An AST editor. There is one method per AST node; override the methods for which you need to change the AST, and leave the others untouched.

The default operation for “bottom“ non-leaf node types’ methods is to check editNode() for that node; if it returns true, the node is copied, editing the children, and if it returns false, the same node is returned without changes (shortcut). The default operation for leaf node types’ methods (nodes with no children) is to return the same node. (The default operation for non-“bottom” node types’ methods is inherited from ImmediateNarrowingTransformer, see there.) By itself, an Editor will not actually edit the AST – it’s only scaffolding that allows you to easily edit parts of the AST without having to bother with the deep nesting of the nodes.

For example:

class TraceFunctionEditor() satisfies Editor {
    shared actual FunctionDefinition transformFunctionDefinition(FunctionDefinition that) {
        value print = BaseExpression(MemberNameWithTypeArguments(LIdentifier("print")));
        value enterArgs = PositionalArguments(ArgumentList([StringLiteral("enter ``that.name.name``")]));
        value leaveArgs = PositionalArguments(ArgumentList([StringLiteral("leave ``that.name.name``")]));
        value enter = InvocationStatement(Invocation(print.copy(), enterArgs));
        value leave = InvocationStatement(Invocation(print.copy(), leaveArgs));
        return that.copy {
            definition = Block(concatenate([enter], that.definition.content, [leave]));
        };
    }
}

will insert a print statement at the beginning and end of every function block in the AST.