? as Boolean type char

[xieguigang]

The ? character was chosen using for the Nullable structure type currently. And all of the stand along ? type char currently was defined as nullable Object type, but the Object is already can be Nothing.

' local flag As Object
Dim flag?

' param flag As Object = True
Sub Test(Optional flag? = True)
End Sub

In my opinion, the ? character have some literal meaning like:

• yes or no?
• true or false?
• success or failure?
• right or wrong?
• exists or not?

So proposal, when user define a variable with a stand along ? character, then it should be defined as a Boolean variable:

' boolean, exists or not?
Dim exists? = list.IndexOf("123") > -1
' nullable integer
Dim n As Integer?

Sub Test(path$, Optional ByRef skip? = False, Optional count As Integer? = 0)
End Sub

[AdamSpeight2008]

I like the usage of ? for Boolean type, but have reservations.
How would it work with null member access eg myobject?.mymember

[xieguigang]

Class Foo

    Public Sub Calls()
    End Sub

End Class

Module Test

    Sub Main()

        ' Boolean
        Dim success? = Test()

        ' If Nothing syntax sugar
        Dim o As New Foo
        Call o?.Calls()

        ' Nullable
        Dim n As Integer? = 5
        Call n?.CompareTo(0)
    End Sub

End Module

@AdamSpeight2008 How would it work with the nullable member access and current If Nothing syntax sugar?

Due to the reason of we just using Boolean its value True/False to indicate the status, function execute result or turn switch on/off, member access of the Boolean type have a very very Very limited scenarios. So that, in my opinion, it wouldn't be a problem to distinguish the member access syntax between the Boolean type char and nullable type.

[AdamSpeight2008]

I see an ambiguity on obj?.Method() is that a null-member access or a member access.
Parsing would be even more complex, likely needing type semantic information.
@xieguigang We not branch the compiler? and try it out.

[AdamSpeight2008]

@xieguigang Initial parsing a type character done. Feature: Boolean TypeCharacter
Loads and loads of unit test failures. Ambiguity. Needs to know identifier's type whilst determining if ? is not a type character, whilst parsing identifier (and said type character.)
I am not sure if it worth the effort to pursuing figuring how to disambiguate.

[xieguigang]

Hi, @AdamSpeight2008 because I'm not so familiar to vbc its operation mechanism, so that I can not give too much help on the code implementation, but I think this issue is very similar to the scenario of dynamics Late-bound member access.

For example, the Late-bound member access and invocation expressions that using candidate ! symbol, which is very similar to this scenario: we use ! symbol for the single type, but it probably have some conflicts between type char and the dynamics calls.

' we not sure this is a method call from a single type
' or dynamics call from an object
Call obj!.method()

Which is very similar to the scenario in current issue:

' we not sure this is a method call from a boolean type
' or method calls from a nullable object
Call obj?.method() 

There are two possible solution for this disambiguate job, as @AnthonyDGreen has pointed out:

there are a couple of solutions to that. 1) decide it's a targeted enough break to accept. 2) quirks it. Basically, on Object expression!.Name will do a late-bound lookup, and on Single expression!.Name will do an early bound lookup for back-compat.

The solution for detects that the statement is a member access from boolean or null-member access, probably could be determined based on the code context: what data type of the parsed identifier it is, this can be solved from the code association between the member access statement and the variable definition.

[AnthonyDGreen]

The problem here is that Dim v? = e already means something in VB. It means infer the type of v to be the nullable form of the type of e. So for the simplest case of assigning a boolean value to a new variable the variable will actually be a nullable bool. That applies most everywhere else one can declare a variable in VB.

I'm not much of a fan of type characters to begin with given that they only apply to the built-in types.