Allowed expressions in optional method args

bgrommes

The docs imply that the optional argument syntax must state a constant as a default value, e.g.,

Optional count As Integer = 0

However, Analyser accepts this list of arguments:

description As String, Optional count As Integer = description.Length

Is this actually valid? If yes, that's fantastic that you can declare optional default values that can be set at runtime, at least from a preceding non-optional argument. If No, then this is a feature request, and if the answer is "no, that's not possible" then this becomes an Analyser bug. ;-)

Garry

Good catch. This wasn't valid but has now been implemented for the next release: https://feedback.objo.dev/feature/682

The intended rule is that optional parameter defaults are runtime expressions. They are evaluated left-to-right, only when the caller omits that argument, and they may reference earlier parameters in the same parameter list.

So this is valid:

Function Describe(description As String, Optional count As Integer = description.Length) As String
    Return description.Left(count)
End Function

Describe("runtime")     # count = 7
Describe("runtime", 3)  # count = 3; default is not evaluated

Defaults cannot reference themselves or later parameters, and the expression must type-check against the optional parameter’s declared type:

Function Bad(Optional count As Integer = other, Optional other As Integer = 1) As Integer
    Return count
End Function

That now produces a compile-time error.

bgrommes

Garry Yay! Thanks. It makes sense that the optional parm's assignment context would just be the prior parameters, but even that will be really handy at times. It is better than strictly limiting it to constants.