Flow functions are not type checked by FlowType

[thejuan]

FlowType sees this inside a flow function as any and so no type checking takes place on this calls.
You can get around it by binding the generator function passed to flow.

flow(function*(){yield 1;}.bind(this))

It's easy to forget todo this and I haven't found a lint for it.
Are there any other suggestions on how to get type checking?
Is there a reason why flow isn't a decorator? Is it dangerous in some way if i added it locally?

[mweststrate]

The primary problem is that arrow functions cannot be combined with generators, so .bind seems to be a nice work around.

flow can be used as decorator when using asyncAction from mobx-utils (no worries, it is just a wrapper, same implementation): https://github.com/mobxjs/mobx-utils#asyncaction. Bu this works only with TypeScript, as it doesn't work for babel legacy. Babel stage 2 non legacy decorators are under way, so will add it as todo there.

[cncolder]

for typescript, you can type this by hand:

class MyStore {
    @observable
    value = ''

    print = flow(function*(this: MyStore) {
        // type checking worked.
        console.log(this.value.toLowerCase())
    })
}

[thejuan]

You still loose the return type if you have one. We went with a Babel Transform to convert async methods to flow during transpilation.

[joshua1]

@mweststrate can We use mobx-task here for async actions then. would that suffice?

[mweststrate]

Closing for now, will be reopened once #1928 happens.

[davidgolden]

It seems to me that functions inside flow are bound to this as an arrow function would be:

const ctx = this
const args = arguments
const runId = ++generatorId
const gen = action(`${name} - runid: ${runId} - init`, generator).apply(
    ctx,
    (args as any) as Args
)

https://github.com/mobxjs/mobx/blob/master/src/api/flow.ts

Am I understanding this wrong?