Add ControlDependence trait family to CTBase.Traits

- New strict opt-in family with tags ControlFree and WithControl for encoding control
  presence in OCPs at the type level, ready for dispatch in downstream packages
- Factor shared contract helpers _throw_missing_trait and _throw_trait_not_implemented to
  eliminate 70% duplication across time_dependence, variable_dependence, and mutability
  families; behaviour and messages unchanged
- Generalise _caller_function_name stacktrace filter to match any has_<family>_trait
  predicate (startswith + endswith pattern) instead of hard-coded names
- Collapse has_variable duplicate to direct alias is_variable(obj)
- Add 25 unit tests mirroring variable_dependence test structure; Traits suite: 301/301 ✓
- Document the two trait templates (strict opt-in vs default-valued capability) in
  module docstring; add control-dependence section to guide/traits.md with rationale
  for abstract-vs-concrete trait types (abstract used for type-param-only axes)
- Update api_reference.jl to include control_dependence.jl in the auto-generated API page

Follows the exact pattern of variable_dependence for consistency. No changes to
existing predicates; _caller_function_name detection now names is_control_free through
the new shared helper chain.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: ocots

docs/api_reference.jl

@@ -174,6 +174,7 @@ function generate_api_reference(src_dir::String)
                 joinpath("Traits", "mutability.jl"),
                 joinpath("Traits", "time_dependence.jl"),
                 joinpath("Traits", "variable_dependence.jl"),
+                joinpath("Traits", "control_dependence.jl"),
             ),
         ),
         (

docs/src/guide/traits.md

@@ -56,6 +56,22 @@ Traits.Fixed <: Traits.VariableDependence
 Traits.NonFixed <: Traits.VariableDependence
 ```
 
+### Control dependence
+
+Does the optimal control problem carry a control input ``u``?
+
+| Type | Meaning |
+|---|---|
+| [`Traits.ControlFree`](@ref CTBase.Traits.ControlFree) | no control: ``ẋ = f(t, x, v)`` |
+| [`Traits.WithControl`](@ref CTBase.Traits.WithControl) | control ``u``: ``ẋ = f(t, x, u, v)`` |
+
+Both are concrete subtypes of [`Traits.ControlDependence`](@ref CTBase.Traits.ControlDependence):
+
+```@repl traits
+Traits.ControlFree <: Traits.ControlDependence
+Traits.WithControl <: Traits.ControlDependence
+```
+
 ### Mutability
 
 Does a function allocate a new output, or write into a pre-allocated buffer?
@@ -78,15 +94,39 @@ below — they are passed directly as type arguments.
 | Automatic differentiation | [`Traits.WithAD`](@ref CTBase.Traits.WithAD), [`Traits.WithoutAD`](@ref CTBase.Traits.WithoutAD) |
 | Variable costate | [`Traits.SupportsVariableCostate`](@ref CTBase.Traits.SupportsVariableCostate), [`Traits.NoVariableCostate`](@ref CTBase.Traits.NoVariableCostate) |
 
-## The trait contract
-
-A type opts in to a trait by implementing **two methods**: one declaring that it
-*has* the trait, and one returning the trait *value*. The boolean predicates
-([`Traits.is_autonomous`](@ref CTBase.Traits.is_autonomous),
-[`Traits.is_variable`](@ref CTBase.Traits.is_variable), …) then follow
-generically — they are not implemented per type.
-
-For time and variable dependence:
+!!! note "Abstract tags vs. concrete tags"
+    Time-dependence tags (`Autonomous`, `NonAutonomous`) are **abstract** types
+    because they are only ever used as type-parameter *values* (e.g. `Model{Autonomous}`)
+    — never instantiated. All other tags (`Fixed`/`NonFixed`, `ControlFree`/`WithControl`,
+    `InPlace`/`OutOfPlace`, …) are **concrete** empty singletons. In practice every tag
+    is compared as a type (`control_dependence(obj) === Traits.ControlFree`), so the
+    two forms are used interchangeably; the distinction is historical, not semantic.
+
+## The trait contract — two templates
+
+Trait families follow one of **two contracts**, and the choice is dictated by a
+single question: **does a safe default value exist?**
+
+- **Strict opt-in (no safe default).** Time-, variable-, and control-dependence,
+  and mutability. An object is *either* autonomous or not, *either* control-free or
+  not — guessing silently would be a correctness bug, so there is no default. A type
+  must opt in by implementing **two methods**: one declaring that it *has* the trait,
+  one returning the trait *value*. The boolean predicates
+  ([`Traits.is_autonomous`](@ref CTBase.Traits.is_autonomous),
+  [`Traits.is_control_free`](@ref CTBase.Traits.is_control_free), …) then follow
+  generically. If a type does not opt in, the predicates throw rather than guess.
+- **Default-valued capability (safe default exists).** Automatic differentiation
+  and variable costate are *capabilities*: a conservative "no" is always safe. The
+  extractor returns a default for any object
+  ([`Traits.ad_trait`](@ref CTBase.Traits.ad_trait) ``→`` `WithoutAD`,
+  [`Traits.variable_costate_trait`](@ref CTBase.Traits.variable_costate_trait) ``→``
+  `NoVariableCostate`) and concrete types override it — no `has_*` method, no
+  predicates.
+
+The "Other families" above (integration mode, dynamics) use neither contract: they
+are read directly from a concrete type's parameters.
+
+For the strict opt-in families — time, variable and control dependence:
 
 ```@repl traits
 struct MyObject end
@@ -97,11 +137,16 @@ Traits.time_dependence(::MyObject) = Traits.NonAutonomous
 Traits.has_variable_dependence_trait(::MyObject) = true
 Traits.variable_dependence(::MyObject) = Traits.Fixed
 
+Traits.has_control_dependence_trait(::MyObject) = true
+Traits.control_dependence(::MyObject) = Traits.WithControl
+
 obj = MyObject()
 Traits.is_autonomous(obj)
 Traits.is_nonautonomous(obj)
 Traits.is_variable(obj)
 Traits.is_nonvariable(obj)
+Traits.is_control_free(obj)
+Traits.has_control(obj)
 ```
 
 For mutability, the same pattern applies with `has_mutability_trait` and `mutability`:
@@ -139,6 +184,7 @@ end # hide
 |---|---|
 | [`Traits.time_dependence`](@ref CTBase.Traits.time_dependence) | `is_autonomous`, `is_nonautonomous` |
 | [`Traits.variable_dependence`](@ref CTBase.Traits.variable_dependence) | `is_variable`, `is_nonvariable`, `has_variable` |
+| [`Traits.control_dependence`](@ref CTBase.Traits.control_dependence) | `is_control_free`, `has_control` |
 | [`Traits.mutability`](@ref CTBase.Traits.mutability) | `is_inplace`, `is_outofplace` |
 | [`Traits.ad_trait`](@ref CTBase.Traits.ad_trait) | — |
 | [`Traits.variable_costate_trait`](@ref CTBase.Traits.variable_costate_trait) | — |

src/Traits/Traits.jl

@@ -12,6 +12,7 @@ no runtime cost.
 - **abstract.jl**: Root abstract type `AbstractTrait` and family abstractions
 - **time_dependence.jl**: Time-dependence traits and the opt-in contract
 - **variable_dependence.jl**: Variable-dependence traits and the opt-in contract
+- **control_dependence.jl**: Control-dependence traits and the opt-in contract
 - **mutability.jl**: Mutability traits and the opt-in contract
 - **mode.jl**: Integration-mode traits (`EndPointMode`, `TrajectoryMode`)
 - **dynamics.jl**: Dynamics-type traits (`StateDynamics`, `HamiltonianDynamics`, `AugmentedHamiltonianDynamics`)
@@ -25,23 +26,39 @@ no runtime cost.
 
 - **Time dependence**: `TimeDependence`, `Autonomous`, `NonAutonomous`
 - **Variable dependence**: `VariableDependence`, `Fixed`, `NonFixed`
+- **Control dependence**: `ControlDependence`, `ControlFree`, `WithControl`
 - **Mutability**: `InPlace`, `OutOfPlace`
 - **Integration mode**: `EndPointMode`, `TrajectoryMode`
 - **Dynamics**: `StateDynamics`, `HamiltonianDynamics`, `AugmentedHamiltonianDynamics`
 - **Automatic differentiation**: `WithAD`, `WithoutAD`
 - **Variable costate**: `SupportsVariableCostate`, `NoVariableCostate`
 
-## Trait contract
-
-For time-dependence, variable-dependence, and mutability, a type opts in by
-implementing two methods — `has_<family>_trait` returning `true`, and an accessor
-(`time_dependence`, `variable_dependence`, `mutability`) returning the trait type.
-Boolean predicates (`is_autonomous`, `is_variable`, `is_inplace`, …) are then
-derived generically.
-
-The remaining families (`EndPointMode`, `StateDynamics`, `WithAD`,
-`SupportsVariableCostate`) are used as type parameters only and do not use the
-two-method contract.
+## Trait contracts — two templates
+
+Trait families follow one of two contracts. The choice is dictated by a single
+question: **does a safe default value exist?**
+
+**1. Strict opt-in (no safe default).** Time-dependence, variable-dependence,
+control-dependence, and mutability. Guessing a value silently would be a
+correctness bug (an object is *either* autonomous or not), so there is no default:
+a type must opt in by implementing two methods — `has_<family>_trait` returning
+`true`, and an accessor (`time_dependence`, `variable_dependence`,
+`control_dependence`, `mutability`) returning the trait type. The fallbacks throw
+loudly ([`CTBase.Exceptions.IncorrectArgument`](@ref) /
+[`CTBase.Exceptions.NotImplemented`](@ref)) via the shared helpers
+`_throw_missing_trait` / `_throw_trait_not_implemented`. Boolean predicates
+(`is_autonomous`, `is_variable`, `is_control_free`, `is_inplace`, …) are derived
+generically.
+
+**2. Default-valued capability (safe default exists).** Automatic differentiation
+(`ad_trait(::Any) = WithoutAD`) and variable-costate
+(`variable_costate_trait(::Any) = NoVariableCostate`). These are *capabilities*: a
+conservative "no" is always safe, so the extractor returns a default for any object
+and concrete types override it. No `has_<family>_trait` guard, no derived predicates.
+
+The remaining families (`EndPointMode`, `StateDynamics`) are used as type
+parameters only (read from a concrete type's parameters, e.g.
+`AbstractSystem{TD,VD,D}`) and use neither contract.
 """
 module Traits
 
@@ -65,6 +82,7 @@ include(joinpath(@__DIR__, "variable_costate.jl"))
 include(joinpath(@__DIR__, "mutability.jl"))
 include(joinpath(@__DIR__, "time_dependence.jl"))
 include(joinpath(@__DIR__, "variable_dependence.jl"))
+include(joinpath(@__DIR__, "control_dependence.jl"))
 
 # ==============================================================================
 # Module exports
@@ -82,10 +100,13 @@ export InPlace, OutOfPlace
 export WithAD, WithoutAD
 export SupportsVariableCostate, NoVariableCostate
 export VariableDependence, Fixed, NonFixed
+export ControlDependence, ControlFree, WithControl
 export ad_trait, variable_costate_trait, dynamics_trait
 export is_inplace, is_outofplace
 export is_autonomous, is_nonautonomous, is_variable, is_nonvariable, has_variable
+export is_control_free, has_control
 export has_time_dependence_trait, time_dependence, has_mutability_trait, mutability
 export has_variable_dependence_trait, variable_dependence
+export has_control_dependence_trait, control_dependence
 
 end # module Traits

src/Traits/control_dependence.jl

@@ -0,0 +1,158 @@
+"""
+$(TYPEDEF)
+
+Abstract supertype for control-dependence traits.
+
+Encodes, at the type level, whether an optimal control problem (or any object
+that opts into the trait) carries a control input.
+
+# Trait Pattern
+
+Objects that have a control-dependence trait must implement two methods:
+- `has_control_dependence_trait(obj::MyType) = true`: Indicates the type has this trait
+- `control_dependence(obj::MyType)`: Returns the specific trait value (`ControlFree` or `WithControl`)
+
+Once these are implemented, the object automatically gains:
+- `is_control_free(obj)`: Returns true if `control_dependence(obj)` is `ControlFree`
+- `has_control(obj)`: Returns true if `control_dependence(obj)` is `WithControl`
+
+If `has_control_dependence_trait` is not implemented or returns `false`,
+calling `is_control_free`, `has_control`, or `control_dependence` will throw an error
+indicating the object does not support control-dependence queries.
+
+See also: [`CTBase.Traits.ControlFree`](@ref), [`CTBase.Traits.WithControl`](@ref).
+"""
+abstract type ControlDependence <: AbstractTrait end
+
+"""
+$(TYPEDEF)
+
+Trait indicating the problem has no control input (control-free).
+
+A control-free optimal control problem has dynamics `ẋ = f(t, x, v)` with no
+control argument; the trajectory is determined by the state (and costate)
+equations alone, without a control law.
+
+See also: [`CTBase.Traits.WithControl`](@ref), [`CTBase.Traits.ControlDependence`](@ref).
+"""
+struct ControlFree <: ControlDependence end
+
+"""
+$(TYPEDEF)
+
+Trait indicating the problem has a control input.
+
+A problem with control has dynamics `ẋ = f(t, x, u, v)` depending on a control
+`u`; closing the loop (e.g. for a flow) requires a control law `u(t, x, p)`.
+
+See also: [`CTBase.Traits.ControlFree`](@ref), [`CTBase.Traits.ControlDependence`](@ref).
+"""
+struct WithControl <: ControlDependence end
+
+# =============================================================================
+# Check has trait
+# =============================================================================
+
+"""
+$(TYPEDSIGNATURES)
+
+Check if the object has the control-dependence trait.
+
+This fallback method throws an error indicating the object does not support
+control-dependence queries. Concrete types that have the trait should implement
+`has_control_dependence_trait(obj::MyType) = true`.
+
+The calling function name is automatically detected from the stacktrace
+for better error messages.
+
+# Arguments
+- `obj::Any`: The object to check.
+
+# Throws
+- [`CTBase.Exceptions.IncorrectArgument`](@ref): Always, indicating the object does not have the trait.
+
+See also: [`CTBase.Traits.ControlDependence`](@ref), [`CTBase.Traits.control_dependence`](@ref).
+"""
+function has_control_dependence_trait(obj::Any)
+    return _throw_missing_trait(
+        obj, :has_control_dependence_trait, :control_dependence, "control-dependence"
+    )
+end
+
+"""
+$(TYPEDSIGNATURES)
+
+Return the control-dependence trait value for the object.
+
+This fallback method throws an error indicating the method is not implemented.
+Concrete types that have the trait should implement `control_dependence(obj::MyType)`
+to return the specific trait value (`ControlFree` or `WithControl`).
+
+# Arguments
+- `obj::Any`: The object to query.
+
+# Throws
+- [`CTBase.Exceptions.NotImplemented`](@ref): Always, indicating the method must be implemented.
+
+See also: [`CTBase.Traits.ControlDependence`](@ref), [`CTBase.Traits.has_control_dependence_trait`](@ref).
+"""
+function control_dependence(obj::Any)
+    has_control_dependence_trait(obj)
+    return _throw_trait_not_implemented(
+        obj, :control_dependence, "control-dependence", "ControlFree or WithControl"
+    )
+end
+
+# =============================================================================
+# Trait accessors
+# =============================================================================
+
+"""
+$(TYPEDSIGNATURES)
+
+Return true if the object is control-free (has no control input).
+
+Checks that the object has the control-dependence trait, then returns true
+if `control_dependence(obj)` is `ControlFree`.
+
+# Arguments
+- `obj::Any`: The object to check.
+
+# Returns
+- `Bool`: true if the object has no control input.
+
+# Throws
+- [`CTBase.Exceptions.IncorrectArgument`](@ref): If the object does not support control-dependence queries.
+- [`CTBase.Exceptions.NotImplemented`](@ref): If `control_dependence` is not implemented for the object type.
+
+See also: [`CTBase.Traits.ControlDependence`](@ref), [`CTBase.Traits.has_control`](@ref).
+"""
+function is_control_free(obj::Any)
+    has_control_dependence_trait(obj)
+    return control_dependence(obj) === ControlFree
+end
+
+"""
+$(TYPEDSIGNATURES)
+
+Return true if the object has a control input.
+
+Checks that the object has the control-dependence trait, then returns true
+if `control_dependence(obj)` is `WithControl`.
+
+# Arguments
+- `obj::Any`: The object to check.
+
+# Returns
+- `Bool`: true if the object has a control input.
+
+# Throws
+- [`CTBase.Exceptions.IncorrectArgument`](@ref): If the object does not support control-dependence queries.
+- [`CTBase.Exceptions.NotImplemented`](@ref): If `control_dependence` is not implemented for the object type.
+
+See also: [`CTBase.Traits.ControlDependence`](@ref), [`CTBase.Traits.is_control_free`](@ref).
+"""
+function has_control(obj::Any)
+    has_control_dependence_trait(obj)
+    return control_dependence(obj) === WithControl
+end