feat: Return already-compiled hugrs from GuppyModule.compile (#247)

Closes #246 

I'm not sure where to add a test for this, as it's not integration nor
error testing.

guppylang/module.py

@@ -36,6 +36,10 @@ class GuppyModule:
     # Whether the module has already been compiled
     _compiled: bool
 
+    # If the hugr has already been compiled, keeps a reference that can be returned
+    # from `compile`.
+    _compiled_hugr: Hugr | None
+
     # Map of raw definitions in this module
     _raw_defs: dict[DefId, RawDef]
     _raw_type_defs: dict[DefId, RawDef]
@@ -61,6 +65,7 @@ def __init__(self, name: str, import_builtins: bool = True):
         self._imported_globals = Globals.default()
         self._imported_compiled_globals = {}
         self._compiled = False
+        self._compiled_hugr = None
         self._instance_func_buffer = None
         self._raw_defs = {}
         self._raw_type_defs = {}
@@ -162,10 +167,11 @@ def _check_defs(
         }
 
     @pretty_errors
-    def compile(self) -> Hugr | None:
+    def compile(self) -> Hugr:
         """Compiles the module and returns the final Hugr."""
         if self.compiled:
-            raise GuppyError("Module has already been compiled")
+            assert self._compiled_hugr is not None, "Module is compiled but has no Hugr"
+            return self._compiled_hugr
 
         # Type definitions need to be checked first so that we can use them when parsing
         # function signatures etc.
@@ -201,6 +207,7 @@ def compile(self) -> Hugr | None:
             defn.compile_inner(graph, all_compiled_globals)
 
         self._compiled = True
+        self._compiled_hugr = graph
         return graph
 
     def contains(self, name: str) -> bool:

tests/integration/test_basic.py

@@ -86,3 +86,16 @@ def func_name() -> None: ...
         if isinstance(n.op.root, ops.FuncDecl)
     ]
     assert def_op.name == "func_name"
+
+
+def test_compile_again():
+    module = GuppyModule("test")
+
+    @guppy(module)
+    def identity(x: int) -> int:
+        return x
+
+    hugr = module.compile()
+
+    # Compiling again should return the same Hugr
+    assert hugr is module.compile()