Missing values

// Purpose: "no value" is always said, never a silent 0 — empty
// statistics, indicators before they have enough data, out-of-range
// reads, text that is not a number.
// Category: pure
// Permissions: none
// Side effects: none
// Run: t0 run examples/16_missing_values.tz

lang ">=0.1.1, <0.2.0";
import std.stats;
import std.ind;
import std.decimal;
import std.text;

fn describe(value: Option<Float>) -> String {
    return match value {
        Some(x) => to_string(x),
        None => "no value",
    };
}

fn main() {
    let empty: List<Float> = [];
    print("mean of nothing: " + describe(stats.mean(empty)));
    let one: List<Float> = [42.0];
    print("stdev of one value: " + describe(stats.stdev(one)));
    let short: List<Float> = [1.0, 2.0];
    print("a 3-period SMA of 2 values: " + to_string(ind.sma(short, 3)));
    print("its latest value: " + describe(ind.latest(ind.sma(short, 3))));
    let xs = [10, 20, 30];
    print("xs.get(7): " + to_string(xs.get(7)));
    print("decimal.parse(\"12,5\"): " + to_string(decimal.parse("12,5")));
    print("text.parse_int(\"seven\"): " + to_string(text.parse_int("seven")));
    let fallback = xs.get(7).unwrap_or(-1);
    print("an explicit default, chosen by the program: " + to_string(fallback));
}
// expect-output: mean of nothing: no value
// expect-output: stdev of one value: no value
// expect-output: a 3-period SMA of 2 values: [None, None]
// expect-output: its latest value: no value
// expect-output: xs.get(7): None
// expect-output: decimal.parse("12,5"): None
// expect-output: text.parse_int("seven"): None
// expect-output: an explicit default, chosen by the program: -1