feat(py): ComposablePass protocol and ComposedPass for hugr-py

hugr-py/src/hugr/passes/__init__.py

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+"""A hugr-py passes module for hugr transformations."""

hugr-py/src/hugr/passes/composable_pass.py

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+"""A Protocol for a composable pass."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Protocol
+
+if TYPE_CHECKING:
+    from hugr.hugr.base import Hugr
+
+
+class ComposablePass(Protocol):
+    """A Protocol which represents a composable Hugr transformation."""
+
+    def __call__(self, hugr: Hugr) -> None:
+        """Call the pass to transform a HUGR."""
+        ...
+
+    def then(self, other: ComposablePass) -> ComposablePass:
+        """Perform another composable pass after this pass."""
+        # Provide a default implementation for composing passes.
+        pass_list = []
+        if isinstance(self, ComposedPass):
+            pass_list.extend(self.passes)
+        else:
+            pass_list.append(self)
+
+        if isinstance(other, ComposedPass):
+            pass_list.extend(other.passes)
+        else:
+            pass_list.append(other)
+
+        return ComposedPass(pass_list)
+
+
+@dataclass
+class ComposedPass(ComposablePass):
+    """A sequence of composable passes."""
+
+    passes: list[ComposablePass]
+
+    def __call__(self, hugr: Hugr):
+        """Call all of the passes in sequence."""
+        for comp_pass in self.passes:
+            comp_pass(hugr)