variant3.re

type length = [ | `Meter | `Inch];
type time = [ | `Second | `Minute | `Hour];
type temperatureLinear = [ | `Kelvin];
type temperatureNonLinear = [ | `Celsius];

type unitLinear = [ length | time | temperatureLinear];
type anyUnit = [ unitLinear | temperatureNonLinear];

let siScale = (unit: unitLinear) =>
  switch (unit) {
  | `Meter => 1.
  | `Inch => 0.0254
  | `Second => 1.
  | `Minute => 60.
  | `Hour => 3600.
  | `Kelvin => 1.
  };

let toSi = (value, unit: anyUnit) =>
  switch (unit) {
  | #unitLinear as linearUnit => value *. siScale(linearUnit)
  | `Celsius => value +. 273.15
  };