[minor] Replace while with a macro pattern

pyiron_workflow/__init__.py

@@ -53,4 +53,3 @@
     inputs_to_list,
     list_to_outputs,
 )
-from pyiron_workflow.nodes.while_loop import while_loop

pyiron_workflow/create.py

@@ -89,13 +89,11 @@ def Workflow(self):
     @lru_cache(maxsize=1)
     def meta(self):
         from pyiron_workflow.nodes.transform import inputs_to_list, list_to_outputs
-        from pyiron_workflow.nodes.while_loop import while_loop
 
         return DotDict(
             {
                 inputs_to_list.__name__: inputs_to_list,
                 list_to_outputs.__name__: list_to_outputs,
-                while_loop.__name__: while_loop,
             }
         )
 

pyiron_workflow/nodes/standard.py

@@ -73,6 +73,43 @@ def process_run_result(self, function_output):
             self.signals.output.false()
 
 
+@as_function_node("list")
+def AppendToList(existing: list | None = None, new_element=NOT_DATA):
+    """
+    Append a new element to a list.
+
+    Args:
+        existing (list | None): The list to append to. Defaults to None, which creates
+            an empty list.
+        new_element: The new element to append. This is mandatory input as the default
+            value of `NOT_DATA` will cause a readiness error.
+
+    Returns:
+        list: The input list with the new element appended
+
+    Examples:
+        This node is particularly useful for acyclic flows, where you want to bend the
+        suggestions of idempotency by looping the node's output back on itself:
+
+        >>> from pyiron_workflow import standard_nodes as std
+        >>>
+        >>> n = std.AppendToList()
+        >>> n.run(new_element="foo")
+        ['foo']
+
+        >>> n.run(existing=n, new_element="bar")
+        ['foo', 'bar']
+
+        >>> n.run(existing=n, new_element="baz")
+        ['foo', 'bar', 'baz']
+
+    """
+    existing = [] if existing is None else existing
+    if new_element is not NOT_DATA:
+        existing.append(new_element)
+    return existing
+
+
 @as_function_node("random")
 def RandomFloat():
     """
@@ -652,6 +689,7 @@ def Round(obj):
     Absolute,
     Add,
     And,
+    AppendToList,
     Bool,
     Bytes,
     Contains,

tests/integration/test_while.py

@@ -0,0 +1,79 @@
+import pickle
+import unittest
+
+from pyiron_workflow import as_macro_node, standard_nodes as std
+
+
+@as_macro_node("greater")
+def AddWhileLessThan(self, a, b, cap):
+    """
+    Add :param:`b` to :param:`a` while the sum is less than or equal to :param:`cap`.
+
+    A simple but complete demonstrator for how to construct cyclic flows, including
+    logging key outputs during the loop.
+    """
+    # Bespoke logic
+    self.body = std.Add(obj=a, other=b)
+    self.body.inputs.obj = self.body.outputs.add  # Higher priority connection
+    # The output is NOT_DATA on the first pass and `a` gets used,
+    # But after that the node will find and use its own output
+    self.condition = std.LessThan(self.body, cap)
+
+    # Universal logic
+    self.switch = std.If()
+    self.switch.inputs.condition = self.condition
+
+    self.starting_nodes = [self.body]
+    self.body >> self.condition >> self.switch
+    self.switch.signals.output.true >> self.body
+
+    # Bespoke logging
+    self.history = std.AppendToList()
+    self.history.inputs.existing = self.history
+    self.history.inputs.new_element = self.body
+    self.body >> self.history
+
+    # Returns are pretty universal for single-value body nodes,
+    # assuming a log of the history is not desired as output,
+    # but in general return values are also bespoke
+    return self.body
+
+
+class TestWhileLoop(unittest.TestCase):
+    def test_while_loop(self):
+        a, b, cap = 0, 2, 5
+        n = AddWhileLessThan(a, b, cap, run_after_init=True)
+        self.assertGreaterEqual(
+            6,
+            n.outputs.greater.value,
+            msg="Verify output"
+        )
+        self.assertListEqual(
+            [2, 4, 6],
+            n.history.outputs.list.value,
+            msg="Verify loop history logging"
+        )
+        self.assertListEqual(
+            [
+                'body',
+                'history',
+                'condition',
+                'switch',
+                'body',
+                'history',
+                'condition',
+                'switch',
+                'body',
+                'history',
+                'condition',
+                'switch'
+            ],
+            n.provenance_by_execution,
+            msg="Verify execution order -- the same nodes get run repeatedly in acyclic"
+        )
+        reloaded = pickle.loads(pickle.dumps(n))
+        self.assertListEqual(
+            reloaded.history.outputs.list.value,
+            n.history.outputs.list.value,
+            msg="Should be able to save and re-load cyclic graphs just like usual"
+        )