docs: update BREAKING.md and CHANGELOG.md for definition refactor in 0.9.15

Author: ocots

BREAKING.md

@@ -32,6 +32,30 @@ The old function names were misleading because they returned the dimension of du
 
 The functions `dim_*_constraints_box(ocp::Model)` (for Model, not Solution) remain unchanged and still refer to constraint dimension in the model.
 
+### Breaking Changes: Definition Getter Removal on PreModel
+
+The `definition` and `expression` getters on `PreModel` have been removed to maintain consistency with the rest of the codebase where `PreModel` fields are accessed directly.
+
+#### Migration Guide
+
+```julia
+# Before (removed getters)
+d = definition(pre)          # No longer available
+e = expression(pre)          # No longer available
+
+# After (direct field access)
+d = pre.definition            # Access the field directly
+e = expression(pre.definition) # Pass the definition to expression()
+```
+
+#### Rationale
+
+The removal aligns with the existing pattern in CTModels where `PreModel` fields are accessed directly (e.g., `pre.state`, `pre.control`, `pre.variable`) rather than through getter functions. Getters on `Model` remain unchanged.
+
+#### Note
+
+The `definition(ocp::Model)` and `expression(ocp::Model)` getters remain available and unchanged. Only the `PreModel` variants were removed.
+
 ### Non-Breaking Changes
 
 This release also introduces consistent variable and control checking functions without breaking existing functionality:

CHANGELOG.md

@@ -27,6 +27,44 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Display improvement**: Dual variables only displayed if model has declared constraints
 - **New exports**: `dim_dual_state_constraints_box`, `dim_dual_control_constraints_box`, `dim_dual_variable_constraints_box`
 
+#### Optional Definition with EmptyDefinition Sentinel
+
+- **Type hierarchy**: Introduced `AbstractDefinition` with concrete types `Definition(expr::Expr)` and `EmptyDefinition` (sentinel)
+- **Optional definition**: `PreModel.definition` defaults to `EmptyDefinition()` instead of `nothing`
+- **Model parametric**: `Model` is now parametric on `DefinitionType<<:AbstractDefinition`
+- **Expression getter**: Added `expression()` function to extract `Expr` from `AbstractDefinition`
+- **Build relaxation**: Removed precondition requiring definition in `build()`
+- **Display refactor**: Split `Display/print.jl` into 5 focused files by responsibility
+- **Code organization**: Moved definition setters to `Components/definition.jl`, Model getters to `Building/model.jl`
+
+#### API Enhancements
+
+```julia
+# Definition is now optional
+pre = PreModel()
+pre.definition isa EmptyDefinition  # true by default
+
+# Set definition via setter (auto-wraps Expr)
+definition!(pre, quote
+    t ∈ [0, 1], time
+    x ∈ R, state
+    u ∈ R, control
+    ẋ(t) == u(t)
+    ∫(0.5u(t)^2) → min
+end)
+
+# Extract expression
+expr = expression(pre.definition)  # Returns the Expr
+
+# Build without definition is now valid
+model = build(pre)  # Works even without definition
+```
+
+#### Breaking Changes
+
+- **PreModel getters removed**: `definition(pre::PreModel)` and `expression(pre::PreModel)` removed; use `pre.definition` and `expression(pre.definition)` instead
+- **Model getters unchanged**: `definition(model::Model)` and `expression(model::Model)` remain available
+
 #### Consistent Variable and Control Checking Functions
 
 - **New functions**: Added `is_variable()` and `is_control_free()` for checking problem properties