Function evaluate_expr_async 

pub fn evaluate_expr_async<'a>(
    expr: &'a Expression,
    env: &'a Environment,
    tools: &'a ToolRegistry,
    ctx: &'a BlockEvalContext<'a>,
) -> Pin<Box<dyn Future<Output = GentResult<Value>> + 'a>>

Evaluate an expression in an async context, handling function calls

This function is similar to evaluate_expr but supports async tool calls. The ctx parameter provides optional LLM client for agent execution.