simple example to evaluate strings

[Josh Stratton]

I'm completely new to guile and am trying to find a simple example of
evaluating a string inside C with some kind of context.  I want to execute
small self-contained strings provided by a user.  I've looked at various
examples, but everything I've seen seems uses the shell, which seems like
more than I need.  The context doesn't need to persist between evals.

Is there an example that would allow me to do something like this?

float evaluateExpression(char* expression)
{
    if (!initialized) {
        scm_init_guile();
        initialized = true;
    }

    // making stuff up here
    some_guile_context = ...
    some_guile_context.add("someDef", 5); // add some definitionos the
expression might expect to be defined
    some_guile_response = some_guile_context.eval(expression); //
expression might use "someDef" like (* someDef 8)

    return scm_arg_to_float(scm_guile_response[0]);
}