feat: Type check array subscripts (#420)

Closes #416. See #415 for context.

* Adds a new `SubscriptAccess` place that will be used to track array
subscripts during linearity checking
* This place is emitted when checking a subscript AST node
* Ensures that subscripts in inout positions also implement a
`__setitem__` method

guppylang/checker/core.py

@@ -60,10 +60,10 @@
 #:
 #: All places are equipped with a unique id, a type and an optional definition AST
 #: location. During linearity checking, they are tracked separately.
-Place: TypeAlias = "Variable | FieldAccess"
+Place: TypeAlias = "Variable | FieldAccess | SubscriptAccess"
 
 #: Unique identifier for a `Place`.
-PlaceId: TypeAlias = "Variable.Id | FieldAccess.Id"
+PlaceId: TypeAlias = "Variable.Id | FieldAccess.Id | SubscriptAccess.Id"
 
 
 @dataclass(frozen=True)
@@ -154,6 +154,45 @@ def replace_defined_at(self, node: AstNode | None) -> "FieldAccess":
         return replace(self, exact_defined_at=node)
 
 
+@dataclass(frozen=True)
+class SubscriptAccess:
+    """A place identifying a subscript `place[item]` access."""
+
+    parent: Place
+    item: Variable
+    ty: Type
+    item_expr: ast.expr
+    getitem_call: ast.expr
+    #: Only populated if this place occurs in an inout position
+    setitem_call: ast.expr | None = None
+
+    @dataclass(frozen=True)
+    class Id:
+        """Identifier for subscript places."""
+
+        parent: PlaceId
+        item: Variable.Id
+
+    @cached_property
+    def id(self) -> "SubscriptAccess.Id":
+        """The unique `PlaceId` identifier for this place."""
+        return SubscriptAccess.Id(self.parent.id, self.item.id)
+
+    @cached_property
+    def defined_at(self) -> AstNode | None:
+        """Optional location where this place was last assigned to."""
+        return self.parent.defined_at
+
+    @property
+    def describe(self) -> str:
+        """A human-readable description of this place for error messages."""
+        return f"Subscript `{self}`"
+
+    def __str__(self) -> str:
+        """String representation of this place."""
+        return f"{self.parent}[...]"
+
+
 PyScope = dict[str, Any]
 
 

guppylang/checker/expr_checker.py

@@ -24,6 +24,7 @@
 import sys
 import traceback
 from contextlib import suppress
+from dataclasses import replace
 from typing import Any, NoReturn, cast
 
 from guppylang.ast_util import (
@@ -35,12 +36,15 @@
     with_loc,
     with_type,
 )
+from guppylang.cfg.builder import tmp_vars
 from guppylang.checker.core import (
     Context,
     DummyEvalDict,
     FieldAccess,
     Globals,
     Locals,
+    Place,
+    SubscriptAccess,
     Variable,
 )
 from guppylang.definition.ty import TypeDef
@@ -56,13 +60,15 @@
     DesugaredListComp,
     FieldAccessAndDrop,
     GlobalName,
+    InoutReturnSentinel,
     IterEnd,
     IterHasNext,
     IterNext,
     LocalCall,
     MakeIter,
     PlaceNode,
     PyExpr,
+    SubscriptAccessAndDrop,
     TensorCall,
     TypeApply,
 )
@@ -447,7 +453,7 @@ def _synthesize_binary(
             node,
         )
 
-    def _synthesize_instance_func(
+    def synthesize_instance_func(
         self,
         node: ast.expr,
         args: list[ast.expr],
@@ -495,16 +501,37 @@ def visit_Compare(self, node: ast.Compare) -> tuple[ast.expr, Type]:
 
     def visit_Subscript(self, node: ast.Subscript) -> tuple[ast.expr, Type]:
         node.value, ty = self.synthesize(node.value)
+        item_expr, item_ty = self.synthesize(node.slice)
+        # Give the item a unique name so we can refer to it later in case we also want
+        # to compile a call to `__setitem__`
+        item = Variable(next(tmp_vars), item_ty, item_expr)
+        item_node = with_type(item_ty, with_loc(item_expr, PlaceNode(place=item)))
+        # Check a call to the `__getitem__` instance function
         exp_sig = FunctionType(
             [
-                FuncInput(ty, InputFlags.NoFlags),
+                FuncInput(ty, InputFlags.Inout),
                 FuncInput(ExistentialTypeVar.fresh("Key", False), InputFlags.NoFlags),
             ],
             ExistentialTypeVar.fresh("Val", False),
         )
-        return self._synthesize_instance_func(
-            node.value, [node.slice], "__getitem__", "not subscriptable", exp_sig
+        getitem_expr, result_ty = self.synthesize_instance_func(
+            node.value, [item_node], "__getitem__", "not subscriptable", exp_sig
         )
+        # Subscripting a place is itself a place
+        expr: ast.expr
+        if isinstance(node.value, PlaceNode):
+            place = SubscriptAccess(
+                node.value.place, item, result_ty, item_expr, getitem_expr
+            )
+            expr = PlaceNode(place=place)
+        else:
+            # If the subscript is not on a place, then there is no way to address the
+            # other indices after this one has been projected out (e.g. `f()[0]` makes
+            # you loose access to all elements besides 0).
+            expr = SubscriptAccessAndDrop(
+                item=item, item_expr=item_expr, getitem_expr=getitem_expr
+            )
+        return with_loc(node, expr), result_ty
 
     def visit_Call(self, node: ast.Call) -> tuple[ast.expr, Type]:
         if len(node.keywords) > 0:
@@ -550,7 +577,7 @@ def visit_MakeIter(self, node: MakeIter) -> tuple[ast.expr, Type]:
         exp_sig = FunctionType(
             [FuncInput(ty, InputFlags.NoFlags)], ExistentialTypeVar.fresh("Iter", False)
         )
-        expr, ty = self._synthesize_instance_func(
+        expr, ty = self.synthesize_instance_func(
             node.value, [], "__iter__", "not iterable", exp_sig
         )
 
@@ -574,7 +601,7 @@ def visit_IterHasNext(self, node: IterHasNext) -> tuple[ast.expr, Type]:
         exp_sig = FunctionType(
             [FuncInput(ty, InputFlags.NoFlags)], TupleType([bool_type(), ty])
         )
-        return self._synthesize_instance_func(
+        return self.synthesize_instance_func(
             node.value, [], "__hasnext__", "not an iterator", exp_sig, True
         )
 
@@ -584,14 +611,14 @@ def visit_IterNext(self, node: IterNext) -> tuple[ast.expr, Type]:
             [FuncInput(ty, InputFlags.NoFlags)],
             TupleType([ExistentialTypeVar.fresh("T", False), ty]),
         )
-        return self._synthesize_instance_func(
+        return self.synthesize_instance_func(
             node.value, [], "__next__", "not an iterator", exp_sig, True
         )
 
     def visit_IterEnd(self, node: IterEnd) -> tuple[ast.expr, Type]:
         node.value, ty = self.synthesize(node.value)
         exp_sig = FunctionType([FuncInput(ty, InputFlags.NoFlags)], NoneType())
-        return self._synthesize_instance_func(
+        return self.synthesize_instance_func(
             node.value, [], "__end__", "not an iterator", exp_sig, True
         )
 
@@ -714,6 +741,8 @@ def type_check_args(
     new_args: list[ast.expr] = []
     for inp, func_inp in zip(inputs, func_ty.inputs, strict=True):
         a, s = ExprChecker(ctx).check(inp, func_inp.ty.substitute(subst), "argument")
+        if InputFlags.Inout in func_inp.flags and isinstance(a, PlaceNode):
+            a.place = check_inout_arg_place(a.place, ctx, a)
         new_args.append(a)
         subst |= s
 
@@ -734,6 +763,43 @@ def type_check_args(
     return new_args, subst
 
 
+def check_inout_arg_place(place: Place, ctx: Context, node: PlaceNode) -> Place:
+    """Performs additional checks for place arguments in @inout position.
+
+    In particular, we need to check that places involving `place[item]` subscripts
+    implement the corresponding `__setitem__` method.
+    """
+    match place:
+        case Variable():
+            return place
+        case FieldAccess(parent=parent):
+            return replace(place, parent=check_inout_arg_place(parent, ctx, node))
+        case SubscriptAccess(parent=parent, item=item, ty=ty):
+            # Check a call to the `__setitem__` instance function
+            exp_sig = FunctionType(
+                [
+                    FuncInput(parent.ty, InputFlags.Inout),
+                    FuncInput(item.ty, InputFlags.NoFlags),
+                    FuncInput(ty, InputFlags.NoFlags),
+                ],
+                NoneType(),
+            )
+            setitem_args = [
+                with_type(parent.ty, with_loc(node, PlaceNode(parent))),
+                with_type(item.ty, with_loc(node, PlaceNode(item))),
+                with_type(ty, with_loc(node, InoutReturnSentinel(var=place))),
+            ]
+            setitem_call, _ = ExprSynthesizer(ctx).synthesize_instance_func(
+                setitem_args[0],
+                setitem_args[1:],
+                "__setitem__",
+                "not allowed in a subscripted `@inout` position",
+                exp_sig,
+                True,
+            )
+            return replace(place, setitem_call=setitem_call)
+
+
 def synthesize_call(
     func_ty: FunctionType, args: list[ast.expr], node: AstNode, ctx: Context
 ) -> tuple[list[ast.expr], Type, Inst]:

guppylang/checker/linearity_checker.py

@@ -18,6 +18,7 @@
     Locals,
     Place,
     PlaceId,
+    SubscriptAccess,
     Variable,
 )
 from guppylang.definition.value import CallableDef
@@ -169,6 +170,7 @@ def _reassign_inout_args(self, func_ty: FunctionType, args: list[ast.expr]) -> N
                 match arg:
                     case PlaceNode(place=place):
                         for leaf in leaf_places(place):
+                            assert not isinstance(leaf, SubscriptAccess)
                             leaf = leaf.replace_defined_at(arg)
                             self.scope.assign(leaf)
                     case arg if inp.ty.linear:

guppylang/nodes.py

@@ -94,6 +94,20 @@ class FieldAccessAndDrop(ast.expr):
     )
 
 
+class SubscriptAccessAndDrop(ast.expr):
+    """A subscript element access on an object, dropping all the remaining items."""
+
+    item: "Variable"
+    item_expr: ast.expr
+    getitem_expr: ast.expr
+
+    _fields = (
+        "item",
+        "item_expr",
+        "getitem_expr",
+    )
+
+
 class MakeIter(ast.expr):
     """Creates an iterator using the `__iter__` magic method.
 
@@ -205,7 +219,7 @@ class InoutReturnSentinel(ast.expr):
     """An invisible expression corresponding to an implicit use of @inout vars whenever
     a function returns."""
 
-    var: "Variable | str"
+    var: "Place | str"
 
     _fields = ("var",)
 

guppylang/prelude/builtins.py

@@ -655,7 +655,15 @@ class Array:
             "ArrayGet", args=[int_arg(), type_arg()], variable_remap={0: 1, 1: 0}
         ),
     )
-    def __getitem__(self: array[T, n], idx: int) -> T: ...
+    def __getitem__(self: array[L, n] @ inout, idx: int) -> L: ...
+
+    @guppy.hugr_op(
+        builtins,
+        custom_op(
+            "ArraySet", args=[int_arg(), type_arg()], variable_remap={0: 1, 1: 0}
+        ),
+    )
+    def __setitem__(self: array[L, n] @ inout, idx: int, value: L) -> None: ...
 
     @guppy.custom(builtins, checker=ArrayLenChecker())
     def __len__(self: array[T, n]) -> int: ...

tests/error/inout_errors/subscript_not_setable.err

@@ -0,0 +1,7 @@
+Guppy compilation failed. Error in file $FILE:24
+
+22:    @guppy(module)
+23:    def test(c: MyImmutableContainer) -> MyImmutableContainer:
+24:        foo(c[0])
+               ^^^^
+GuppyTypeError: Expression of type `MyImmutableContainer` is not allowed in a subscripted `@inout` position since it does not implement the `__setitem__` method

tests/error/inout_errors/subscript_not_setable.py

@@ -0,0 +1,28 @@
+from guppylang.decorator import guppy
+from guppylang.module import GuppyModule
+from guppylang.prelude.builtins import inout
+from guppylang.prelude.quantum import qubit, quantum
+
+module = GuppyModule("test")
+module.load(quantum)
+
+
+@guppy.declare(module)
+def foo(q: qubit @inout) -> None: ...
+
+
+@guppy.struct(module)
+class MyImmutableContainer:
+    q: qubit
+
+    @guppy.declare(module)
+    def __getitem__(self: "MyImmutableContainer" @inout, idx: int) -> qubit: ...
+
+
+@guppy(module)
+def test(c: MyImmutableContainer) -> MyImmutableContainer:
+    foo(c[0])
+    return c
+
+
+module.compile()

tests/error/type_errors/not_subscriptable.err

@@ -0,0 +1,7 @@
+Guppy compilation failed. Error in file $FILE:9
+
+7:    @guppy(module)
+8:    def foo(x: int) -> None:
+9:        x[0]
+          ^
+GuppyTypeError: Expression of type `int` is not subscriptable

tests/error/type_errors/not_subscriptable.py

@@ -0,0 +1,12 @@
+from guppylang.decorator import guppy
+from guppylang.module import GuppyModule
+
+module = GuppyModule("test")
+
+
+@guppy(module)
+def foo(x: int) -> None:
+    x[0]
+
+
+module.compile()

tests/error/type_errors/subscript_bad_item.err

@@ -0,0 +1,7 @@
+Guppy compilation failed. Error in file $FILE:10
+
+8:     @guppy(module)
+9:     def foo(xs: array[int, 42]) -> int:
+10:        return xs[1.0]
+                     ^^^
+GuppyTypeError: Expected argument of type `int`, got `float`