Communications of the ACM

guage creation toolkits, developed by Anthony Bau for use in Pencil Code, and also used by Code.org in their App Lab. It is newer and less mature than OpenBlocks and Blockly, but takes a unique approach that allows seamless bidirectional transformation between all blocks and textual code.

Summary

When a programming language is provided as a user interface that welcomes novice users, rather than as a technical tool only for experienced developers, we arrive at a new picture of what the programming environment should provide:

˲ Vocabulary should derive from recognition, not recall;

˲ Cognitive load should be lowered by chunking code;

˲ Grammar rules and types should be made visible;

˲ Program chunks should be directly manipulable;

˲ Low-viscosity editing should also be possible;

˲Coding environments should be available without the need for tool installation;

˲Simple concepts should be described with clear words and high-level abstractions;

˲ Runtime state and behavior should be visible; and,

˲ Examples should be easy to find and apply.

In short, for a programming tool to be usable by new or casual programmers, its design must focus on learnability. Blocks have proven to be effective at solving many of these problems. Although programming is still not nearly as widely learned as it should be, the progress made by blocks language interfaces can inspire us all to see that programming can be made more learnable. The art of programming is the original human-computer interaction, and it remains an unsolved usability challenge. We can still do more to make programming available to all.

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David Bau (davidbau@csail.mit.edu) is a Ph. D. student at MIT’s Computer Science and Artificial Intelligence Lab, Cambridge, MA.

Jeff Gray (gray@cs.ua.edu) is a professor in the Department of Computer Science at the University of Alabama, Tuscaloosa.

Caitlin Kelleher (ckelleher@cse.wustl.edu) is an assistant professor in the Department of Computer Science at Washington University, St. Louis, MO.

Josh Sheldon (jsheldon@csail.mit.edu) is Director of Programs and MI T App inventor at MIT’s Computer Science and Artificial Intelligence Lab, Cambridge, MA.