# Debugging in Music Blocks
*Learning is hard fun.*—Marvin Minsky
*Make the complicated comprehensible*—Arthur Miller
*Debugging is the learning opportunity of the 21st Century.* —
Cynthia Solomon
*The important message that comes from ideas about debugging is that
we learn from our mistakes; that the intricate process of making
things work or learning new skills has to do with hypothesizing,
testing, revising, etc.*—Cynthia Solomon
*Sometimes bugs are serendipitously adopted as features worth
perpetuating, sometimes procedures must be constructed to deal with
the phenomena caused by their appearance, and sometimes the bugs and
their side effects need to be removed. But in this pursuit, children
become creative researchers studying behavior, making up theories,
trying out ideas, etc.*—Cynthia Solomon
*6 Stages of Debugging*—Anonymous
1. That can't happen.
2. That doesn't happen on my machine.
3. That shouldn't happen.
4. Why does that happen?
5. Oh, I see.
6. How did that ever work?
----
Programming is hard. Composing music is also hard. Both programming
and composing involve some trial and error and serendipity. Inevitably
you will make mistakes along the way. Music Blocks provides a number
of mechanisms, reviewed below, to help you explore ideas and find
mistakes.
## TABLE OF CONTENTS
1. [Clicking on an Individual Stack of Blocks](#1-clicking-on-an-individual-stack-of-blocks)
2. [Print and Comment Blocks](#2-print-and-comment-blocks)
3. [Status Widget](#3-status-widget)
4. [Playback Modes](#4-playback-modes)
5. [Show and Hide blocks](#5-show-and-hide-blocks)
6. [Browser Console](#6-browser-console)
## 1. Clicking on an Individual Stack of Blocks
The *Play* button (in top left corner) will run all of the *Start*
blocks simultaneously. (Every Music Blocks project has at least one
*Start* block). But you can also run an individual stack of code by
clicking on a stack. This lets you test and debug small sections of
code, or, as in the example below, you can play a single voice by
clicking on one of the *Start* blocks or single phase by clicking on
one of the *Action* blocks. [RUN LIVE](https://musicblocks.sugarlabs.org/index.html?id=1732463245651983&run=True)
## 2. Print and Comment Blocks
[Back to Table of Contents](#table-of-contents)
The *Print* block (found on the *Extras* palette) can be used to print
a message while running a program. It is useful to determine if a
section of code is being executed when expected or if a box or
parameter contains an expected value.
The *Print* block is used to display the number of whole notes played,
in this case, `1/4 + 1/4 + 1/2`, which adds up to `1`, which is
displayed at the top of the browser window. [RUN LIVE](https://musicblocks.sugarlabs.org/index.html?id=1732474452574359&run=True)
The *Comment* block (also found on the *Extras* palette) is similar to
the *Print* block, except it only prints a message when the program is
being run in *Playback Slow* mode (See below). Comments are also
written to the browser console. [RUN LIVE](https://musicblocks.sugarlabs.org/index.html?id=1732825564345176&run=True)
## 3. Status Widget
[Back to Table of Contents](#table-of-contents)
The *Status widget* is a tool for inspecting the status of Music
Blocks as it is running. By default, the key, BPM, and volume are
displayed. Also, each note is displayed as it is played. There is one
row per voice in the status table. [RUN LIVE](https://musicblocks.sugarlabs.org/index.html?id=1732541757152077&run=True)
Additional *Print* blocks can be added to the *Status* widget to
display additional music factors, e.g., duplicate, transposition,
skip, [staccato](#MORE-TRANSFORMATIONS),
[slur](#MORE-TRANSFORMATIONS), and [graphics](#GRAPHICS) factors,
e.g., x, y, heading, color, shade, grey, and pensize.
You can do additional programming within the status block. In the
example above, `whole notes played` is multiplied by `4` (e.g. quarter notes)
before being displayed. [RUN LIVE](https://musicblocks.sugarlabs.org/index.html?id=1732553086132345&run=True)
## 4. Playback Modes
[Back to Table of Contents](#table-of-contents)
Clicking on the Play button will play your program at full speed.
(It will also hide the blocks while the program runs, which improves
performance.) But there are two other playback modes.
On the Secondary Menu, there are two other Play buttons.
During *Playback Slow* mode the program will pause between the execution
of each block and the block being executed will be highlighted. This is
useful for following program flow, ensuring that the sequence of blocks
being executed is what you expect. In addition, the value stored in any
box or parameter is displayed on the block as the program runs, so you
can "inspect" program elements as the program runs.
*Run Step by Step* advances one block per button press.
## 5. Show and Hide blocks
[Back to Table of Contents](#table-of-contents)
The *Show* and *Hide* blocks (found on the *Extras* palette) are
useful for setting
"[breakpoints](https://en.wikipedia.org/wiki/Breakpoint)" in your
program to debug a specific section of code. By putting a *Show* block
at the start of a problematic section of code and a *Hide* block at
the end of the section, your program can be run full speed until it
gets to the *Show* block. Then the blocks are displayed and
run in *Playback Slow* mode. When the *Hide* block is encountered, the
blocks are hidden and the program resumes running at full speed.
## 6. Browser Console
As Music Blocks runs, some debugging information is written to the
browser console, such as the notes being played and comments (See the
*Comment* block above). The console can be accessed by typing
`Ctrl-Shift-J` on most web browsers.
Shown above is the console output from three notes: `sol mi sol`.
[Back to Table of Contents](#table-of-contents)