12. What the check rejects

The language is built so that a dangerous guard cannot even be written. A file that breaks a rule does not pass check and cannot be put to work.

Errors that protect PSE

These three bans protect the mathematical core. No syntax gets around them.

Line	Message	Why
w_s: 0.5	w_s is read-only	weights are computed, not set; that keeps the sum at 1
theta: 0.3	theta is read-only	θ comes from the relation class and from the conversation
remove attractor borderline	there is no "remove"	base attractors and the game set cannot be reduced

Errors in rules

Line	Message	Why
on pressure > 0.7	rule needs ": action"	a rule without an action does nothing
on pressure > high: deny	"high" is not a number	a threshold must be a number
on pressure > 0.7: block	unknown action "block"	only four actions exist
on w_f >= 0.9: deny	cannot read condition	only < and > exist
on dwell(ghost) -> infinity: deny	unknown attractor "ghost"	the attractor is neither a base one nor added with add

Errors in settings

Line	Message	Why
mode: strict	unknown mode "strict"	the modes are observe and enforce
mode: written twice	mode given twice	one setting, one line
learn: max 2	learn max must be above 0 and at most 1	a weight cannot move by more than 1
relation: best-friends	unknown relation class	the class must be in the specification’s list

Errors in the shape of the file

What is written	Message	Why
protect my agent from bad people	expected "guard NAME"	free text is not a program
relation: before the first guard line	expected "guard NAME"	a setting must belong to a guard
two guards with the same name	duplicate guard	a name must identify one guard
a line indented four spaces	indent must be two spaces	one way to write, one way to read

Example output

guard bad
  w_s: 0.5
  on pressure > 0.7

$ pse check bad.pse
bad.pse:2: w_s is read-only
bad.pse:3: rule needs ": action"
bad.pse: 1 guard, 2 errors

The number after the file name is the line number. The check reports all errors at once.

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Next: 13. Complete examples Part of The PSE Language, a textbook by Andrej Ristić. Try the examples in the PSE Playground.