Acknowledgements

We have been extremely lucky in our mentors. Jens cut his teeth in the company of the Smalltalk pioneers: Alan Kay , Dan Ingalls , and the rest of the gang who invented personal computing and object oriented programming in the great days of Xerox PARC . He worked with John Maloney , of the MIT Scratch Team , who developed the Morphic graphics framework that’s still at the heart of Snap!.

The brilliant design of Scratch, from the Lifelong Kindergarten Group at the MIT Media Lab , is crucial to Snap!. Our earlier version, BYOB, was a direct modification of the Scratch source code. Snap! is a complete rewrite, but its code structure and its user interface remain deeply indebted to Scratch. And the Scratch Team, who could have seen us as rivals, have been entirely supportive and welcoming to us.

Brian grew up at the MIT and Stanford Artificial Intelligence Labs , learning from Lisp inventor John McCarthy , Scheme inventors Gerald J. Sussman and Guy Steele , and the authors of the world’s best computer science book, Structure and Interpretation of Computer Programs , Hal Abelson and Gerald J. Sussman with Julie Sussman , among many other heroes of computer science. (Brian was also lucky enough, while in high school, to meet Kenneth Iverson , the inventor of APL .)

In the glory days of the MIT Logo Lab, we used to say, “Logo is Lisp disguised as BASIC.” Now, with its first class procedures, lexical scope, and first class continuations, Snap! is Scheme disguised as Scratch.

Four people have made such massive contributions to the implementation of Snap! that we have officially declared them members of the team: Michael Ball and Bernat Romagosa, in addition to contributions throughout the project, have primary responsibility for the web site and cloud storage . Joan Guillén i Pelegay has contributed very careful and wise analysis of outstanding issues, including help in taming the management of translations to non-English languages. Jadga Hügle , has energetically contributed to online mini-courses about Snap! and leading workshops for kids and for adults. Jens, Jadga, and Bernat are paid to work on Snap! by SAP, which also supports our computing needs.

We have been fortunate to get to know an amazing group of brilliant middle school(!) and high school students through the Scratch Advanced Topics forum, several of whom (since grown up) have contributed code to Snap!: Kartik Chandra , Nathan Dinsmore , Connor Hudson , Ian Reynolds , and Deborah Servilla . Many more have contributed ideas and alpha-testing bug reports. UC Berkeley students who’ve contributed code include Achal Dave . Kyle Hotchkiss , Ivan Motyashov , and Yuan Yuan . Contributors of translations are too numerous to list here, but they’re in the “About…” box in Snap! itself.

This material is based upon work supported in part by the National Science Foundation under Grants No. 1138596, 1143566, and 1441075; and in part by MioSoft, Arduino.org, SAP, and YC Research. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or other funders.

Snap! Reference Manual

Version 8.0

Snap! (formerly BYOB) is an extended reimplementation of Scratch (https://scratch.mit.edu) that allows you to Build Your Own Blocks. It also features ﬁrst class lists, ﬁrst class procedures, first class sprites, first class costumes, first class sounds, and first class continuations. These added capabilities make it suitable for a serious introduction to computer science for high school or college students.

In this manual we sometimes make reference to Scratch, e.g., to explain how some Snap! feature extends something familiar in Scratch. It’s very helpful to have some experience with Scratch before reading this manual, but not essential.

To run Snap!, open a browser window and connect to https://snap.berkeley.edu/run. The Snap! community web site at https://snap.berkeley.edu is not part of this manual’s scope.