MSDN Magazine - October 2007 - (Page 19)

and then it maps this tree to a DLR AST. Some languages have their own intermediate tree so that they can perform analysis or support tools (code editors) over their ASTs. Tools need a tree that Block of statements for file is close to source code that users have typed in an editor. Many languages may have ASTs that will be similar to the DLR’s, but the DLR Function definition with ASTs will have more information body and name “yo” about semantics explicitly represented. You might think of the DLR AST as one of the various transformations that later stages of a compiler need. It is interesting to note that the DLR lazily compiles portions of the ASTs when the code runs. This is similar to the way the CLR uses JIT compilation only when code runs. You pay a little Figure 1 IronPython AST in the Debugger more in performance the first time the code runs, but subsequent executions are fast. Later on, has a Name member which is the name that will be bound to the you’ll see that pieces of the AST are not compiled until specific resulting function object. lines of code execute. Pictorially, with some details elided, the IronPython AST for the entire msdnmag.py file looks like Figure 2. Looking at the IronPython AST There is a mixture of visual representation in the tree. For the Let’s get back to running the program so we can look at the IronPython AST. First, hit F5 (Debug | SuiteStatement Continue) to stop at the breakpoint again when the IronPython engine starts parsing the contents of site.py to execute it. If you look in the Visual Studio FunctionDefinition PrintStatement Locals window, you can expand “cc”, then “SourceUnit”, Name: SymbolID {“yo”} Expressions: and then look at either Name or DisplayName to see Body: SuiteStatement CallExpression what ParseFile is processing. {Target: NameExpression {“yo”}} Hit F5 again. We stopped the last time at this {Args: “bill”} breakpoint. If you look in Locals now, you’ll see that SuiteStatement cc.SourceUnit.DisplayName is msdnmag.py. This is the file I added as a command-line argument to ipy.exe. In the Locals window, cc.SourceUnit.Name is AssignmentStatement ReturnStatement “__main__” because Python names the primary exLeft: Expression[] Expression: BinaryExpression ecuting module this way. {NameExpression {“text”}} {text + yourname} Right: ConstantExpression {“hello, ”} Now, hit F10 (Debug | Step Over) three times to step over the line ast = parser.ParseFileInput() BinaryExpression Now you can look at the IronPython AST in the deOp: Add Left: NameExpression {text} bugger, as shown in Figure 1. Right: NameExpression {yourname} You can see the AST root node is a SuiteStatement, representing a series of statements, and it has a NameExpression ConstantExpression Statements member. Expanding the first statement Name: {text} Value: {“hello, ”} (index zero), you see it holds a FunctionDefinition node. It has a Body member that is a SuiteStatement for the statements of the function’s definition, and it Figure 2 IronPython Abstract Syntax Tree for msdnmag.py CLR Inside Out october2007 19

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

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