Tag side effects with opaque types

index.js

/* @flow */
import type { Wrapped } from './wrap';
import { wrapWithSideEffect } from './wrap';

function addOne(a) {
  return a + 1;
}

type AddOne = typeof addOne;

function withAddOne (fn: AddOne) {}
function withWrappedAddOne (fn: Wrapped<AddOne>) {}

withAddOne(addOne)
withWrappedAddOne(addOne) // Error

withAddOne(wrapWithSideEffect(addOne)) // Error
withWrappedAddOne(wrapWithSideEffect(addOne))

// This can be made generic too:

function onlyBareFns(fn: Function) {}
function onlyWrappedFns(wrappedFn: Wrapped<Function>) {}

onlyBareFns(addOne)
onlyWrappedFns(addOne) // Error

onlyBareFns(wrapWithSideEffect(addOne)) // Error
onlyWrappedFns(wrapWithSideEffect(addOne))

wrap.js

/* @flow */
export opaque type Wrapped<T> = T;

export function wrapWithSideEffect<T: Function>(fn: T): Wrapped<T> {
  const wrapped: Wrapped<*> = (...args) => {
  	const result = fn(...args);

    // Now do a side effect:
  	console.log("Dispatch side effect!", result)

    return result
  }
  return (wrapped: Wrapped<*>)
}