expose_functions() method of CmdStanModel fails with mysterious error message

[ksvanhorn]

Describe the bug
Calling model$expose_functions() with a CmdStanModel object results in the error message

Error in function_env$external && cmdstan_version() < "2.32" :
  invalid 'x' type in 'x && y'

To Reproduce

1. Create a model object m.
2. m$expose_functions()

Expected behavior
Either expose the functions or give an informative error message.

Operating system
macOS Ventura 13.4

CmdStanR version number
0.6.0.9000

Additional context
cmdstan_version() returns 2.31.0, so there should be an error message printed here. Investigating with the debugger, I find that function_env only has one name in it, compiled.

[jgabry]

I think this is the same issue as #784, where the error only occurs if the executable already exists, not anytime you create a model object. But your To Reproduce section seems to imply this always happens for you, is that right?

Does this example work?

stan_file <- write_stan_file(
 "
 functions {
   real a_plus_b(real a, real b) {
     return a + b;
   }
 }
 parameters {
   real x;
 }
 model {
   x ~ std_normal();
 }
 "
)
mod <- cmdstan_model(stan_file)
mod$expose_functions()
mod$functions$a_plus_b(1, 2)

For me this works fine. But if the executable already exists and then I create a model object I get the error you're mentioning (which we need to fix).

[ksvanhorn]

Ah, no, I didn't check for that. Your example does in fact work for me. You're right, the model I was using was already compiled.

[andrjohns]

Closed by #831