065 phantom compound unit

// Test: compound phantom labels carry the result of dimensional arithmetic.
//
// `i32[meter]` divided by `i32[second]` returns `i32[meter/second]`. The
// phantom checker doesn't derive that relationship — the author declares it
// in the converter event's signature. The `/` in the label is just opaque
// characters; what makes it meaningful is that a producer event has it as
// an output and that producer's input/output declarations match physical
// units.

~import std/io

~event read-distance { trip_id: u8 }
| traveled i32<meter>

~read-distance => traveled 100

~event read-time { trip_id: u8 }
| elapsed i32<second>

~read-time => elapsed 4

~event compute-velocity { d: i32<meter>, t: i32<second> }
| result i32<meter/second>

~compute-velocity => result d / t

~read-distance(trip_id: 1)
| traveled m |> read-time(trip_id: 1)
    | elapsed s |> compute-velocity(d: m, t: s)
        | result v |> std/io:print.ln("{{ v:d }} m/s")