go/ast: adding a new ast node

[Abhishek Kulkarni]

I couldn't find any examples or any tests/fixes (in gofix) that

do this, but how do I add new nodes to an AST while walking

it?

more specifically, i want to, let's say, change:

   var a Foo

to

   var a Foo

   a = 42

Is it possible to do this?

Thanks,

Abhishek

[John Asmuth]

It doesn't compile anymore (since I don't maintain it), but https://github.com/skelterjohn/gorf does this kind of thing, and I imagine the relevant bits for your query still make sense. It's a refactoring tool, so it rewrites go code by reading in the ast, manipulating it, and pretty-printing the result.

[Evan Shaw]

Yes, you can do this. Say you have an *ast.Block b that's populated
with statements and you want to insert a statement at index i.

// first put together the statement "a = 42"
identA := ast.NewIdent("a")
fortyTwo := &ast.BasicLit{Kind: token.INT, Value: "42"}
assignment := &ast.AssignStmt{Lhs: []ast.Expr{identA}, Rhs:
[]ast.Expr{fortyTwo}}

// allocate a new statement list big enough for our new statement
list := make([]ast.Stmt, len(b.List) + 1)
copy(list, b.List[:i])
list[i] = assignment
copy(list[i+1:], b.List[i:])
b.List = list

(In some cases you could use append instead of explicitly allocating a
new list if you know you're adding to the end of the block.)

- Evan

[John Asmuth]

On Thursday, August 4, 2011 6:16:40 PM UTC-4, Evan Shaw wrote:

(In some cases you could use append instead of explicitly allocating a
new list if you know you're adding to the end of the block.)

 The whole point of append is that it will do the allocation for you.

http://code.google.com/p/go-wiki/wiki/SliceTricks

Though the insert trick is a bit cumbersome, admittedly.

[Evan Shaw]

On Fri, Aug 5, 2011 at 1:46 PM, John Asmuth <jas...@gmail.com> wrote:
>  The whole point of append is that it will do the allocation for you.
> http://code.google.com/p/go-wiki/wiki/SliceTricks

That's a neat trick, but I think my way is clearer even though it's more code.

- Evan

[Russ Cox]

> // allocate a new statement list big enough for our new statement
> list := make([]ast.Stmt, len(b.List) + 1)
> copy(list, b.List[:i])
> list[i] = assignment
> copy(list[i+1:], b.List[i:])
> b.List = list

var list []ast.Stmt // or = make([]ast.Stmt, 0, len(b.List)+1) if you care
list = append(list, b.List[:i]...)
list = append(list, assignment)
list = append(list, b.List[i:]...)
b.List = list

Russ