Hello World¶

Welcome to the Dyna tutorial!

It is traditional to start by writing and running a program that prints hello world. Downlad Dyna and follow the instructions in README.md to build it. Then, look at the file examples/helloworld.dyna (or here). It should contain:

goal += hello*world.        % an inference rule for deriving values
hello := 6.                 % some initial values
world := 7.

This does not print hello world. It was the closest we could come. Dyna is a pure language. It focuses on computation, and sniffs haughtily at mundane concerns like input and output.

Running Hello World¶

After building Dyna, you may ask our interpreter to run helloworld by executing

./dyna examples/helloworld.dyna

At this point, you should see:

Charts
============
goal/0
=================
goal                           := 42

hello/0
=================
hello                          := 6

world/0
=================
world                          := 7

What has happened? Dyna has compiled and executed the program requested and printed out its conclusions. Notably, the item goal is seen to have value 42. Whenever the runtime prints all of its conclusions, they are organized by functor

The Interactive Interpreter¶

Dyna also comes with an interactive interpreter. This mode allows you to

append new rules to the program and observe the consequences
make custom queries of the conclusions
visualize the information flow within the program

To run a program interactively, add -i to the dyna command line:

./dyna -i examples/helloworld.dyna

In addition to the chart printout above, you will be greeted with the interpreter’s prompt, :-. Interactive help is available by typing help at the prompt.

Let’s try adding a new rule to the program. Suppose that our goal is not merely to multiply hello by world but to additionally square hello. At the prompt, type:

goal += hello**2.

The interpreter will respond with:

goal := 78

Here you can see that goal‘s value has changed to be 78. But wait, is that right? We can check by typing at the prompt:

query hello**2

bug

The output for the query is not especially friendly. There’s a bug filed about that and it’s being worked on.

If we modify one of the inputs hello or world, by typing:

hello += 1.

The interpreter will respond with:

goal := 120
hello := 8
out(3) := [(64, {})]

So not only is it telling us that hello has changed, and that goal now takes on a new value as a result, but it reminds us that the query we ran earlier also has a new value.

At this point, we invite you to continue the tutorial by finding the shortest path.