Text that is kept: names for channels, and a log of what happened

# Text that is kept: names for channels, and a log of what happened.
#
#   zeno run examples/labels.zeno
#   zeno build --no-collector --memory examples/labels.zeno && ./labels
#
# With no collector, whatever keeps text has a copy of its own and frees
# it. docs/text.md goes through what the compiler writes for this file.

owned class Channel {
    id: int
    label: str
    notes: owned list[str]

    function rename(self, label: str) {
        self.notes.append(f"was {self.label}")
        self.label = label
    }

    function describe(self) -> str {
        return f"{self.label} (channel {self.id}, {len(self.notes)} notes)"
    }
}

class TooHot {
    label: str
    reading: int
}

function label_for(id: int) -> str {
    if id == 0 { return "ground" }
    return f"ch{id}"
}

function check(channel: borrow Channel, reading: int) -> int raises TooHot {
    if reading > 90 { raise TooHot(channel.label, reading) }
    return reading
}

# The last thing that happened. Any function can change it.
var last = "started " + label_for(1)

function note(what: str) -> str {
    let before = last
    last = what
    return before
}

let channel = Channel(3, label_for(3), [])
print(channel.describe())
channel.rename(label_for(0))
channel.rename("probe " + channel.label)
print(channel.describe())
print(channel.notes)

# `last` is read, then changed by `note`, then read again, in one statement.
print(last, note("renamed twice"), last)

var hottest = ""
var total = 0
for reading in [40, 95, 60, 99] {
    try {
        total += check(channel, reading)
    } except TooHot as hot {
        hottest = f"{hot.label} at {hot.reading}"
        print(note(hottest))
    }
}
print(total, hottest, last)

match label_for(total % 2) {
    case "ground" { print("an even total") }
    case other { print("an odd total, so", other) }
}

ch3 (channel 3, 0 notes)
probe ground (channel 3, 2 notes)
["was ch3", "was ground"]
started ch1 started ch1 renamed twice
renamed twice
probe ground at 95
100 probe ground at 99 probe ground at 99
an even total