MSDN Magazine - October 2007 - (Page 20)

Block of statements in file Empty statement for IPy file comment Block of statements for rest of file ExpressionStatement wraps BoundAssignment to dispose of value Assignment of MethodCallExpression’s result to bound (known storage) variable named “yo” Figure 3 The DLR AST most part, if an AST node has a member that points to another AST node, the picture shows an arrow pointing to the subnode. Usually the subnode is named as the type of a member in the parent node. For example, the FunctionDefinition node has a member of type SuiteStatement, and the next node is a SuiteStatement. Sometimes a node contains a description of the subnodes using indentation and curly braces. I chose to show these nodes that way to simplify the diagram. Looking at the DLR AST IronPython is now ready to build the DLR AST so that the final stages of compilation can run. Below our breakpoint in ParseFile you can see the call to BindAndTransform, which transforms the IronPython AST into a DLR tree. During that process, IronPython resolves all identifiers so that their representation in the DLR tree points to the declaration/allocation information for the variables they represent. Of course, for some identifiers, they need to compile to an AST node that represents searching for the value at run time; for example, the identifier could resolve to a module member added at runtime or to a late-bound value supplied by a host via the module’s dictionary. There is a lot of detail in this section, perhaps of interest only if you’re looking to port a language to the DLR. If you only look at the picture of the DLR tree, note there are MethodCallExpression nodes and ActionExpression nodes. MethodCallExpression nodes compile to calls to an IronPython runtime function that performs an operation. Every time this function gets called, it performs the same work given the arguments passed to it. The ActionExpression node is particularly interesting because 20 msdnmagazine CLR Inside Out it represents an abstract operation or action. Abstract actions, for example, include fetching a member, indexing an object, calling an object, performing addition on objects, and so on. The DLR compiles ActionExpression nodes into dynamic call sites. Dynamic sites are a runtime mechanism that caches methods appropriate to specific combinations of action and argument types. These allow dynamic code to run faster with little or no static type information at compile time. I’ll explain dynamic sites in more detail later. Let’s get back to stepping through the program’s execution. Hit Shift+F11 (Debug | Step Out) twice to stop in SourceUnit.Compile, then use F10 (Step Over) to perform the assignment to the variable “block”, which will hold the root of the DLR AST for the entire file. Now we can look at the DLR AST. In the Visual Studio Locals window, expand the node called block, which is a CodeBlock AST node. This is the root node for the file of Python code. Part of translating a language’s specific syntax tree to the DLR’s AST is clearly designating where variables are declared (explicitly or implicitly) and which variables identifiers refer to. If you expand block’s variables member, you’ll see two variables declared in the module’s scope, “__name__” and “yo”. (Expand elements 0 and 1 to see the Variable object’s name members.) The first variable is a bound name that Python requires and creates for modules. The second is the module variable that holds our function definition. Before drilling into the body, take a quick look at a partial expansion of the DLR AST in Figure 3, which reflects a similar representation to the IronPython AST you saw earlier. Expand the Body property under the block local. Due to the Visual Studio debugger’s display characteristics, you need to expand the

Table of Contents Feed for the Digital Edition of MSDN Magazine - October 2007

Cover Contents Toolbox CLR Inside Out Basic Instincts Data Points Cutting Edge Pooled Threads WPF Threads Parallel Linq Parallel Performance Mobile Apps Test Run Foundations Windows with C++ Netting C++ .NET Matters { End Bracket } Net Nuptials

MSDN Magazine - October 2007

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.