This is an ability we lost when we fixed the semantics of inline parameters. Currently, we can only do these checks using macros.

const(n) // static assertion that the parameter n is a constant

I find it weird that the same syntax is used in if as a condition and as an assertion. I think the assertion should be more explicit, like requireConst(n)

There is only one method, that happens to be used in both use cases. If I rename it we would end up with

  if requireConst(n == 0) then 1L
  else if requireConst(n % 2 == 1) then x * power(x, n - 1)

which is a bit too long.

I think that requireConst is better than const. I had shared the same misunderstanding as @smarter, so clearly this is not an isolated problem. If we, who are very used to imagine what it could mean for a compiler, were confused by this in the same way, it's quite likely that non-compiler-developers will be even more confused.

The fact that it's bit longer should not stand in the way of clarity.

Ok. I changed const to requireConst.

I still find it weird that the same method call does something different (return a value or emitting an error) depending on where it's used syntactically. I don't think we've ever had something like that in Scala.

That's not what's happening here. In both cases it a) emits an error if the argument is not constant, and b) returns the constant value of the argument if it is constant.

In the first example, the returned value is just discarded.

Ah I see, I misread if const(foo) and if requireConst(foo) as "if foo is const" which is maybe a sign this isn't the best syntax.

The doc should explicitly state that the value is returned.

Alternative names

• requiredConst
• assertedConst

These names seem to indicate that the value is returned

@liufengyun I updated this PR to match what we agreed during the meeting. Now requireConst returns Unit and I removed the special handling in the inline if. I also made the argument type more precise. Unlike other intrinsic macros, there is only a check done but the code generation is just the normal inlining of the method.

inline def requireConst(inline x: Boolean | Byte | Short | Int | Long | Float | Double | Char | String): Unit = ()