When things go wrong

Every diagnostic points at the source with a caret. These are the ones you will meet most often, each captured from a real run.

Type mismatch

The type checker is strict and has no implicit conversions:

error: type mismatch: expected f64, found bool
  --> mismatch.mag:2:9
  |
2 |     y = u > 0.0;
  |         ^^^^^^^

Undefined variable

Often a parameter used without self.. Parameters are read as self.kp, not kp:

error: undefined variable `kp`
  --> undef.mag:2:13
  |
2 |     u_out = kp * e;
  |             ^^

Missing semicolon

The caret sits at the end of the statement that needs the ;, not at the token that revealed it:

error: missing `;` before `y`
  --> semi.mag:3:28
  |
3 |     let scaled = self.k * u
  |                            ^

Unknown parameter path

A self. path walks the declared parameter tree and reports the model where the path ran out:

error: unknown parameter `self.motor.omega_init`: model `Motor` has no field `omega_init`
  --> path.mag:5:27
  |
5 |     let prev = self.motor.omega_init fby y;
  |                           ^^^^^^^^^^

Non-constant fby initial value

The initial value is evaluated before the first tick, so it may use parameters but not signals:

error: `fby` initial value must be a constant or an expression of parameters
  --> fbyinit.mag:2:9
  |
2 |     y = u fby (y + dt * u);
  |         ^

Unstepped model instance

Every declared instance is stepped exactly once per tick:

error: model instance `self.pid` is never stepped
  --> unstepped.mag:3:14
  |
3 | model Loop { pid: Pid, }
  |              ^^^

Unknown module

The first segment of a use path is a dependency from Magnet.toml, core, or crate:

error: unknown module `motors`; the first segment is a `Magnet.toml` dependency (or `core`), or `crate` for a module of this package, and the rest are directories under its `src/`. A hyphenated name is imported with underscores (`control-lib` → `control_lib`)
  --> unknown_pkg.mag:1:5
  |
1 | use motors::Dc;
  |     ^^^^^^

Ambiguous model

magnet simulate runs the one model that no other model instantiates. With several such models, it needs --model:

the package declares several models; pick one with --model: Logger, Plant

Exit codes

A diagnostic is exit code 1: the model is at fault. Code 2 is magnet’s own fault. The table is on the CLI reference.