type Easing = 'ease-in' | 'ease-out' | 'ease-in-out';

function getEasingFunction(easing: Easing): (val: number) => number {
    switch (easing) {
        case 'ease-in':     return t => t * t;
        case 'ease-out':    return t => t * (2-t);
        case 'ease-in-out': return t => t<0.5 ? (2*t*t) : -1+(4-2*t)*t;
    }
    // `easing` should be of type `never` here
    // however the actual type is `'ease-in' | 'ease-out' | 'ease-in-out'`
}