Writing a generic max function

[Bill Cox]

Rune's polymorphism is primarily based on Python-like function definitions that do not needlessly restrict polymorphism.  Consider the basic "max" function.  Rune has this in builtin/misc.rn

// Return the max of |a| and |b|.  Safe to use with secrets.
func max(a, b) {
  return a >= b? a : b
}

Who would have trouble writing this in under a minute?  The ?: operator being constant-time for secrets is a nice bonus.  Here's the Rust version:

pub fn max_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
    match compare(&v1, &v2) {
        Ordering::Less | Ordering::Equal => v2,
        Ordering::Greater => v1,
    }
}

Who could write this in 5 minutes?  Traits as a vehicle for polymorphism are, IMO, a failed experiment.

Bill

[Aiden Hall]

IMO the only reasonable polymorphism I have seen is the system of implicit inlining that Go uses.

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[Andrew Wilson]

max is an interesting example because it only works if the type supports a total ordering, so it's not "completely polymorphic" in the sense that, say, reversing a list, can be.

You need the notion of a type hierarchy or type class, subtypes, e.g., 'int' is a subclass of 'totally ordered type".

For a functional programming example, (say, Standard ML)  we can write fun max(a, b) = if a < b then b else a;  and that looks polymorphic, but in fact, if declared at the top-level, SML will give it the int type, because "<" is not polymorphic, it is a built-in 'overloaded' operator.  *However*, if the surrounding context of the definition showed that a, b or both would be a string type, then it would be typed string * string -> bool.

The interesting question here for Rune is - what is the type of  '>='?  does Rune support type classes?

Cheers

Andrew

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Andrew Wilson

Software Engineer, Android TV Eng Prod