Command language. For frequent users, command languages (discussed in
Chapter 8) provide a strong feeling of being in control. Users learn the syntax and
can often express complex possibilities rapidly, without having to read distracting
prompts. However, error rates are typically high, training is necessary, and
retention may be poor. Error messages and online assistance are hard to provide
because of the diversity of possibilities and the complexity of mapping from
tasks to interface concepts and syntax. Command languages and lengthier query
or programming languages are the domain of expert frequent users, who often
derive great satisfaction from mastering a complex set of semantics and syntax.
Powerful advantages include easy history keeping and simple macro creation.
Natural language. The hope that computers will respond properly to arbitrary
natural-language sentences or phrases engages many researchers and system
developers, in spite of limited success thus far. Natural-language interaction
usually provides little context for issuing the next command, frequently requires
clarification dialog, and may be slower and more cumbersome than the