Stream: beginners

Topic: ✔ Handling List.walk Result Bool

Brendan Hansknecht (Dec 28 2024 at 18:56):

I think you want Ok is_file if is_file -> as the first when branch and _ -> as the second when branch

Barton Hammond (Dec 28 2024 at 19:06):

Sorry - never used Zulip before...I deleted my original message by mistake.

So - trying to determine if a entry is a File or a Directory using "if" rather than "when"

Code: https://github.com/bartonhammond/pullNightly/blob/main/list-walk.roc

Here's the code snipped:

 foo = List.walk dir_list { file: [], dir: [] } \state, elem ->
        if File.is_file! elem then
            { state & file: List.append state.file elem }
        else
            { state & dir: List.append state.dir elem }

And the error is

This if condition needs to be a Bool:

15│          if File.is_file! elem then
                ^^^^^^^^^^^^^^^^^^

This File.is_file! call produces:

    Result Bool [PathErr InternalIOErr.IOErr]

But I need every if condition to evaluate to a Bool—either Bool.true
or Bool.false.

Brendan Hansknecht (Dec 28 2024 at 19:08):

so File.is_file! returns a Result Bool [PathErr InternalIOErr.IOErr]. Essentially, if the path is bad or if something else goes wrong, it will return a PathErr.

Brendan Hansknecht (Dec 28 2024 at 19:09):

Otherwise, it returns a boolean indicating if something is a file or not

Brendan Hansknecht (Dec 28 2024 at 19:09):

In the example, you have to decide what to do with the error.

Brendan Hansknecht (Dec 28 2024 at 19:10):

Probably the easiest is just propagating the error with ?

Brendan Hansknecht (Dec 28 2024 at 19:10):

Something like:

foo = List.walk? dir_list { file: [], dir: [] } \state, elem ->
        if File.is_file!? elem then
            { state & file: List.append state.file elem }
        else
            { state & dir: List.append state.dir elem }

I think should work an propagate the error up the stack

Brendan Hansknecht (Dec 28 2024 at 19:10):

I think that is the right syntax...

Brendan Hansknecht (Dec 28 2024 at 19:12):

Otherwise, you can handle the error locally:

foo = List.walk? dir_list { file: [], dir: [] } \state, elem ->
    when File.is_file! elem is
        Ok is_file if is_file ->
            { state & file: List.append state.file elem }
        Ok _ ->
            { state & dir: List.append state.dir elem }
        _ ->
            # Just ignore failures
            state

Barton Hammond (Dec 28 2024 at 19:16):

Sorry - this is very confusing...

With this:

foo = List.walk dir_list { file: [], dir: [] } \state, elem ->
    when File.is_file! elem is
        Ok is_file if is_file ->
            { state & file: List.append state.file elem }
        Ok _ ->
            { state & dir: List.append state.dir elem }
        _ ->
            # Just ignore failures
            state

I get this:

I was partway through parsing a function, but I got stuck here:

14│      foo = List.walk dir_list { file: [], dir: [] } \state, elem ->
                                                                       ^

I just saw a pattern, so I was expecting to see a -> next.%

Sam Mohr (Dec 28 2024 at 19:19):

Try putting some more newlines and indents in there

Sam Mohr (Dec 28 2024 at 19:19):

Seems like it doesn't know where the block starts?

Brendan Hansknecht (Dec 28 2024 at 19:22):

sorry, I may have mixed up some tabbing. Didn't actually run the example, just typed it out

Brendan Hansknecht (Dec 28 2024 at 19:26):

Oh, we don't have List.walk! so this function can't work.

Barton Hammond (Dec 28 2024 at 19:39):

wrt to formatting, my VSCode is not formatting the file on save. I have the Roc Extension installed and have the settings.json updated.
The roc_nightly directory has the roc and roc_language_server.

When I am in any roc file, I do not have a menu option to "format" so I don't know if there is problem with file formatting.

settings.json

{
  "roc-lang.language-server.exe": "/Users/bartonhammond/projects/roc/roc_nightly",
  "editor.formatOnSave": true
}

Brendan Hansknecht (Dec 28 2024 at 19:43):

So to give a concrete solution here. Due to File.is_file! being effectful (e.g. having a ! in the name), it can't be used with non-effectful functions like List.walk. Currently, roc does not have List.walk!, so this code must be done manually through a recursive functions (growing pains of having just switched to purity inference).

So the solution right now is likely this:

dirs_and_files! = \list, state ->
    when list is
        [elem, .. as rest] ->
            next_state =
                when File.is_file! elem is
                    Ok is_file if is_file ->
                        { state & file: List.append state.file elem }

                    Ok _ ->
                        { state & dir: List.append state.dir elem }

                    _ ->
                        # Just ignore failures
                        state
            dirs_and_files! rest next_state

        _ ->
            state

Barton Hammond (Dec 28 2024 at 19:50):

I'll have to learn more to understand what you're talking about.

Notification Bot (Dec 28 2024 at 19:52):

Barton Hammond has marked this topic as resolved.

Brendan Hansknecht (Dec 28 2024 at 19:53):

Yeah, roc is currently in flux with a lot of changes that greatly affect syntax and direct code.

Brendan Hansknecht (Dec 28 2024 at 19:53):

Purity inference is one of them. Original design doc at the top of this thread: #ideas > Purity inference proposal v3

Last updated: Jul 26 2025 at 12:14 UTC