Do what I mean (DWIM)

Do what I mean (DWIM) describes a style of user interface design in which a system tries to infer what the user intended and act on that, rather than executing the literal input verbatim. The system corrects trivial errors automatically, fills in omitted parameters with reasonable defaults, and picks an interpretation when the input is ambiguous.

The term was coined by Warren Teitelman in his DWIM package for BBN Lisp, part of his PILOT system, in the mid-1960s. It was adopted and elaborated in Interlisp, where DWIM was less a single feature than a pervasive philosophy. At the user interface, the system should make reasonable interpretations of unrecognized input, default omitted parameters to sensible values, and let the user converse with it as if with an agent that reads context.

The same idea survives in GNU Emacs. comment-dwim comments out a selected region if it is uncommented, and uncomments it when it is already commented, choosing comment syntax and indentation to suit the language at point. Magit’s magit-diff-dwim shows changes for the thing at point, guessing what to diff from cursor position alone. In both cases the command name advertises the behaviour. The function does the obvious thing in context, without further prompting.

Trade-offs

DWIM trades predictability for convenience. When the inference is right, the user is saved a step. When it is wrong, the system silently does something other than what was asked, and the user may not notice. Early critics of Teitelman’s package joked that DWIM stood for "Do What Teitelman Means" or "Do What Interlisp Means", because the corrections were tuned to the mistakes its author happened to make and no others.

This is the tension with the principle of least astonishment. A system that guesses intent can surprise the user when the guess is off. DWIM is best applied where the inference is cheap to verify and cheap to undo, and where the literal interpretation would be obviously wrong. It is risky where the wrong guess is destructive, hard to notice, or hard to reverse.

The robustness principle is a cousin. Be liberal in what you accept. DWIM extends that instinct from parsing to intent, guessing not just what the input said but what the user meant by it.

See also

References

  • Teitelman, W. (1966). PILOT: A Step toward Man-Computer Symbiosis. MIT Ph.D. Dissertation, Project MAC MAC-TR-32.
  • Teitelman, W. and Masinter, L. (1981). The Interlisp Programming Environment. Computer (IEEE) 14:4:25–33.