Allow predicate refinements for split

[abliarsar]

const source = createEvent<string | number | null | {}>()

const events = split(source, {
    numbers: (a): a is number => typeof a === 'number',
    strings: (a): a is string => typeof a === 'string',
    not_refined: (a) => !!a,
})

// Event<number>, no error
events.numbers.map(a => a.toFixed())

// Event<string>, no error
events.strings.map(a => a.charCodeAt(0))

// still fails, object is possibly null
events.not_refined.map(a => a.toString())

[vercel]

This pull request is automatically deployed with Now.
To access deployments, click Details below or on the icon next to each push.

Latest deployment for this branch: https://effector-git-fork-abliarsar-split-typings.zerobias.now.sh

[zerobias]

Good improvement 👍 👍