feat!: Turn py expression lists into arrays (#697)

Closes #545.

BREAKING CHANGE: Lists in `py(...)` expressions are now turned into
Guppy arrays instead of lists.

guppylang/checker/expr_checker.py

@@ -106,6 +106,7 @@
 from guppylang.span import Span, to_span
 from guppylang.tys.arg import TypeArg
 from guppylang.tys.builtin import (
+    array_type,
     bool_type,
     get_element_type,
     is_bool_type,
@@ -1193,7 +1194,7 @@ def _python_list_to_guppy_type(
     representable in Guppy.
     """
     if len(vs) == 0:
-        return list_type(ExistentialTypeVar.fresh("T", False))
+        return array_type(ExistentialTypeVar.fresh("T", False), 0)
 
     # All the list elements must have a unifiable types
     v, *rest = vs
@@ -1207,4 +1208,4 @@ def _python_list_to_guppy_type(
         if (subst := unify(ty, el_ty, {})) is None:
             raise GuppyError(PyExprIncoherentListError(node))
         el_ty = el_ty.substitute(subst)
-    return list_type(el_ty)
+    return array_type(el_ty, len(vs))

guppylang/compiler/expr_compiler.py

@@ -13,7 +13,6 @@
 from hugr import val as hv
 from hugr.build.cond_loop import Conditional
 from hugr.build.dfg import DP, DfBase
-from hugr.std.collections import ListVal
 from typing_extensions import assert_never
 
 from guppylang.ast_util import AstNode, AstVisitor, get_type
@@ -55,8 +54,8 @@
 from guppylang.tys.builtin import (
     get_element_type,
     int_type,
+    is_array_type,
     is_bool_type,
-    is_list_type,
 )
 from guppylang.tys.const import BoundConstVar, ConstValue, ExistentialConstVar
 from guppylang.tys.subst import Inst
@@ -601,10 +600,18 @@ def python_value_to_hugr(v: Any, exp_ty: Type) -> hv.Value | None:
             if doesnt_contain_none(vs):
                 return hv.Tuple(*vs)
         case list(elts):
-            assert is_list_type(exp_ty)
+            assert is_array_type(exp_ty)
             vs = [python_value_to_hugr(elt, get_element_type(exp_ty)) for elt in elts]
             if doesnt_contain_none(vs):
-                return ListVal(vs, get_element_type(exp_ty).to_hugr())
+                # TODO: Use proper array value: https://github.com/CQCL/hugr/issues/1497
+                return hv.Extension(
+                    name="ArrayValue",
+                    typ=exp_ty.to_hugr(),
+                    # The value list must be serialized at this point, otherwise the
+                    # `Extension` value would not be serializable.
+                    val=[v._to_serial_root() for v in vs],
+                    extensions=["unsupported"],
+                )
         case _:
             # TODO replace with hugr protocol handling: https://github.com/CQCL/guppylang/issues/563
             # Pytket conversion is an experimental feature

tests/error/py_errors/list_empty.err

@@ -4,6 +4,6 @@ Error: Cannot infer type (at $FILE:6:9)
 5 | def foo() -> None:
 6 |     xs = py([])
   |          ^^^^^^ Cannot infer type variables in expression of type
-  |                 `list[?T]`
+  |                 `array[?T, 0]`
 
 Guppy compilation failed due to 1 previous error

tests/integration/test_py.py

@@ -76,7 +76,7 @@ def foo() -> int:
 
 def test_list_basic(validate):
     @compile_guppy
-    def foo() -> list[int]:
+    def foo() -> array[int, 3]:
         xs = py([1, 2, 3])
         return xs
 
@@ -85,7 +85,7 @@ def foo() -> list[int]:
 
 def test_list_empty(validate):
     @compile_guppy
-    def foo() -> list[int]:
+    def foo() -> array[int, 0]:
         return py([])
 
     validate(foo)
@@ -94,15 +94,15 @@ def foo() -> list[int]:
 def test_list_empty_nested(validate):
     @compile_guppy
     def foo() -> None:
-        xs: list[tuple[int, list[bool]]] = py([(42, [])])
+        xs: array[tuple[int, array[bool, 0]], 1] = py([(42, [])])
 
     validate(foo)
 
 
 def test_list_empty_multiple(validate):
     @compile_guppy
     def foo() -> None:
-        xs: tuple[list[int], list[bool]] = py([], [])
+        xs: tuple[array[int, 0], array[bool, 0]] = py([], [])
 
     validate(foo)