control-toolbox · ocots · Apr 3, 2026 · Apr 3, 2026 · Apr 3, 2026
diff --git a/BREAKING.md b/BREAKING.md
@@ -4,6 +4,38 @@
 
 This document describes breaking changes in CTModels releases and how to migrate your code.
 
+## [0.9.12-beta] - 2026-04-03
+
+### No Breaking Changes
+
+This release introduces enhancements without breaking existing functionality:
+
+#### New Default Behavior (Non-Breaking)
+
+- **Enhanced `time_grid()` function**: `time_grid(sol)` now works for `MultipleTimeGridModel` solutions without explicit component
+- **Default component**: Automatically uses `:state` when no component specified
+- **Full backward compatibility**: All existing code continues to work unchanged
+
+#### What Changed
+
+```julia
+# These still work exactly as before
+time_grid(sol_unified)              # UnifiedTimeGridModel (unchanged)
+time_grid(sol_multi, :state)        # MultipleTimeGridModel with explicit component
+time_grid(sol_multi, :control)      # MultipleTimeGridModel with explicit component
+
+# This now works (previously threw IncorrectArgument)
+time_grid(sol_multi)                # MultipleTimeGridModel without component (NEW)
+```
+
+#### Migration
+
+- **No action required**: Existing code continues to work
+- **Optional simplification**: Can use `time_grid(sol)` instead of `time_grid(sol, :state)` for state grid access
+- **Explicit still preferred**: Use explicit component specification when accessing non-state grids
+
+---
+
 ## [0.9.11] - 2026-03-31
 
 ### No Breaking Changes

diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -7,6 +7,58 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.9.12-beta] - 2026-04-03
+
+### 🚀 Enhancements
+
+#### Default Component for Multiple Time Grid Solutions
+
+- **Default time grid access**: `time_grid(sol)` now works for `MultipleTimeGridModel` without explicit component specification
+- **Sensible default**: When no component is specified, defaults to `:state` grid (most commonly used)
+- **Consistent API**: Same `time_grid(sol)` syntax works for both unified and multiple time grid solutions
+- **Backward compatibility**: All existing code with explicit component specification continues to work unchanged
+
+#### Improved Developer Experience
+
+- **Reduced verbosity**: No need to specify `:state` component for most common use case
+- **Intuitive behavior**: State trajectory is the natural default for optimal control problems
+- **Cleaner code**: Simplified access to time grids in multi-grid solutions
+
+### 📊 API Changes
+
+```julia
+# Before (required component specification)
+time_grid(sol_multi, :state)  # Required for MultipleTimeGridModel
+time_grid(sol_unified)        # Worked for UnifiedTimeGridModel
+
+# After (consistent behavior)
+time_grid(sol_multi)          # Now works! Defaults to :state
+time_grid(sol_multi, :state)  # Still works (explicit)
+time_grid(sol_unified)        # Still works (unchanged)
+```
+
+### 🔧 Internal Changes
+
+- **Default function**: Added `__time_grid_default_component()::Symbol = :state` in `defaults.jl`
+- **Method signature**: Updated `time_grid` method for `MultipleTimeGridModel` with default parameter
+- **Removed exception**: Eliminated method that threw `IncorrectArgument` for missing component
+- **Enhanced tests**: Updated test suites to verify new default behavior
+
+### 🧪 Testing
+
+- **Comprehensive coverage**: All existing tests pass with new behavior
+- **Default behavior tests**: Added tests for automatic component selection
+- **Compatibility verification**: Confirmed backward compatibility with explicit specifications
+- **Integration testing**: End-to-end testing of multi-grid workflows
+
+### 📝 Migration Notes
+
+- **No breaking changes**: Existing code continues to work unchanged
+- **Optional enhancement**: Can adopt new shorter syntax when convenient
+- **Explicit still supported**: `time_grid(sol, :component)` syntax remains fully functional
+
+---
+
 ## [0.9.11] - 2026-03-31
 
 ### 🔧 Internal Improvements

diff --git a/Project.toml b/Project.toml
@@ -1,6 +1,6 @@
 name = "CTModels"
 uuid = "34c4fa32-2049-4079-8329-de33c2a22e2d"
-version = "0.9.11"
+version = "0.9.12-beta"
 authors = ["Olivier Cots <olivier.cots@toulouse-inp.fr>"]
 
 [deps]

diff --git a/src/OCP/Building/solution.jl b/src/OCP/Building/solution.jl
@@ -985,7 +985,7 @@ function time_grid(
         <:AbstractDualModel,
         <:AbstractSolverInfos,
     },
-    component::Symbol,
+    component::Symbol = __time_grid_default_component(),
 )::TimesDisc
     # Clean and validate component symbol
     component_clean = clean_component_symbols((component,))[1]
@@ -1011,50 +1011,6 @@ end
 """
 $(TYPEDSIGNATURES)
 
-Return the time grid for solutions with multiple time grids (component must be specified).
-
-# Throws
-- `IncorrectArgument`: Always thrown for MultipleTimeGridModel without component specification
-
-# Notes
-This method enforces explicit component specification for solutions with multiple time grids
-to avoid ambiguity about which grid is being accessed.
-
-# Examples
-```julia-repl
-julia> time_grid(sol)  # ❌ Error for MultipleTimeGridModel
-julia> time_grid(sol, :state)  # ✅ Correct usage
-```
-"""
-function time_grid(
-    sol::Solution{
-        <:MultipleTimeGridModel,
-        <:AbstractTimesModel,
-        <:AbstractStateModel,
-        <:AbstractControlModel,
-        <:AbstractVariableModel,
-        <:AbstractModel,
-        <:Function,
-        <:ctNumber,
-        <:AbstractDualModel,
-        <:AbstractSolverInfos,
-    },
-)
-    # ⚠️ Applying Exception Rule: Missing component specification
-    throw(
-        CTBase.Exceptions.IncorrectArgument(
-            "Component must be specified for solutions with multiple time grids";
-            got="no component specified",
-            expected="time_grid(sol, :component) where component ∈ {:state, :control, :path}",
-            suggestion="Specify which time grid to access, e.g., time_grid(sol, :state)",
-            context="time_grid for MultipleTimeGridModel",
-        ),
-    )
-end
-
-"""
-$(TYPEDSIGNATURES)
-
 Return the objective value.
 
 """

diff --git a/src/OCP/Core/defaults.jl b/src/OCP/Core/defaults.jl
@@ -112,3 +112,13 @@ The default value is `:constant` for piecewise constant interpolation (direct me
 The other possible value is `:linear` for piecewise linear interpolation (indirect methods).
 """
 __control_interpolation()::Symbol = :constant
+
+"""
+$(TYPEDSIGNATURES)
+
+Return the default component for time grid access in multiple time grid solutions.
+
+The default value is `:state` since the state trajectory is typically the most
+commonly accessed component in optimal control problems.
+"""
+__time_grid_default_component()::Symbol = :state
diff --git a/test/suite/ocp/test_defaults.jl b/test/suite/ocp/test_defaults.jl
@@ -52,6 +52,7 @@ function test_defaults()
         Test.@testset "time and criterion defaults" begin
             Test.@test CTModels.OCP.__time_name() == "t"
             Test.@test CTModels.OCP.__criterion_type() == :min
+            Test.@test CTModels.OCP.__time_grid_default_component() == :state
         end
 
         Test.@testset "variable naming defaults" begin

diff --git a/test/suite/ocp/test_solution_multi_grids.jl b/test/suite/ocp/test_solution_multi_grids.jl
@@ -356,8 +356,8 @@ function test_solution_multi_grids()
                     successful=true,
                 )
 
-                # Should require component specification
-                Test.@test_throws Exceptions.IncorrectArgument CTModels.time_grid(sol)
+                # Should work with default component (state)
+                Test.@test CTModels.time_grid(sol) == T_state
 
                 # Should work with component specification
                 Test.@test CTModels.time_grid(sol, :state) == T_state
@@ -605,8 +605,10 @@ function test_solution_multi_grids()
                 )
             end
 
-            Test.@testset "Missing component specification" begin
-                Test.@test_throws Exceptions.IncorrectArgument CTModels.time_grid(sol)
+            Test.@testset "Default component specification" begin
+                # Should default to state grid
+                Test.@test CTModels.time_grid(sol) == T_state
+                Test.@test CTModels.time_grid(sol) == CTModels.time_grid(sol, :state)
             end
         end
 

diff --git a/test/suite/serialization/test_multi_grids.jl b/test/suite/serialization/test_multi_grids.jl
@@ -234,8 +234,9 @@ function test_multi_grids()
                 successful=CTModels.successful(sol_unified),
             )
 
-            # time_grid without component should throw error
-            Test.@test_throws Exceptions.IncorrectArgument CTModels.time_grid(sol_multi)
+            # time_grid without component should return state grid (default behavior)
+            Test.@test CTModels.time_grid(sol_multi) == T_state
+            Test.@test CTModels.time_grid(sol_multi) == CTModels.time_grid(sol_multi, :state)
 
             # Invalid component should throw error
             Test.@test_throws Exceptions.IncorrectArgument CTModels.time_grid(