In the "type select on unions" thread, Kyle said:
Longer term I think it could be valuable in looking at changing unions to
> a ML/Haskell/rust style tagged union. Going down this route could help
> with the syntax questions as well. But that is probably a topic for
> another thread.
>
I'd like to kick this discussion off, because I think it's a great idea.
I'll toss out some thoughts on what this might look like in chapel, in the
form of commented code. This is heavily inspired by rust's enum.
// Simple enumerated type
enum Color { Red, Green, Blue }
// (Contrived) sum-of-products type
enum Shape {
Circle { var radius: real; },
Rectangle { var length, width: real; },
Point
}
proc area(s: Shape) {
select s {
when Shape.Circle{radius} do return pi * radius**2;
when Shape.Rectangle{length, width} do return length * width;
when Shape.Point do return 0;
}
// In rust, the field names here need to match the definition;
// e.g. `when Circle{r} do` would be an error.
// We could do the same, or we could match based on position,
// and allow _ for ignored fields, e.g. `when Rectangle{len, _} do`.
// I prefer the former, to avoid bugs if the enum definition changes.
}
var s = Shape.Rectangle{length:4, width:5};
writeln(area(s));
// Aside: I also like this syntax for record initialization;
// it feels more in-tune with the value semantics.
// Haskell's Maybe type (generic)
enum Maybe {
Just { var value; },
Nothing
}
var i: Maybe(int) = parseInt(readString());
select i {
when Just{value} do writeln("success: ", value);
when Nothing do writeln("parsing failed");
}
enum Expected {
Good { var value; },
Bad { var error: string; }
}
var x: Expected(real) = fallibleComputation();
select x {
when Good{value} do writeln("success: ", value);
when Bad{error} do writeln("failed: ", error);
}
Thoughts?
- Sean
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