# `Monty.Frameworks.Environments.PositioningProcedures.PositioningProcedure`
[🔗](https://github.com/d-led/ex.tbp.monty/blob/v0.1.0/packages/ex_tbp_monty_core/lib/monty/frameworks/positioning_procedures/positioning_procedure.ex#L1)

How an experiment puts the agent somewhere sensible before the first step.

Mirrors
`tbp/monty/frameworks/environments/positioning_procedures.py::PositioningProcedure`,
a `Protocol` in Monty and a behaviour here, which is how this port spells every
abstract class (`docs/monty-differences.md` §2.6).

A procedure is asked for one step at a time and answers with actions the
environment should take, plus whether it is finished. It is *not* a motor
policy: it runs before the motor system does, against the environment directly,
because a run has to see something before a policy has anything to decide.

Monty's protocol also declares `depth_at_center` as a static method; that is a
function of `PositioningProcedures`, not of an implementation, so it is not a
callback here.

## State

Monty's procedures mutate themselves — the number of turns attempted, whether
the extra orientation step for multi-object scenes has been done. Here a step
answers `{result, procedure}`, and the caller keeps the procedure it was given
back, as everything else in this port threads state.

# `call`

```elixir
@callback call(procedure :: term(), observation :: map(), state :: map()) ::
  {Monty.Frameworks.Environments.PositioningProcedures.PositioningProcedureResult.t(),
   term()}
```

One step of positioning: actions for the environment, and the procedure's new state.

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*Consult [api-reference.md](api-reference.md) for complete listing*