Typed Meiosis with Function Patches and mithril-stream

state.ts

import stream from 'mithril/stream'
import * as Stream from 'mithril/stream'

export type StatePatch<S> = (state: S) => S

export type AppState = {
  step: string
}

export type AppActions = {
  show: (step: string) => void
}

const app = ({
  initial: (): AppState => ({
    step: '',
  }),
  actions: (update: Stream<StatePatch<AppState>>): AppActions => {
    return ({
      show: (step) => {
        update((state: AppState) => {
          return Object.assign(state, { step })
        })
      }
    })
  }
})

const update = stream<StatePatch<AppState>>()

const states = Stream.scan<StatePatch<AppState>, AppState>(
  (state, patch) => patch(state),
  app.initial(),
  update
)

const actions = app.actions(update)

export {
  actions,
  states,
}