examples/gallery/12-interpolation.hero

## Text with holes in it
#
# An `f` before the opening quote lets a string carry values: `{n}` writes `n`
# as `to_str` would. Any expression may stand in a hole — a sum, a call, a
# field, another `f"…"` — and the hole ends at the `}` that closes it, so a
# map index or a nested literal inside it is read as it would be anywhere.
# `{{` and `}}` each write one brace, and a `}` alone is an error. A literal
# without the `f` prints its braces as they stand, and is refused where they
# hold what would be a hole naming only what is bound there: `"{n}"` is a
# forgotten `f` whenever `n` is.
#
# What a hole may hold is what `to_str` renders: an integer, a float, `bool`
# or `str`. A record or an array has no text of its own, and a map index
# answers a `V?` that must be made whole first — `check` says so, at the hole.

record Star
    name: str
    magnitude: f64

function line(at: i64, of: i64, star: Star) -> str
    return f"{at}/{of} {star.name} at magnitude {star.magnitude}"

function main()
    stars = [Star(name: "Sirius", magnitude: -1.46), Star(name: "Vega", magnitude: 0.03)]
    at: i64 @= 1

    for star in stars
        print(line(at: at, of: stars.len(), star: star))
        at @ at + 1

    seen: {str: i64} = {"Sirius": 2}
    print(f"seen Sirius {seen["Sirius"].must()} times, {{like this}}")
    print(f"nested: {f"{stars.len()} stars"}")