Expr supports concurrent execution, but there are key differences in how its components handle concurrency. Here's what you need to know:

Thread-Safe: *program.Program

• Compiled programs are thread-safe and read-only. You can safely reuse a compiled *program.Program across multiple goroutines.
• Example:

program, err := expr.Compile(`X + Y`)
if err != nil {
    panic(err)
}

// Safe to run in multiple goroutines
go expr.Run(program, Env{X: 1, Y: 2})
go expr.Run(program, Env{X: 3, Y: 4})

Not Thread-Safe: *vm.VM

• VMs are not thread-safe. Each goroutine should create its own *vm.VM instance.
• You can manually create a pool of vm machines with *vm.VM.
• Each expr.Run() call creates a new VM.

Best Practices

1. Compile once, run many times: Reuse compiled programs to save on performance.
2. Use one VM per goroutine: Create a new VM for each concurrent execution to avoid conflicts.

In short: programs are reusable across threads, but VMs need to be per goroutine. Keep these in mind to safely and efficiently use Expr in concurrent environments.