Fix symbolic to numeric conversion using Symbolics.unwrap (#559)

[ChrisRackauckas-Claude]

Summary

This PR fixes issue #559 where solve throws a MethodError: cannot convert BasicSymbolic to a Float64 when creating an ODEProblem from a Basis.

Problem

When creating an ODEProblem from a recovered Basis, get_parameter_values was returning symbolic values (SymbolicUtils.BasicSymbolic{Real}) instead of numeric values. This caused the ODE solver to fail when trying to convert symbolic values for use in the integrator.

The issue occurred because:

1. Symbolics.getdefaultval() can return Num types which wrap symbolic values
2. zero(Symbolics.symtype(p)) returns a symbolic zero (type SymbolicUtils.BasicSymbolic{Real}), not a numeric zero

Solution

Modified both get_parameter_values and get_parameter_map functions in src/basis/type.jl to use Symbolics.unwrap() to extract numeric values from symbolic wrappers. This approach is cleaner and more appropriate than explicit Float64 conversion as it properly handles the symbolic unwrapping.

Changes

src/basis/type.jl

• Updated get_parameter_values to use Symbolics.unwrap(val)
• Updated get_parameter_map to use Symbolics.unwrap(val)
• Added documentation notes about unwrapping for ODEProblem compatibility

test/basis/basis.jl

• Added comprehensive test case "ODEProblem from Basis (Issue solve throws MethodError: cannot convert BasicSymbolc to a Float64 for ODEProblem created from Basis #559)"
• Tests parameters without defaults return unwrapped numeric zeros
• Tests parameters with defaults return unwrapped numeric values
• Verifies ODEProblem creation without MethodError
• Confirms ODE solver can run without symbolic conversion errors

Testing

All 261 tests pass ✅

The new test specifically validates:

• get_parameter_values returns non-symbolic values (not Num types)
• get_parameter_map returns non-symbolic values
• ODEProblem(basis, u0, tspan, params) works without MethodError
• solve(problem, Tsit5()) doesn't throw symbolic conversion errors

Notes

This approach using Symbolics.unwrap() is preferred over Float64() conversion as it:

• Properly handles the symbolic type system
• Maintains type flexibility where appropriate
• Directly addresses the root cause (symbolic wrapping)

Fixes #559

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com