Optional syntax

Originally from: https://c3.handmade.network/blog/p/8460-optional_syntax

In C3 optionals are built into the language. They're not the run of the mill optionals as they carry a "optional result value". This makes them more like "Result" types than optionals.

In C3 you declare a variable holding an optional using the ! suffix:

int! x = ...

We can now assign either to the real value, or to the optional result:

int! x = 1; // x is a real value
x = MyRes.MISSING!; // x is assigned an optional
// x = MyRes.MISSING; <- Error: cannot assign "MyRes" to int

If we think of it in terms of a "Result":

Result<int> x;
x.result = 1; // x = 1
x.error = MyRes.ERR; // x = MyRes.ERR!
x.result = MyRes.ERR; // x = MyRes.ERR - fails

So the "clever" ! suffix here is used to assign to the "error" part of the Result. Unfortunately, the suffix is hard to read at the end of a line, where ! and ; often blurs together. For that reason I regularly try to revisit this syntax to see if I can improve on it.

It's used in two cases:

assignment: x = MyRes.ERR!
return: return MyRes.ERR!

While (2) could be replaced by something like return! MyRes.ERR or even raise MyRes.ERR, the assignment is not as easily tackled.

Naive ideas could be to use some symbol salad like:

int! x !!= MyRes.MISSING;
int! x <!= MyRes.MISSING;
int! x <- MyRes.MISSING;
// I'm going to exclude
// int! x := MyRes.MISSING
// as it is used as regular assign in most languages.

Or we could allow those return statements to have a different meaning in an assignment:

int! x = raise MyRes.MISSING;
int! x = return! MyRes.MISSING;

While it's possible, it creates an odd effect if we consider this example:

int! x;
return x = return! MyRes.MISSING;

This should also illustrate that using x = MyRes.MISSING! should be thought of as implicitly doing x = { 0, MyRes.MISSING }.

Understanding that we see how it works:

x = MyRes.MISSING!; // x = { 0, MyRes.MISSING }
return MyRes.MISSING!; // return { 0, MyRes.MISSING }

So really the proper way would be to always translate the !, like this:

x = fault MyRes.ERR;
return fault MyRes.ERR;

Which is a mouthful. One could of course contract that return fault into something like:

x = fault MyRes.ERR;
throw MyRes.ERR;

Unfortunately, this builds the assumption that a return may not return an optional, which it of course can:

int! x = ...
return x; // Optional, so it may be like a "throw" or not

If we want to be super clear we can do something like this:

int! x = ...
if (y) return? x; // Might return an optional
if (z) return! MyRes.MISSING; // Will return an optional.
if (w) return w; // Will not return an optional

Due to the ? being a rethrow, we could require this:

int! x = ...
if (y) return x?; // Use rethrow to make the type int
if (z) return! MyRes.MISSING; // Will return an optional.
if (w) return w; // Will not return an optional

So the question here is if this adds anything over the original:

int! x = ...
if (y) return x;
if (z) return MyRes.MISSING!; 
if (w) return w;

These are questions that need quite a bit of C3 error handling code to decide, so for now things have to stay as they are.

Comments¶

Comment by Christoffer Lernö¶