expression-level code, it hides syntactic structure and allows traditional text editing, providing high-density display and lower viscosity. Visible tree structure and drag-and-drop manipulation are used for higher-level code such as control flow and class declarations.

Bidirectional Mode Switching. Some blocks environments provide a bidirectional transformation between a traditional text language and a blocks representation of that language. These include Pencil Code (CoffeeScript) 2 (Figure 7), Code.org’s App Labb ( JavaScript), BlockEditor (Java), 21 and Tiled Grace (Grace). 13 Alice and Blockly provide non-editable views of text code.

The hypothesis that motivates the design of dual-mode tools is that users may benefit from the learnability of blocks in one mode, while they learn syntax and get the efficiency of text in the other mode. This goal requires the views be linked. For text to be safe for users who may want to return blocks, it must be possible to switch between modes.

In dual-mode editors, the text code is the primary representation of the program, and blocks are a projected user interface view derived by parsing. This approach allows the editor to fully represent text information such as spacing, but it also means the editor must allow syntax errors that would not have been possible in blocks. Error-recovery heuristics convert simple text syntax errors to special error blocks, but complex errors can prevent mode switching.

Comparing approaches. There is a trade-off between the two approaches to unifying blocks and text. While the dual-mode editors provide direct support for learning traditional text syntax such as JavaScript or Java, they also impose the cognitive overhead of working with syntax errors that can only be introduced in text mode. Visualization research on multiple coordinated views suggests the benefits of providing more than one view need to be balanced against the cognitive overhead imposed by switching between views. 37

Single mode structure editors have the advantage of a conceptual model free of syntax errors, because the primary object being edited is the abstract syntax tree. However, to main- b https://code.org/educate/applab

Figure 7. Pencil Code provides bidirectional switching between blocks and text. Mode switching allows users to learn with blocks and edit quickly with text.

Figure 8. Blocks programming in MadeUp. 3D printing is an area of rapid innovation, and blocks make it possible to use new 3D modeling languages without a steep learning curve.

Figure 9. The SPARQL playground is a blocks-based query execution tool that provides blocks for constructing queries of RDF data. Query results (bottom) are also provided as blocks, and they can be dragged to build into other queries.