1

Consider the following simple @resultBuilder adding integers:

@resultBuilder struct SumBuilder<Result: SignedInteger> {
  typealias Term = Result

  static func buildBlock(_ components: Term...) -> Term {
    return components.reduce(0) { $0 + $1 }
  }
  
  static func buildExpression<T: SignedInteger>(_ expression: T) -> Term {
    Term(expression)
  }
}

With it, I can write things like:

var x1 = SumBuilder.buildBlock(2, 3, 462534252412524)
var x2: Int64 = SumBuilder.buildBlock(2, 3, 462534252412524)

but in

@SumBuilder<Int32> var y: Int32 {
  10
  Int32(-1)
}

the result type needs to be repeated twice—taking either Int32 out is an error. The following

@SumBuilder<Int32> var y {
...

is also an error: computed property must have an explicit type.

Is there a way around that? Or, maybe I am doing this all wrong in which case what would be better (assuming I want to have the generic builder)?

Cheers,

2

The result builder would expand like this:

var x = SumBuilder.buildExpression(2)
var y = SumBuilder.buildExpression(3)
var z = SumBuilder.buildExpression(462534252412524)
var r: Int64 = SumBuilder.buildBlock(x, y, z)

and this is where you get type mismatch.