Stream: beginners

Topic: Missing method errors

cerk (Aug 27 2026 at 13:40):

Here is some code:

Gap := {lo: U64, hi: U64}.{
    mid : Gap -> U64
    mid = |g| {
        (g.lo + g.hi) / 2
    }

    min_max : Gap, Gap -> Gap
    min_max = |a, b| {
        {lo: a.lo.min(b.lo), hi: a.hi.max(b.hi)}
    }
}

main! = |_args| {
    a = {lo: 0, hi: 10}
    b = {lo: 11, hi: 20}
    dbg a.mid()
    dbg a.min_max(b)
    Ok({})
}

With Roc compiler version nightly-2026-08-26-b29bef3, I am getting missing method errors for the a.mid() and a.min_max(b) calls.

If I change the dbg a.mid() line to dbg Gap.mid(a), then the program runs successfully:

Gap := {lo: U64, hi: U64}.{
    mid : Gap -> U64
    mid = |g| {
        (g.lo + g.hi) / 2
    }

    min_max : Gap, Gap -> Gap
    min_max = |a, b| {
        {lo: a.lo.min(b.lo), hi: a.hi.max(b.hi)}
    }
}

main! = |_args| {
    a = {lo: 0, hi: 10}
    b = {lo: 11, hi: 20}
    dbg Gap.mid(a)
    dbg a.min_max(b)
    Ok({})
}

I'm not sure what the correct behavior is supposed to be here but by changing that one line, the missing method error for a.min_max(b) disappears. Should I report a bug on github?

Jared Ramirez (Aug 27 2026 at 19:40):

this is correct behavior, unless roc knows that the a and b are explicitly typed at Gap, it infers as plain records, which do not have those methods defined

you can give a/b an annotation, or construct like Gap.{ .. } to tell roc the type

Last updated: Sep 03 2026 at 15:16 UTC