actions.js

export const fetchByPostId = id => {
  const data = API.getPostById(id);
  return {
    type: FETCH_POST_BY_ID,
    payload: data,
  }
}

index.js

import React, { Component } from 'react';
import { connect, bindActionCreators } from 'react-redux';
import * as actions from '../some/actionCreatorFile.js';

class FooContainer extends Component {
  componentDidMount() {
    this.props.fetchByPostId(this.props.match.params.id);
  }
  
  render() {
    return (
      <SomeStateLessComponent post={this.props.postFoo} />
    )
  }
}

// State => Props
mapStateToProps = (state) => {
  return {
    postFoo: state.postConatiner.post,
  }
}

export default connect(mapStateToProps, { actions })(FooContainer);

reducer.js

//import types (FETCH_POST_BY_ID) 

const initialState = {
  post: {},
}

const postReducer = (state = initialState, action) => {
  switch(action.type) {
    case FETCH_POST_BY_ID: {
      return {
        ...state,
        post: action.payload,
      }
    }
    default:
      return state;
  }
}

export default initialState;