Stream: beginners

Topic: ✔ match list patterns

removewingman (Sep 04 2026 at 16:21):

This works:

b = |lst| {
    match lst {
        [h, ..] if h < 4 => "a"
        _ => ""
    }
}
expect [3] |> b |> Str.is_eq("a")
expect []  |> b |> Str.is_eq("")

But if I want to have tail as an variable it does not work:
Error: I was parsing a pattern, and this token cannot start a pattern here. [h, .. as _t] if h < 4 => "a"

b = |lst| {
    match lst {
        [h, .. as _tail] if h < 4 => "a"
        _ => ""
    }
}

expect [3] |> b |> Str.is_eq("a")
expect []  |> b |> Str.is_eq("")

this should work right, or is there any other way to have an if with head and tail as variable?

Richard Feldman (Sep 04 2026 at 16:38):

oh it's just that the syntax is [h, ..tail]

Richard Feldman (Sep 04 2026 at 16:39):

no as needed, although I do think that's an obvious thing to reach for, and we should give a nicer error message for that - would you mind opening a github issue for that @removewingman?

removewingman (Sep 04 2026 at 16:46):

Im confused, when was the change I am on: nightly-2026-09-02-d2609e2

And got this error message:

 ✗ old list rest pattern ─────────────────────────────────────────────────────────────────────────── /tmp/xdd.roc:4:13

I was parsing a list pattern, and this uses the old rest syntax.

[h, ..tail] if h < 4 => "a"
    ^^^^^^

List rest patterns now use .. as name. The name is optional, but if it is present it must come after as.

For example:
    [first, .. as rest]

(Will upgrade to latest and try again)

Anton (Sep 04 2026 at 17:00):

That as syntax is supported on latest nightly and latest main branch:
https://github.com/roc-lang/roc/blob/main/test/echo/all_syntax_test.roc

Richard Feldman (Sep 04 2026 at 17:03):

oh it used to be ..tail - maybe we changed it for the new compiler :sweat_smile:

removewingman (Sep 04 2026 at 17:09):

This example does not work tho:

b = |lst| {
    match lst {
        [h, .. as _tail] if h < 4 => "a"
        _ => ""
    }
}

expect [3] |> b |> Str.is_eq("a")
expect []  |> b |> Str.is_eq("")

It is probably a bug right? Opened this issue: https://github.com/roc-lang/roc/issues/11109

I actually like, the ..tail syntax a little bit more.

Notification Bot (Sep 04 2026 at 17:28):

removewingman has marked this topic as resolved.

Luke Boswell (Sep 04 2026 at 23:08):

See https://github.com/roc-lang/roc/issues/7091

Last updated: Sep 24 2026 at 15:59 UTC