The Rule of Least Surprise

From: Shalom Bresticker (Shalom.Bresticker@motorola.com)
Date: Sun Sep 21 2003 - 08:46:40 PDT

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    I saw this in Eric Raymond's "The Art of Unix Programming".

    He did not originate it, but no matter.

    He was referring to interface design,
    but we can apply it to language design as well.
    Just generalize it.

    This is what he says:

    Rule of Least Surprise: In interface design, always do the least surprising
    thing.

    (This is also widely known as the Principle of Least Astonishment.)

    The easiest programs to use are those that demand the least new learning from
    the user - or, to put it another way, the easiest programs to use are those that
    most effectively connect to the user's pre-existing knowledge.

    Therefore, avoid gratuitous novelty and excessive cleverness in interface
    design. If you're writing a calculator
    program, `+' should always mean addition! When designing an interface, model it
    on the interfaces of functionally
    similar or analogous programs with which your users are likely to be familiar.

    Pay attention to your expected audience. They may be end users, they may be
    other programmers, or they may be system administrators. What is least
    surprising can differ among these groups.

    Pay attention to tradition. The Unix world has rather well-developed conventions
    about things like the format of
    configuration and run-control files, command-line switches, and the like. These
    traditions exist for a good reason: to tame the learning curve. Learn and use
    them.

    (We'll cover many of these traditions in Chapter 5 and Chapter 10.)

             The flip side of the Rule of Least Surprise is to avoid making things
    superficially similar but
             really a little bit different. This is extremely treacherous because
    the seeming familiarity
             raises false expectations. It's often better to make things distinctly
    different than to make
             them ALMOST the same. -- Henry Spencer 

    --
Shalom Bresticker                           Shalom.Bresticker@motorola.com
Design & Reuse Methodology                             Tel: +972 9 9522268
Motorola Semiconductor Israel, Ltd.                    Fax: +972 9 9522890
POB 2208, Herzlia 46120, ISRAEL                       Cell: +972 50 441478

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