Notes on Snap! Sample Code¶

Following are notes on each sketch used in the workshop.

Finch-Chrome¶

Finch-Chrome is the default sketch provided by BirdBrain and loaded by the Finch Connection App for ‘Level 4: Regular Snap!’. It includes all the Finch blocks which communicate with the robot, and so is the ancestor of all the other Finch sketches.

Note that the visible program appears on the Scripts tab for the default Sprite, and no code appears on the tab for the Stage. This is recommended so that your scripts can show debugging information on the canvas using ‘say’ and ‘think’, which can sometimes be helpful.

Finch-Animation-Deck¶

Finch-Animation-Deck provides an interface for driving the robot and recording complex actions into primitives. It is built using Finch-Drama-Library. It can be used as a starting point for creating scripted performances composed from motion primitives.

Finch-Box-Navigation¶

Finch-Box-Navigation is the code used in the Finch Box Navigation Demo video. It uses the plain Finch-Chrome library to implement two different strategies for navigating around a Finch Box placed on the floor. It is helpful to weight the box with something dense and heavy, as the Finch does make contact.

The first strategy uses the proximity sensors to detect a face, than performs an open-loop sequence of driving movements to drive around a corner and attempt to find the next face.

The second strategy adds deliberate collision to reduce positioning error, and works better.

Finch-Coordination-Intro¶

Finch-Coordination-Intro is used in Exercise - Robot Synchronization. It uses the plain Finch-Chrome library to implement multi-robot coordination using communication over a network server.

Finch-Drama-Library¶

Finch-Drama-Library is a reference library to serve as the foundation for more advanced robot scripting. It includes a number of new blocks to extend the basic Finch-Chrome library. The sketch includes a number of demonstration scripts on the Stage Scripts tab to test library features.

The library includes the following:

A precision teleoperation interface with action recorder. Keyboard commands can be used to puppet the robot while simultaneous building a list data structure which can be used for precise playback.
Action player to execute action compositions represented as list data. Individual actions can be scripted into sequences since actions can invoke other actions (non-recursively).
Operators to convert action recordings to and from a readable text form. This allows extracting action sequences out of Snap for sharing or direct editing using a text editor.
Object-oriented communications server interface. The interface maintains estimates of client-server clock skew for managing precision synchronization of multiple robots.

Finch-Precise-Coordination¶

Finch-Precise-Coordination is a followup to Finch-Coordination-Intro which adds an estimation of client-server clock skew, built using the plain Finch-Chrome library. It is largely superseded by Finch-Drama-Library except as a simpler example for understanding how time works across the network.

Finch-Sensorless-Parking¶

Finch-Sensorless-Parking is used in Exercise - Sensorless Parking. It uses the plain Finch-Chrome library to implement simple open-loop strategies for using collision with an obstacle for localization. It isn’t autonomous; it requires user input to initiate the different phases of the movement.

Finch-Sequencing¶

Finch-Sequencing needs to be rewritten to use Finch-Drama-Library, it is really a precursor to the library and not as capable. It implements an action recording and playback system to demonstrate list data structures. The data is stored on a per-Sprite basis, which isn’t the most obvious to use.

Finch-State-Machine¶

Finch-State-Machine is used in Exercise - State Transition Diagrams. It implements a single-loop polled state machine coding structure which can be used to implement arbitrary state transition graphs.

Snap-Value-Quirks¶

Snap-Value-Quirks is a technical exploration of some quirks and corner cases of the Snap! language. The actual language is partially defined in the Snap Manual and the help available within the interface, but many conditions are not well-specified. This sketch uses assertion tests to document a number of specific operator properties.

Some of the implicit properties complicate writing clean code for string and list processing. For example:

An empty input field has zero length, but is identified as a number, not a string.
A input field 0 is equal to either a computed number zero or a computed character ‘0’. I.e., input fields have ambiguous type as evaluated by the equality operator.
A single ‘space’ character identifies a number, not a string. Furthermore,the equality operator does not treat this value as equal to zero, but the multiplication operator does.
Splitting an empty string on whitespace yields a list with one element containing a zero-length value.

Sprite-Communication-Intro¶

Sprite-Communication-Intro is used in Exercise - Network Communication. It doesn’t use the robots; it demonstrates the use of the network communications server to coordinate graphical sprites across different computers.