feat(checker): add python scope resolver

adapted from Pebble

midas/resolver/resolver.py

@@ -0,0 +1,112 @@
+import midas.ast.python as p
+
+
+class ResolverError(Exception): ...
+
+
+class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
+    def __init__(self):
+        self.locals: dict[p.Expr, int] = {}
+        self.scopes: list[dict[str, bool]] = []
+
+    def resolve(self, *objects: p.Stmt | p.Expr) -> None:
+        for obj in objects:
+            obj.accept(self)
+
+    def begin_scope(self):
+        self.scopes.append({})
+
+    def end_scope(self):
+        self.scopes.pop()
+
+    def declare(self, name: str) -> None:
+        if len(self.scopes) == 0:
+            return
+        scope: dict[str, bool] = self.scopes[-1]
+        if name in scope:
+            raise ResolverError(
+                f"A variable with the name {name} is already declared in this scope"
+            )
+        scope[name] = False
+
+    def define(self, name: str) -> None:
+        if len(self.scopes) == 0:
+            return
+        self.scopes[-1][name] = True
+
+    def resolve_local(self, expr: p.Expr, name: str) -> None:
+        for i, scope in enumerate(reversed(self.scopes)):
+            if name in scope:
+                self.locals[expr] = i
+                return
+
+    def resolve_function(self, function: p.Function) -> None:
+        self.begin_scope()
+        for param in function.all_args:
+            self.declare(param.name)
+            self.define(param.name)
+        self.resolve(*function.body)
+        self.end_scope()
+
+    def visit_expression_stmt(self, stmt: p.ExpressionStmt) -> None:
+        stmt.expr.accept(self)
+
+    def visit_function(self, stmt: p.Function) -> None:
+        # Declare before resolving body to allow recursion
+        self.declare(stmt.name)
+        self.define(stmt.name)
+        self.resolve_function(stmt)
+
+    def visit_type_assign(self, stmt: p.TypeAssign) -> None:
+        self.declare(stmt.name)
+        # NOTE: resolve type here?
+        self.define(stmt.name)
+
+    def visit_assign_stmt(self, stmt: p.AssignStmt) -> None:
+        self.resolve(stmt.value)
+        for target in stmt.targets:
+            match target:
+                case p.VariableExpr(name=name):
+                    self.resolve_local(target, name)
+                    # TODO: declare if not found
+                case _:
+                    raise Exception(f"Unsupported assignment to {target}")
+
+    def visit_binary_expr(self, expr: p.BinaryExpr) -> None:
+        self.resolve(expr.left)
+        self.resolve(expr.right)
+
+    def visit_compare_expr(self, expr: p.CompareExpr) -> None:
+        self.resolve(expr.left)
+        self.resolve(expr.right)
+
+    def visit_unary_expr(self, expr: p.UnaryExpr) -> None:
+        self.resolve(expr.right)
+
+    def visit_call_expr(self, expr: p.CallExpr) -> None:
+        self.resolve(expr.callee)
+        for arg in expr.arguments:
+            self.resolve(arg)
+        for arg in expr.keywords.values():
+            self.resolve(arg)
+
+    def visit_get_expr(self, expr: p.GetExpr) -> None:
+        self.resolve(expr.object)
+
+    def visit_literal_expr(self, expr: p.LiteralExpr) -> None:
+        pass
+
+    def visit_variable_expr(self, expr: p.VariableExpr) -> None:
+        if len(self.scopes) != 0 and self.scopes[-1].get(expr.name) is False:
+            raise ResolverError(
+                f"Cannot use local variable '{expr.name}' in its own initializer"
+            )  # aka. UnboundLocalError
+        self.resolve_local(expr, expr.name)
+
+    def visit_logical_expr(self, expr: p.LogicalExpr) -> None:
+        self.resolve(expr.left)
+        self.resolve(expr.right)
+
+    def visit_set_expr(self, expr: p.SetExpr) -> None:
+        self.resolve(expr.value)
+        self.resolve(expr.object)