Function Fn 

pub fn Fn<R: Runtime>(
    _ApplicationHandle: &AppHandle<R>,
    MethodName: &str,
    Parameters: Value,
) -> Result<MappedEffect, String>

Maps a string-based method name (command or RPC) to its corresponding effect constructor, returning a boxed closure (MappedEffect) that can be executed by the ApplicationRunTime.

Delegates to domain modules in priority order. The first module that returns Some(result) wins; unknown methods fall through to an error.