onCond

onCond(predicate, fn)

Like .cond, but calls fn(value) if matched.

match("Hello world!" as string)
    .onCond(msg => msg.length > 250, msg => `Message "${msg}" is too long`)
    .onCond(msg => msg.length < 100, msg => `Message "${msg}" is too short`)
    .or(false); //=> `Message "Hello world!" is too short`