at 19:45):

Yes, one block at a time due to indentation.

Brendan Hansknecht (Dec 12 2023 at 19:46):

Though you can always do:

task =
        x <- ...
        y <- ...
        ...

# continue on outside of that scope

Brendan Hansknecht (Dec 12 2023 at 19:46):

So one level of indentation for a whole chain of calls

Steven Chen (Dec 12 2023 at 19:47):

gotcha. the callback chains can be one block

Notification Bot (Dec 15 2023 at 00:20):

Steven Chen has marked this topic as resolved.

Last updated: Jul 26 2025 at 12:14 UTC