Command Language Interfaces

Design Goals for Any Language

• Precision
• Compactness
• Ease in writing and reading
• Speed of learning
• Simplicity to reduce errors
• Ease of retention over time

Higher Level Language Design Goals

• Close correspondence between reality and language notation
• Convenience in carrying out manipulations relevant to user tasks
• Compatibility with existing notations
• Flexibility to accommodate both novice and expert users
• Expressiveness to encourage creativity
• Visual appeal

Constraints on Language

• Capacity for human beings to record notation
• Match between the recording and the display media
• Convenience in speaking

Successful languages evolve to serve goals within the constraints.

GOMS Model for Command Language Interfaces

basic goal - speed and flexibility

Step1. Think of and enter command verb

Step2. Think of and enter next argument

Step3. if more arguments then
         go to Step2

Step4. if command is incorrect then
         correct the command

Step5. Signal computer to process the command.

Step6. go to Step1

What makes a command language easy to learn and use?

Easy Command Synthesis

• User can think up command by analogy from previously learned commands
• Commands conform to simple rules rather than lots of unique special cases

Command Language Organization Strategies

• Simple command set
• Commands plus arguments
• Commands plus options and arguments
• Hierarchical command structure (command action, object argument, destination)

Benefits of Structures

• Ease of learning, use, and recall
• Easy to construct guidelines documents
• Permits discussion of impact of allowing exceptions

Command Language Abbreviation Strategies

• Simple truncation
• Drop vowel and use simple truncation
• First and last letters
• Standard abbreviations from other contexts
• First letter of each work or phrase
• Phonics (e.g. XQT for execution)
• Cross product languages {verbs} x {objects}

Command Language Guidelines

• Make command terms easy to remember
• Provide easy command synthesis method and abbreviation strategy
• Provide simple, consistent command structure
• Commands should be right grain size
• Parsimony (no more commands than really needed)
• Studies show a few commands are used a lot by most users
• Provide for command reuse (replay, re-entry, macros)
• Avoid unnecessary distinctions among commands