# `BB.Dsl.ParamRef`
[🔗](https://github.com/beam-bots/bb/blob/main/lib/bb/dsl/param_ref.ex#L5)

A reference to a parameter for use in DSL fields.

Instead of providing a literal unit value in the DSL, users can reference
a parameter that will be resolved at runtime. This enables runtime-adjustable
configuration for values that would otherwise be compile-time constants.

## Usage

```elixir
parameters do
  group :motion do
    param :max_effort, type: {:unit, :newton_meter}, default: ~u(10 newton_meter)
  end
end

topology do
  link :base do
    joint :shoulder do
      limit do
        effort(param([:motion, :max_effort]))
      end
    end
  end
end
```

The `param/1` function creates a reference that:
- Is validated at compile-time to ensure the parameter exists
- Is resolved at robot startup to get the current parameter value
- Subscribes to parameter changes to keep the robot struct updated

# `t`

```elixir
@type t() :: %BB.Dsl.ParamRef{expected_unit_type: atom() | nil, path: [atom()]}
```

# `param`

```elixir
@spec param([atom()]) :: t()
```

Create a parameter reference for DSL fields.

The path should match a parameter defined in the `parameters` section of
the robot DSL.

## Examples

    param([:motion, :max_speed])
    param([:limits, :shoulder, :effort])

---

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