Allow top level generator types for actions

This commit adds a short-hand for generator actions. For example:

```javascript
let count = n => n

function * range (repo, start, end) {
  while (start <= end)
    yield repo.push(count, start++)
  }
}

repo.push(range, 1, 10) // 1,2,3,4,5,6,7,8,9,10
```

examples/react-router/app/actions/lists.js

@@ -1,16 +1,12 @@
 import Lists from '../domains/lists'
 import { visit } from './routing'
 
-export function addList(params) {
-  return function*(repo) {
-    let list = yield repo.push(Lists.create, params)
+export function* addList(repo, params) {
+  let list = yield repo.push(Lists.create, params)
 
-    yield repo.push(visit, `/lists/${list.id}`)
-  }
+  yield repo.push(visit, `/lists/${list.id}`)
 }
 
-export function removeList(id) {
-  return function*(repo) {
-    yield repo.push(Lists.destroy, id)
-  }
+export function* removeList(repo, id) {
+  yield repo.push(Lists.destroy, id)
 }

src/coroutine.js

@@ -6,10 +6,10 @@ import { isFunction, isPromise, isGeneratorFn } from './utils'
  * order.
  * @private
  */
-function processGenerator(action, body, repo) {
-  action.open()
+function processGenerator(action, body, repo, params) {
+  action.open(...params)
 
-  let iterator = body(repo)
+  let iterator = body(repo, ...params)
 
   function step(payload) {
     let next = iterator.next(payload)
@@ -46,6 +46,10 @@ function processGenerator(action, body, repo) {
  * the body of their associated command.
  */
 export default function coroutine(action, command, params, repo) {
+  if (isGeneratorFn(command)) {
+    return processGenerator(action, command, repo, params)
+  }
+
   let body = command.apply(null, params)
 
   /**
@@ -73,7 +77,7 @@ export default function coroutine(action, command, params, repo) {
    * in order
    */
   if (isGeneratorFn(body)) {
-    return processGenerator(action, body, repo)
+    return processGenerator(action, body, repo, params)
   }
 
   /**

test/unit/middleware/generator-middleware.test.js

@@ -1,6 +1,22 @@
 import Microcosm from '../../../src/microcosm'
 
 describe('Generator Middleware', function() {
+  it('opens with the parameters', function() {
+    let stepper = n => n + 1
+    let repo = new Microcosm()
+
+    function stall(n) {
+      return function*(repo) {
+        yield repo.push(() => new Promise(() => {}))
+      }
+    }
+
+    let action = repo.push(stall, 2)
+
+    expect(action).toHaveStatus('open')
+    expect(action.payload).toEqual(2)
+  })
+
   it('processes actions sequentially', function() {
     expect.assertions(1)
 
@@ -235,4 +251,28 @@ describe('Generator Middleware', function() {
       })
     })
   })
+
+  it('allows the top level action description to be a generator', function() {
+    let count = n => n
+
+    function* sequence(repo, start) {
+      yield repo.push(count, 1)
+      yield repo.push(count, 2)
+      yield repo.push(count, 3)
+    }
+
+    class Repo extends Microcosm {
+      register() {
+        return {
+          [count]: (_last, next) => next
+        }
+      }
+    }
+
+    let repo = new Repo()
+
+    repo.push(sequence, 1)
+
+    expect(repo.state).toEqual(3)
+  })
 })