# Hello World!

The first programming book that I read cover to cover was The C Programming Language (first edition) by Brian W. Kernighan and Dennis M. Ritchie, which was around 1980. I did not know at the time that 10 years later Dennis, the designer of the C programming language, would be my boss at AT&T Bell Labs in Murray Hill, NJ, while Brian would be my colleague in the same lab. The first C program in the book printed the string "hello, world". Since then, most programming tutorials for pretty much any programming language start with that example.

Harmony, too, has a Hello World program. Figure 2.1 shows the program and the corresponding output. After installation (see https://harmony.cs.cornell.edu), you can run it as follows from the command line:

$harmony -o hello1.png code/hello1.hny  For this to work, make sure harmony is in your command shell's search path. The code for examples in this book can be found in the code folder under the name listed in the caption of the example. If you need to, you can download the sources separately from https://harmony.cs.cornell.edu/sources.zip. In this case, the file code/hello1.hny contains the code in Figure 2.1. The -o hello1.png arguments tell harmony to write the output in the file hello1.png. The output is a Deterministic State Machine (DFA). The green circle represents the initial state and the double circle represents the final state. There is one transition, labeled with the string "hello world". The DFA describes (or recognizes) all possible outputs that the program can generate. In this case, there is only one. hello1.hny print "hello world"  hello3.hny print choose { "hello", "world" }  hello4.hny while choose { False, True }: print "hello world"  But programs can usually have more than one execution and produce multiple different outputs as a result. This is usually as a result of different inputs, but Harmony programs do not have inputs. Instead, Figure 2.2 demonstrates nondetermistic choice in Harmony programs. In this case, the program chooses to print either "hello" or "world". The corresponding DFA captures both possibilities. You can think of the choose operator as enumerating all possible inputs to the program. Figure 2.3 shows a program that has an infinite number of outputs by using a loop. Harmony usually requires that any program must be able to terminate, so the loop is conditioned on a nondeterministic choice between False and True. The possible outputs consist of zero or more copies of the string "hello world". Note that this single state DFA (where the initial state and the final state happen to be the same) captures an infinite number of possible outputs. Figure 2.4 demonstrates methods and threads in Harmony. In Figure 2.4(a), the code simply prints the strings "hello" and "world", in that order. Notice that this leads to an intermediate state after "hello" is printed but before "world" is. However, there is still only one execution possible. Figure 2.4(b) shows two threads, one printing "hello" and one printing "world". Because the threads run concurrently, the program can either output "hello world" or "world hello". Printing in Harmony is atomic, so "hweolrllod" is not a possible output. hello7.hny def hello(name): print "hello" print name spawn hello("Lesley") spawn hello("Robbert")  hello8.hny def hello(name): atomically: print "hello" print name spawn hello("Lesley") spawn hello("Robbert")  Figure 2.5 shows two threads, one printing the strings "hello" and "Robbert", while the other prints "hello" and "Lesley". Now there are four possible outputs depending on how the two threads are interleaved, including "hello hello Lesley Robbert". This is probably not what the programmer wanted. Figure 2.6 shows another important feature of Harmony: atomic blocks. The program is similar to Figure 2.5, but the programmer specified that the two print statements in a thread should be executed as an atomic unit. As a result, there are only two thread interleavings possible. Harmony is a programming language that borrows much of Python's syntax. Like Python, Harmony is an imperative, dynamically typed, and garbage collected programming language. There are also some important differences: • Harmony only supports basic operator precedence or associativity. Use parentheses liberally to remove ambiguity. • Harmony does not support floating point; • Python is object-oriented, supporting classes with methods and inheritance; Harmony has objects but does not support classes. On the other hand, Harmony supports pointers to objects and methods. There are also less important differences that you will discover as you get more familiar with programming in Harmony. triangle.hny const N = 10 def triangle(n): # computes the n'th triangle number result = 0 for i in {1..n}: # for each integer from 1 to n inclusive result += i # add i to result x = choose {0..N} # select an x between 0 and N inclusive assert triangle(x) == ((x * (x + 1)) / 2)  Figure 2.7 shows another example of a Harmony program. The example is a sequential program and has a method triangle that takes an integer number as argument. Each method has a variable called result that eventually contains the result of the method (there is no return statement in Harmony). The method also has a bound variable called n containing the value of the argument. The { x..y } notation generates a set containing the numbers from x to y (inclusive). (Harmony does not have a range operator like Python.) The last two lines in the program are the most interesting. The first assigns to x some unspecified value in the range 0..N and the second verifies that triangle(x) equals $$x(x+1)/2$$. Running this Harmony program will try all possible executions, which includes all possible values for x. Try it out (here $ represents a shell prompt):

$harmony triangle.hny #states 13 13 components, 0 bad states No issues$


The assert statement checks that the output is correct. If the program is correct, Harmony reports the size of the "state graph" (13 states in this case). If not, Harmony also reports what went wrong, typically by displaying a summary of an execution in which something went wrong.

In Harmony, constants have a default specified value, but those can be overridden on the command line using the -c option. For example, if you want to test the code for N = 100, run:

$harmony -c N=100 triangle.hny #states 103 103 components, 0 bad states No issues$


## Exercises

2.1 Write a Harmony program that uses choose instead of spawn to create the same output DFA as Figure 2.4(b).

2.2 Add the line print(x, triangle(x)) to the end of the program and create an output png file. Before you look at it, what do you think it should look like?

2.3 See what happens if, instead of initializing result to 0, you initialize it to 1. (You do not need to understand the error report at this time. They will be explained in more detail in Chapter 4.)

2.4 Write a Harmony program that computes squares by repeated adding. So, the program should compute the square of x by adding x to an initial value of 0 x times.