Stream: beginners

Topic: extreme beginner question - chaining List.map

Bucsi (Nov 30 2023 at 11:16):

Hi! I stumbled upon Roc yesterday and immediately fell in love, eve though I have very limited experience with purely functional languages.

I'm trying to work with a string, where separators are ".." and ".", and converting it to a nested list:

"a..b.c" ~> [["a"], ["b", "c"]]

I have written the following in the REPL:

» "a..b.c" |> Str.split ".." |> List.map \item -> Str.split item "."

[["a"], ["b", "c"]] : List (List Str) # as expected


» "a..b.c" |> Str.split ".." |> List.map \item -> Str.split item "." |> List.map \sublist -> Str.joinWith sublist "_"
#expecting output ["a", "b_c"]
This Roc code crashed with: "Erroneous: Expr::Closure"

── TYPE MISMATCH ─────────────

This 2nd argument to |> has an unexpected type:

4│      "a..b.c" |> Str.split ".." |> List.map \item -> Str.split item "." |> List.map \sublist -> Str.joinWith sublist "_"
                                                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The argument is an anonymous function of type:

    List Str -> Str

But |> needs its 2nd argument to be:

    Str -> Str

I understand the issue is that the latest map in my chain receives strings as its input, but based on my experience, I think it should be ["a"] and ["b", "c"], as these are the elements of the list of the previous expression's output. What am I missing?

Bucsi (Nov 30 2023 at 11:19):

And just as I pressed send, I have realized the pipe is being considered as part of my first callback :face_palm: Calling it wrapped in parens provides the expected result:

» "a..b.c" |> Str.split ".." |> List.map (\item -> Str.split item ".") |> List.map \sublist -> Str.joinWith sublist "_"

["a", "b_c"] : List Str

Bucsi (Nov 30 2023 at 11:20):

Is there a recommended way of using parens? Should a callback be always wrapped?

Sven van Caem (Nov 30 2023 at 12:57):

Usually each part of the pipeline will be on its own line, that fixes it. But if you want it to be on one line, you'll have to use parentheses

Declan Joseph Maguire (Dec 02 2023 at 02:32):

This sounds like something that might benefit from a custom compiler error? It sounds easy to detect and might be a common newbie mistake.

Anton (Dec 02 2023 at 09:58):

I'll make an issue for it

Anton (Dec 02 2023 at 10:03):

#6150

Last updated: Jul 26 2025 at 12:14 UTC