Fn, function

In Rust the fn keyword can be used to declare a function. But we can also use fn in a type expression to store function pointers in a vector.

With a vec of function pointers, we can call functions based on an index. This eliminates the need for a match statement or if-else chain.

Example program

Consider this Rust example program—it has 4 functions declared at the top. In main, we create a Vec of function pointers.

fn no_action() -> &'static str {
    "no action"
}

fn core() -> &'static str {
    "core"
}

fn art() -> &'static str {
    "art"
}

fn top() -> &'static str {
    "top"
}

fn main() {
    // Create vector of function pointers.
    let mut lookup: Vec<fn() -> &'static str> = vec![];
    for _ in 0..128 {
        lookup.push(no_action);
    }
    // Store functions as function pointers at specific indexes.
    lookup[b'c' as usize] = core;
    lookup[b'a' as usize] = art;
    lookup[b't' as usize] = top;

    // Loop over this value and call function for each byte.
    let value = "cat";
    println!("VALUE: {}", value);
    for c in value.bytes() {
        println!("RESULT: {}, {}", c as char, lookup[c as usize]());
    }
}
VALUE: cat
RESULT: c, core
RESULT: a, art
RESULT: t, top

Lookup table notes

It is possible to use a lookup table of function pointers to replace match or if-statements in Rust. This will affect performance and should be tested.

A summary

With a function pointer lookup table, we can develop a simple bytecode interpreter. Each byte can be handled with a function call based on the byte value.