Feature request: opt-in Assert<T> type for arguments

[jedwards1211]

This is a:

• Bug Report
• Feature Request
• Question
• Other

Which concerns:

• flow-runtime
• babel-plugin-flow-runtime
• flow-runtime-validators
• flow-runtime-mobx
• flow-config-parser
• The documentation website

---

This is a PR I'd like to work on someday after fixing more important issues.

What is the current behaviour?

I typically use babel-plugin-flow-runtime with "optInOnly": true for API argument validation. The code looks like the following:

// @flow
// @flow-runtime enable

import reify from 'flow-runtime'
import type {Type} from 'flow-runtime'

type InitUserAttributes = {
  email: string,
  password: string,
}
type InitUserAttributesType = (reify: Type<InitUserAttributes>)

export function createUser(attributes: InitUserAttributes) {
  InitUserAttributesType.assert(attributes)
  ...
}

---

What is the expected behaviour?

It would be nice if I could shorten this to

// @flow
// @flow-runtime enable

import type {Assert} from 'flow-runtime'

type InitUserAttributes = {
  email: string,
  password: string,
}

export function createUser(attributes: Assert<InitUserAttributes>) {
  ...
}

Basically type Assert<T> = T, but babel-plugin-flow-runtime would insert runtime assertions for any arguments using the Assert type.

Which package versions are you using?

babel-plugin-flow-runtime: 0.18.0