utils.ts

type Predicate<T> = (value: T) => boolean;
type Mapper<T, R> = (value: T) => R;
type CondPair<T, R> = [Predicate<T>, Mapper<T, R>];

// recursive cond that can breaks early or returns undefined
const cond = <T, R>(
  [[pred, mapr] = [] as unknown as CondPair<T, R>, ...rest]: CondPair<T, R>[] = []
): ((val: T) => R | undefined) => (val: T): R | undefined =>
  !pred ? undefined :
  pred(val) ? mapr(val) : cond(rest)(val);

const stateOfWaterAt = cond<number, string>([
  [(n) => n >= 100, () => `gas`],
  [(n) => n <= 0,   () => `solid`],
  [(n) => true,     () => `liquid`]
]);

// nicer w ramda utility fns
const _stateOfWaterAt = cond<number, string>([
  [lte(100), always('gas')],
  [gte(0),   always('solid')],
  [T,        always('liquid')]
]);

[
  [ -10, 0, 10, 90, 100, 110 ].map(stateOfWaterAt),
  [ -10, 0, 10, 90, 100, 110 ].map(_stateOfWaterAt),
]