[Pitch #2] Function builders

[Douglas_Gregor]

@Paulo_Faria

Schema<String> {
    Component()
    Component()
}

Per the syntactic transformation, this closure becomes:

Schema<String> {
    let v0 = Component()
    let v1 = Component()
    return Builder<String>.buildBlock(v0, v1)
}

Note that there's no "context" for the initializers of v0 and v1, hence the error. If you add buildExpression to your function builder, like this:

static func buildExpression(_ component: Component<Root>) -> Component<Root> {
    component
}

then the closure is transformed to:

Schema<String> {
    let v0 = Builder<String>.buildExpression(Component())
    let v1 = Builder<String>.buildExpression(Component())
    return Builder<String>.buildBlock(v0, v1)
}

and all your examples work. (Note: I also had to fix your buildBlock to be variadic with ... rather than take an array).

@Paulo_Faria

This is exactly the kind of API that function builders should solve and unfortunately they did not help us here.

Please try with buildExpression---which is, intentionally, the only way in which one can have the builder type influence the types of the subexpressions---and see if your statement above still holds.

Doug