[cfe-dev] Writing C++ refactoring tools in Common Lisp

Christian Schafmeister chris.schaf at verizon.net
Tue Jan 7 09:52:27 PST 2014

On Jan 7, 2014, at 12:12 PM, Manuel Klimek <klimek at google.com> wrote:

> On Tue, Jan 7, 2014 at 5:16 PM, Christian Schafmeister <chris.schaf at verizon.net> wrote:
> Hey folks,
> I’ve written a new Common Lisp environment/compiler over the past two years.  
> It uses LLVM as the back-end (by exposing the LLVM C++ library to Common Lisp) and interoperates with C++ in that it has a template library like boost::python for exposing arbitrary C++ classes/functions to Common Lisp.  
> I want to do some automated refactoring to improve my C++ code so I’m exposing the Clang ASTMatcher and Refactoring libraries.
> I also think it would be really cool to write refactoring tools in a dynamic language like Lisp - without all the exhausting boilerplate required by the C++ approach.
> C++ refactoring tools actually don't require that much boilerplate… 

Sorry, my statement sounded like a criticism - I never mean to criticize - these languages are what they are. 
I was just speaking about my feelings from writing a few simple refactoring tools in C++ and my motivations for implementing a Common Lisp compiler and writing refactoring tools in Common Lisp.  I’m a new convert to Common Lisp - I just discovered it two years ago and most of my experience with it is writing a self-hosting compiler.

> So I’m exposing the Clang AST library to this Common Lisp environment with the goal of writing a general tool for writing C++ refactoring tools in Common Lisp.
> My goal is to mimic C++ ASTMatchers like:
> recordDecl(hasDescendant(
>     ifStmt(hasTrueExpression(
>         expr(hasDescendant(
>             ifStmt()))))))
> Using S-expressions: (record-decl (has-descendent (if-stmt (has-true-expression (expr (has-descendant( if-stmt)))))))    
> And the source-to-source translation code will be small lambda functions that use the resulting MatchResults.
> That way I don’t have to write a lot of documentation :-).
> Currently I’m looking at calling the ASTMatcher/Dynamic/VariantValue interface to build ASTMatchers from S-expressions - does that sound like a good idea - or is there a better approach I should be exploring?
> Well, if your boost shim can already match "arbitrary C++ classes / functions", then it should in principle be able to expose the AST matchers as is (without the need to go through the dynamic parsing).
> After all, the matchers are mostly just template functions :)

I was trying to figure out how to do that at first but the ASTMatchers looked more exotic than just simple functions.

For instance “namedDecl” has the type internal::VariadicDynCastAllOfMatcher<Decl,NamedDecl>
VariadicDynCastAllOfMatcher is defined as:

template <typename SourceT, typename TargetT>
class VariadicDynCastAllOfMatcher
    : public llvm::VariadicFunction<
        BindableMatcher<SourceT>, Matcher<TargetT>,
        makeDynCastAllOfComposite<SourceT, TargetT> > {
  VariadicDynCastAllOfMatcher() {}

llvm::VariadicFunction has many templated overloads of "operator()"
I haven’t written template code to automatically wrap variadic functions (as in functions that take … ellipses).

I can wrap functions that take ArrayRef’s though - is there a function like namedDecl(ArrayRef<ASTMatcher>) for ASTMatchers?

I’ve only been getting deep into C++ template programming for about two months so I’m still struggling with reading complex template code.

> Otherwise I think going the dynamic matcher route is a good approach.
> Cheers,
> /Manuel

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