4. First program

The smallest valid program has one line. Create a file first.pse and type:

guard my-agent

The word guard opens the description of one guard. my-agent is a name you choose.

What that one line implies

Everything you did not write gets a default value.

Setting	Default	Meaning
relation	not set	weights start from first contact, 0.15/0.15
sources	all untrusted	text from files, the web and tools is data, not instruction
mode	enforce	the guard really blocks
learning	frozen	criterion weights do not change
oscillation	auto	both checks turn on by themselves
attractors	the four base ones	always on
games	the default set	always on

Two built-in rules also run. They are not written and cannot be turned off:

1. If any tracked quantity tends to infinity, the step goes to a human (chapter 7).
2. If the oscillation verdict is “unknown” and the action is irreversible, the step goes to a human (chapter 9).

So even a one-line guard protects.

Checking the file

The check command reads the file and reports whether it is valid. Expected output:

$ pse check first.pse
first.pse: 1 guard, 0 errors
  guard my-agent
    relation     (none, prior 0.15/0.15)
    sources      all untrusted
    mode         enforce
    learn        frozen
    oscillation  auto
    attractors   4 base
    rules        0 written, 2 built-in

Writing rules

• A guard name uses lowercase letters, digits and hyphens, and starts with a letter: my-agent, payments2.
• Every line inside a guard is indented exactly two spaces.
• A comment starts with #. The guard does not read it.
• One file can hold several guards. Their names must differ.

# my first guard
guard my-agent

Try it

Change the name to My Agent and check the file. The check fails, because the name has a capital letter and a space:

$ pse check first.pse
first.pse:1: expected "guard NAME"

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Next: 5. Setting lines: relation, source, mode Part of The PSE Language, a textbook by Andrej Ristić. Try the examples in the PSE Playground.