Diagnostics cookbook

Every entry below starts from one of two small working models and breaks one thing. The broken and fixed snippets show only the sections that change; the rest of the model stays as written here. Each compiler message is copied verbatim from compiling the broken variant.

Base model A is a patch-only fire spread. Base model B is a pet model: grazing sheep with one action.

In base model A, params: was moved between setup: and step staged:.

Declaration sections (world:, params:, monitors:, patches:, pets:, …) must all come before the lifecycle sections (setup:, step:, stop when). Once the parser is inside a lifecycle section, a bare params: line can only be read as an agent-update header, so the message talks about agentsets rather than ordering. If a diagnostic complains about an agentset on a line where you wrote a section header, check the section order first.

In base model B, the sheep: block header lost its indentation under step:.

Sections own their bodies through indentation. A line at column zero is a new top-level section header, and sheep is not a section name, so the parser reports the un-indented line instead. Indent the whole agent block two spaces so it belongs to the step.

In base model A, the parentheses around the stop condition were dropped.

The stop condition must be in parentheses: stop when (...). Without them the grammar rejects the whole line, and because the line contains a bare =, the message guesses you meant an assignment, which is misleading here. If this message points at a stop when line, the fix is the parentheses, not the operator.

In base model B, the sheep step tests the patch underfoot with the old function form.

patch is a property of the current pet, like x, y, and heading, not a function call. The old patch-here()form was removed because the parentheses were misleading: it took no arguments and returned a writable reference. Older MathPets sources and habits from NetLogo's patch-here still produce it, so the compiler keeps a targeted message.

In base model B, the sheep step removes a sheep on the same patch with the old bundled predation helper.

kill-one hid three decisions in one call: which agent to pick, whether one was there at all, and what the killer gains. The replacement spells out each one: one-of picks, where (prey): handles the empty-patch case, and the energy gain is a plain assignment you can change on its own.

In base model B, the graze action was declared with an empty parameter list.

The rule across the language is parentheses if and only if the call has arguments. graze takes none, so it is declared and called bare, like the built-in commands die and set-random-position. The opposite mistake is caught too: calling an action that takes parameters without parentheses reports Parameterized action "graze" must be called with parentheses.

In base model B, the step calls munch but the action is declared as graze.

A bare identifier used as a statement inside a pet block is read as an action call. So this message covers more than typos: it also appears when you call an action declared on a different breed, or when a field name stands alone on a line. Check the actions: block of the breed named in the message; the action must be declared there or in the shared pets:-level actions: block.

In base model B, the sheep step was declared step staged: while still calling forward(1) and die.

step staged: is the two-phase mode: every read sees the state from before the section, and writes commit together at the end, which is what cellular-automaton rules need. Pet commands such as forward, hatch, and die are not supported there. Keep patch rules staged and put movement in a plain step:; a model can declare both, and they run in source order each tick.

In base model B, a monitor tries to read a field through self.

Monitors, observer-level setup statements, and stop when run as the observer, so there is no current agent for self to refer to. self is only valid inside an agent update block or an action body. At the observer level, aggregate over the agentset instead.

In base model B, the grass check was written the way a JavaScript author would guard a possibly-missing value.

MathPets has no undefined or null; the value for a missing agent is nobody. A declared field always holds a value of its type, so test it directly. A lookup that can miss, such as one-of sheep here or pet-at(...), is false when it finds nothing, so guard it by testing the result itself in a where clause.