Fix variance specification for GraphFunction (#270)

golem/core/optimisers/objective/objective.py

@@ -1,15 +1,21 @@
 import itertools
 from dataclasses import dataclass
 from numbers import Real
-from typing import Any, Optional, Callable, Sequence, TypeVar, Dict, Tuple, Union
+from typing import Any, Optional, Callable, Sequence, TypeVar, Dict, Tuple, Union, Protocol
 
 from golem.core.dag.graph import Graph
 from golem.core.log import default_log
 from golem.core.optimisers.fitness import Fitness, SingleObjFitness, null_fitness, MultiObjFitness
 
-G = TypeVar('G', bound=Graph, covariant=True)
-R = TypeVar('R', contravariant=True)
-GraphFunction = Callable[[G], R]
+G = TypeVar('G', bound=Graph, contravariant=True)
+R = TypeVar('R', covariant=True)
+
+
+class GraphFunction(Protocol[G, R]):
+    def __call__(self, graph: G) -> R:
+        ...
+
+
 ObjectiveFunction = GraphFunction[G, Fitness]