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Takuya Matsuyama
Takuya Matsuyama

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Making HOC with prop type inference from Recompose and Redux connector in Flow

It seems like the type annotation for react-redux’s connect is not compatible with the recompose’s HOC type declaration. I often encountered errors when connect is specified in compose function like this:

const enhance: HOC<*, Props> = compose(
  connect(),
  pure, // <-- Flow error - Component: This type is incompatible with the expected param type of Component
  withHandlers({
    ...
  })
)
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If I removed connect() from parameters, the flow error disappears. Huh? But the app with this code works fine so I guess there are some bugs in the Flow-typed definitions. I don’t wanna waste time for this problem.

So I made simple utility functions to make connect compatible with compose function which yet prop type inference is working in base components. Below code is getDispatch function that calls connect with no parameters so it will simply add dispatch to props of the base component:

// @flow
import { type HOC } from 'recompose'
import { connect } from 'react-redux'
import type { Dispatch } from '../types'

type CHOC<E: {}> = HOC<{ ...$Exact<E>, dispatch: Dispatch }, E>

export default function getDispatch<Enhanced: {}>(): CHOC<Enhanced> {
  return (connect(): Function)
}
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You can use it like so:

const enhance: HOC<*, Props> = compose(
  withDispatch(),
  pure,
  withHandlers({
    ...
  })
)
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And you will get props.dispatch.

When you want to get store mapped to props, you can use below connectStore function:

// @flow
import { type HOC } from 'recompose'
import { connect } from 'react-redux'
import type { Dispatch, State } from '../types'

type F<M> = (state: State) => M
type CHOC<E: {}, M> = HOC<{ ...$Exact<E>, dispatch: Dispatch, ...M }, E>

export default function connectStore<Enhanced: {}, M: *>(
  mapper: F<M>
): CHOC<Enhanced, M> {
  return (connect(mapper): Function)
}
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It forces the type of connector function casted as recompose’s HOC so it will work without problem:

const enhance: HOC<*, Props> = compose(
  connect(({ editingNote }) => ({ editingNote })),
  pure,
  withHandlers({
    ...
  })
)
const EnhancedComponent = enhance(props => {
  console.log(props.editingNote) // <-- type inference works!
})
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Obviously it is just a workaround and it even may break in the future, but it simplifies my codebase and works fine for now.
The type inference in Flow is pretty great but type annotations tend to be very complicated. It reminds me of the macro hell in C/C++ 🙄

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