expressions: Add casts

These are sometimes necessary. In the future the compiler should be able
to detect when they will be necessary so we don't have to find out at
runtime.

blueprintcompiler/language/expression.py

@@ -19,6 +19,7 @@
 
 
 from .common import *
+from .types import ClassName, TypeName
 
 
 expr = Pratt()
@@ -27,6 +28,10 @@ expr = Pratt()
 class Expr(AstNode):
     grammar = expr
 
+    @property
+    def gir_type(self):
+        return self.children[-1].gir_type
+
     def emit_xml(self, xml: XmlEmitter):
         self.children[-1].emit_xml(xml)
 
@@ -45,6 +50,13 @@ class IdentExpr(AstNode):
     def is_this(self):
         return self.parent_by_type(Scope).this_name == self.tokens["ident"]
 
+    @property
+    def gir_type(self):
+        if self.is_this:
+            return self.parent_by_type(Scope).this_type
+        else:
+            return None
+
     def emit_xml(self, xml: XmlEmitter):
         if self.is_this:
             raise CompilerBugError()
@@ -55,17 +67,33 @@ class IdentExpr(AstNode):
 class LookupOp(InfixExpr):
     grammar = [".", UseIdent("property")]
 
+    @property
+    def gir_type(self):
+        return None
+
     def emit_xml(self, xml: XmlEmitter):
         if isinstance(self.lhs, IdentExpr) and self.lhs.is_this:
             xml.put_self_closing("lookup", name=self.tokens["property"], type=self.parent_by_type(Scope).this_type)
         else:
-            xml.start_tag("lookup", name=self.tokens["property"])
+            xml.start_tag("lookup", name=self.tokens["property"], type=self.lhs.gir_type)
             self.lhs.emit_xml(xml)
             xml.end_tag()
 
 
+class CastExpr(AstNode):
+    grammar = ["(", TypeName, ")", Expr]
+
+    @property
+    def gir_type(self):
+        return self.children[TypeName][0].gir_type
+
+    def emit_xml(self, xml: XmlEmitter):
+        self.children[Expr][0].emit_xml(xml)
+
+
 expr.children = [
     Prefix(IdentExpr),
+    Prefix(CastExpr),
     Prefix(["(", Expr, ")"]),
     Infix(10, LookupOp),
 ]