feat(async): add is_async

Author: kollhof

src/async.fnk

@@ -1,6 +1,8 @@
 {new} = import '@fink/js-interop/reflect.fnk'
 {Promise, setTimeout} = import '@fink/js-interop/globals.fnk'
 
+{is_fn} = import './fn.fnk'
+
 
 
 parallel = fn tasks:
@@ -19,3 +21,9 @@ sleep = fn ms:
   new Promise, fn resolve:
     setTimeout resolve, ms
 
+
+
+is_async = fn obj:
+  match obj:
+    {then: is_fn ?}: true
+    else: false

src/async.test.fnk

@@ -1,6 +1,6 @@
 {describe, it, expect, to_equal, mock_fn, was_called, was_not_called, run_all_timers} = import '@fink/jest/test.fnk'
 
-{parallel, sequential, sleep} = import './async.fnk'
+{parallel, sequential, sleep, is_async} = import './async.fnk'
 
 
 run_task = fn name, handler:
@@ -64,3 +64,14 @@ describe 'async', fn:
       handler
       was_called
 
+
+
+describe 'reflective', fn:
+  it 'is_async', fn:
+    expect
+      is_async sleep 123
+      to_equal true
+
+    expect
+      is_async 1234
+      to_equal false