add a config to reducer, to enable custom ish reducers

src/wf_core/__init__.py

@@ -7,6 +7,7 @@ from .models import (
     NodeDef,
     NodeResult,
     NodeUse,
+    ReducerRef,
     ReducerSpec,
     SchemaRef,
     StateField,
@@ -52,6 +53,7 @@ __all__ = [
     "NodeDef",
     "NodeResult",
     "NodeUse",
+    "ReducerRef",
     "ReducerSpec",
     "SchemaRef",
     "StateField",

src/wf_core/models/__init__.py

@@ -9,7 +9,7 @@ from wf_core.models.conditions import (
     VariadicCondition,
 )
 from wf_core.models.results import NodeResult
-from wf_core.models.reducers import ReducerSpec
+from wf_core.models.reducers import ReducerRef, ReducerSpec
 from wf_core.models.schemas import NodeDef, SchemaRef, StateField, StateSchema
 from wf_core.models.steps import (
     ConditionNode,
@@ -33,6 +33,7 @@ __all__ = [
     "LiteralOperand",
     "NodeDef",
     "NodeResult",
+    "ReducerRef",
     "ReducerSpec",
     "NodeUse",
     "NotCondition",

src/wf_core/models/reducers.py

@@ -1,10 +1,28 @@
 from __future__ import annotations
 
-from pydantic import BaseModel
+from typing import Any
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class ReducerRef(BaseModel):
+    """Reference to one reducer plus JSON-compatible configuration."""
+
+    name: str
+    config: dict[str, Any] = Field(default_factory=dict)
 
 
 class ReducerSpec(BaseModel):
     """Inspectable metadata for one named pure state reducer."""
 
+    model_config = ConfigDict(extra="allow")
+
     name: str
     description: str | None = None
+    config_schema: dict[str, Any] = Field(
+        default_factory=lambda: {
+            "type": "object",
+            "properties": {},
+            "additionalProperties": False,
+        }
+    )

src/wf_core/models/schemas.py

@@ -2,7 +2,9 @@ from __future__ import annotations
 
 from typing import Any
 
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel, ConfigDict, Field, field_validator
+
+from wf_core.models.reducers import ReducerRef
 
 
 class SchemaRef(BaseModel):
@@ -20,10 +22,19 @@ class StateField(BaseModel):
     """Declared state path plus its runtime merge behavior."""
 
     type: str
-    reducer: str = "wf.std.replace"
+    reducer: ReducerRef = Field(
+        default_factory=lambda: ReducerRef(name="wf.std.replace")
+    )
     trace: bool = True
     default: Any = None
 
+    @field_validator("reducer", mode="before")
+    @classmethod
+    def _coerce_reducer(cls, value: object) -> object:
+        if isinstance(value, str):
+            return {"name": value}
+        return value
+
 
 class StateSchema(BaseModel):
     """Workflow state schema keyed by declared exact state path."""

src/wf_core/runtime/ops/merges.py

@@ -4,16 +4,22 @@ from collections.abc import Callable, Mapping
 from typing import Any
 
 from wf_core.errors import WorkflowExecutionError
+from wf_core.models.reducers import ReducerRef, ReducerSpec
+from wf_core.runtime.ops.schemas import validate_payload_against_schema
 
-Reducer = Callable[[Any, Any], Any]
+Reducer = Callable[[Any, Any, Mapping[str, Any]], Any]
 
 
-def replace_reducer(_current_value: Any, incoming_value: Any) -> Any:
+def replace_reducer(
+    _current_value: Any, incoming_value: Any, _config: Mapping[str, Any]
+) -> Any:
     """Replace the current state value with the incoming value."""
     return incoming_value
 
 
-def append_reducer(current_value: Any, incoming_value: Any) -> Any:
+def append_reducer(
+    current_value: Any, incoming_value: Any, _config: Mapping[str, Any]
+) -> Any:
     """Append one value or many values into a list-valued state path."""
     if current_value is None:
         return (
@@ -28,7 +34,9 @@ def append_reducer(current_value: Any, incoming_value: Any) -> Any:
     )
 
 
-def merge_object_reducer(current_value: Any, incoming_value: Any) -> Any:
+def merge_object_reducer(
+    current_value: Any, incoming_value: Any, _config: Mapping[str, Any]
+) -> Any:
     """Shallow-merge object values at one exact state path."""
     if current_value is None:
         if not isinstance(incoming_value, dict):
@@ -39,7 +47,9 @@ def merge_object_reducer(current_value: Any, incoming_value: Any) -> Any:
     return current_value | incoming_value
 
 
-def set_union_reducer(current_value: Any, incoming_value: Any) -> Any:
+def set_union_reducer(
+    current_value: Any, incoming_value: Any, _config: Mapping[str, Any]
+) -> Any:
     """Merge list values while preserving stable first-seen order."""
     if current_value is None:
         current_items: list[Any] = []
@@ -58,7 +68,9 @@ def set_union_reducer(current_value: Any, incoming_value: Any) -> Any:
     return merged
 
 
-def max_reducer(current_value: Any, incoming_value: Any) -> Any:
+def max_reducer(
+    current_value: Any, incoming_value: Any, _config: Mapping[str, Any]
+) -> Any:
     """Keep the larger of the current and incoming values."""
     return (
         incoming_value if current_value is None else max(current_value, incoming_value)
@@ -73,20 +85,33 @@ DEFAULT_REDUCERS: Mapping[str, Reducer] = {
     "wf.std.max": max_reducer,
 }
 
+DEFAULT_REDUCER_SPECS: Mapping[str, ReducerSpec] = {
+    name: ReducerSpec(name=name) for name in DEFAULT_REDUCERS
+}
+
 
 def apply_reducer(
     *,
-    reducer_name: str,
+    reducer: ReducerRef,
     current_value: Any,
     incoming_value: Any,
     destination_path: str,
     reducers: Mapping[str, Reducer] = DEFAULT_REDUCERS,
+    reducer_specs: Mapping[str, ReducerSpec] = DEFAULT_REDUCER_SPECS,
 ) -> Any:
     """Apply one named pure reducer to a state write."""
-    reducer = reducers.get(reducer_name)
-    if reducer is None:
-        raise WorkflowExecutionError(f"unknown reducer {reducer_name!r}")
+    reducer_fn = reducers.get(reducer.name)
+    if reducer_fn is None:
+        raise WorkflowExecutionError(f"unknown reducer {reducer.name!r}")
+    spec = reducer_specs.get(reducer.name)
+    if spec is None:
+        raise WorkflowExecutionError(f"unknown reducer spec {reducer.name!r}")
+    validate_payload_against_schema(
+        spec.config_schema,
+        reducer.config,
+        f"reducer config for {reducer.name!r}",
+    )
     try:
-        return reducer(current_value, incoming_value)
+        return reducer_fn(current_value, incoming_value, reducer.config)
     except TypeError as exc:
         raise WorkflowExecutionError(f"{exc} at {destination_path!r}") from exc

src/wf_core/runtime/ops/state.py

@@ -4,6 +4,7 @@ from typing import Any
 
 from wf_core.errors import WorkflowExecutionError
 from wf_core.local_paths import LocalPathError, get_local_value, has_overlapping_paths
+from wf_core.models.reducers import ReducerRef
 from wf_core.models.steps import NodeUse
 from wf_core.models.workflow import Workflow
 from wf_core.paths import (
@@ -73,11 +74,11 @@ def write_state_value(
 
     declared_path = ".".join(parts)
     declared_field = workflow.state_schema.fields.get(declared_path)
-    reducer_name = declared_field.reducer if declared_field else "wf.std.replace"
+    reducer = declared_field.reducer if declared_field else ReducerRef(name="wf.std.replace")
     key_path = parts
     current_value = get_nested_value(state, key_path)
     merged_value = apply_reducer(
-        reducer_name=reducer_name,
+        reducer=reducer,
         current_value=current_value,
         incoming_value=value,
         destination_path=destination_path,