  We present a structure-aware code editor, called Deuce, that is equipped with
direct manipulation capabilities for invoking automated program
transformations. Compared to traditional refactoring environments, Deuce
employs a direct manipulation interface that is tightly integrated within a
text-based editing workflow. In particular, Deuce draws (i) clickable widgets
atop the source code that allow the user to structurally select the
unstructured text for subexpressions and other relevant features, and (ii) a
lightweight, interactive menu of potential transformations based on the current
selections. We implement and evaluate our design with mostly standard
transformations in the context of a small functional programming language. A
controlled user study with 21 participants demonstrates that structural
selection is preferred to a more traditional text-selection interface and may
be faster overall once users gain experience with the tool. These results
accord with Deuce's aim to provide human-friendly structural interactions on
top of familiar text-based editing.
