Snap! (formerly BYOB) is a visual, drag-and-drop programming language. It is an extended reimplementation of Scratch (a project of the Lifelong Kindergarten Group at the MIT Media Lab) that allows you to Build Your Own Blocks. It also features first class[1] lists, first class procedures, and continuations[2]. These added capabilities make it suitable for a serious introduction to computer science for high school or college students.
Snap! runs in your browser. It is implemented using Javascript, which is designed to limit the ability of browser-based software to affect your computer, so it’s safe to run even other people’s projects, even if you don’t trust our competence or good intentions.
Snap! is presented by the University of California at Berkeley. It was developed by Jens Mönig at MioSoft Corporation (now at SAP), with design input and documentation by Brian Harvey at Berkeley, and contributions by students at Berkeley and elsewhere.
First Class Data Types
A data type is considered first class in a programming language if instances of that type can be
the value of a variable
a member of an aggregate (array, list, etc.)
an argument (input) to a procedure
the value returned by a procedure
used without having a name (being the value of a variable)
For example, numbers are first class in every language. Text strings are first class in many languages, but not in C, in which the relevant first class type is “pointer to a character.”
One of the limitations of Scratch as a language for computer science education is that its lists are not first class, so you can’t have a list of lists. Making lists first class is enough of an extension to Scratch to allow the creation of any other data structures (trees, heaps, hash tables, dictionaries, and so on) in user-defined Snap! code, implemented as lists of lists.
